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Nutrition and Human Metabolism Q&A, Lecture notes of Nutrition

Coventry CollegeNutrition

Nutrition and Human Metabolism Q&A. 1. What major digestive enzyme is secreted in the saliva? The main enzyme in saliva is salivary amylase, ...

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Nutrition and Human Metabolism Q&A
1. What major digestive enzyme is secreted in the saliva?
The main enzyme in saliva is salivary amylase, which hydrolyzes α 1-4 bonds in
starch. Salivary amylase has some but not much effect because of the short time
of food in the mouth; lingual lipase (infants digest buttermilk).
2. What cells secrete pepsinogen and where are these cells located?
Chief (peptic or zymogenic) cells, located in the oxyntic glands found in the body
of the stomach.
3. What cells secrete HCl and where are these cells located?
Parietal (oxyntic) cells, located in the oxyntic glands found in the body of the
stomach.
4. What stimulates the release of gastrin?
Gastrin release occurs in response to vagal stimulation, ingestion of specific
substances or nutrients, gastric distention, hydrochloric acid in contact with
gastric mucosa, as well as local and circulating hormones (i.e., ingested foods,
hormones, and neurotransmitters). Foods such as coffee and alcohol, as well as
nutrients such as calcium, amino acids, and peptides, present in the GI tract lumen
stimulate gastrin release. Epinephrine in the blood and gastrin-releasing peptide,
released by some nerves, also stimulate gastrin release.
5. What are the SPECIFIC functions of gastrin?
Gastrin stimulates the release of hydrochloric acid, but it also stimulates gastric
and intestinal motility and pepsinogen release. Gastrin stimulates the cellular
growth of (has trophic action on) the stomach, and both small and large intestine.
6. Is there some sort of feedback mechanism that controls how much gastrin is secreted?
Yes, when the lumen pH gets too acidic, a feedback mechanism reduces acid
secretion by inhibiting gastrin release. Somatostatin inhibits gastrin release from
the G-cells (as well as inhibiting HCL secretion
at the parietal cell.)
7. Name an enzyme that digests protein in the stomach.
Pepsin is the principal proteolytic enzyme in the stomach.
8. Name an enzyme that digests lipids in the stomach.
Lingual lipase, produced by lingual serous glands in the mouth, hydrolyzes
dietary triacylglycerols in the stomach and small intestine.
9. What word ending is associated with enzymes?
-ase
10. Is there major digestion of carbohydrate in the stomach?
No. In the mouth, the enzyme salivary amylase (ptylin), which operates at a
neutral or slightly alkaline pH, starts the digestive action on starch, hydrolyzing it
into smaller molecules. The activity of amylase is halted by contact with
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Nutrition and Human Metabolism Q&A

  1. What major digestive enzyme is secreted in the saliva? The main enzyme in saliva is salivary amylase, which hydrolyzes α 1-4 bonds in starch. Salivary amylase has some but not much effect because of the short time of food in the mouth; lingual lipase (infants digest buttermilk).
  2. What cells secrete pepsinogen and where are these cells located? Chief (peptic or zymogenic) cells, located in the oxyntic glands found in the body of the stomach.
  3. What cells secrete HCl and where are these cells located? Parietal (oxyntic) cells, located in the oxyntic glands found in the body of the stomach.
  4. What stimulates the release of gastrin? Gastrin release occurs in response to vagal stimulation, ingestion of specific substances or nutrients, gastric distention, hydrochloric acid in contact with gastric mucosa, as well as local and circulating hormones (i.e., ingested foods, hormones, and neurotransmitters). Foods such as coffee and alcohol, as well as nutrients such as calcium, amino acids, and peptides, present in the GI tract lumen stimulate gastrin release. Epinephrine in the blood and gastrin-releasing peptide, released by some nerves, also stimulate gastrin release.
  5. What are the SPECIFIC functions of gastrin? Gastrin stimulates the release of hydrochloric acid, but it also stimulates gastric and intestinal motility and pepsinogen release. Gastrin stimulates the cellular growth of (has trophic action on) the stomach, and both small and large intestine.
  6. Is there some sort of feedback mechanism that controls how much gastrin is secreted? Yes, when the lumen pH gets too acidic, a feedback mechanism reduces acid secretion by inhibiting gastrin release. Somatostatin inhibits gastrin release from the G-cells (as well as inhibiting HCL secretion at the parietal cell.)
  7. Name an enzyme that digests protein in the stomach. Pepsin is the principal proteolytic enzyme in the stomach.
  8. Name an enzyme that digests lipids in the stomach. Lingual lipase, produced by lingual serous glands in the mouth, hydrolyzes dietary triacylglycerols in the stomach and small intestine.
  9. What word ending is associated with enzymes? -ase
  10. Is there major digestion of carbohydrate in the stomach? No. In the mouth, the enzyme salivary amylase (ptylin), which operates at a neutral or slightly alkaline pH, starts the digestive action on starch, hydrolyzing it into smaller molecules. The activity of amylase is halted by contact with

hydrochloric acid. If the digestible carbohydrate were to remain in the stomach long enough, the acid hydrolysis would eventually reduce much of it to the monosaccharides. However, the stomach usually empties itself before significant digestion can take place, and carbohydrate digestion occurs almost entirely in the proximal small intestine.

  1. What regulates how much “food” passes into the small intestine? Approximately 1 to 5 ml (< 1 tsp.) of chyme enters the duodenum about twice per minute. Contraction of the pylorus and proximal duodenum is thought to be coordinated with contraction of the antrum. Gastric emptying is also partially affected by the macronutrient composition of the food. Carbohydrate and protein appear to empty at approximately the same rate from the stomach; fat, however, slows gastric emptying into the duodenum. Salts and monosaccharides inhibit gastric emptying, as do many free amino acids like tryptophan and phenylalanine. Complex carbohydrates, especially soluble fiber, decrease (slow down) the rate of gastric emptying. Neural gastrointestinal reflexes, along with the release of regulatory peptides such as secretin by the duodenal bulb, also influence gastric emptying, which following a meal usually takes between 2 and 6 hours.
  2. What causes the release of cholecystokinin (CCK)? CCK is released in response to the presence of lipids (fat) in chyme.
  3. What does CCK do? CCK stimulates secretion of pancreatic juices and enzymes in response to fat. It causes contraction of the gallbladder (i.e., causes the gallbladder to squirt bile into the small intestine) and slows stomach emptying.
  4. What causes the release of secretin? Secretin is secreted into the blood by S-cells of the proximal small intestine in response to the release of acid chyme into the duodenum. (Secretin's major action is to increase the pH of the small intestine by stimulating secretion of water and bicarbonate (pancreatic juice) by the pancreas. It also inhibits gastric acid secretion and gastric emptying.)
  5. Where do pancreatic secretions enter the intestine? Pancreatic secretions (fluid, electrolytes, bicarbonate, and enzymes) are released into the duodenum.
  6. Pancreatic secretions contain two distinctly different types of material. What are they?
    1. Enzymes: are responsible for the digestion of approximately half of all ingested carbohydrates, half of all proteins, and almost all of ingested fat.
    2. Bicarbonate: in pancreatic juice released into the duodenum is important for neutralizing the acid chyme passing into the duodenum from the stomach and for maximizing enzyme activity within the duodenum.
  7. Where does bile enter the intestine? Bile flows into the duodenum from the gallbladder.
  8. Describe the pH differences as “food” passes from the mouth to the stomach to

contractions (longitudinal) mix the intestinal contents with the digestive juices. Standing contractions (segmentation) of circular muscles, produces bidirectional flow of the intestinal contents, and serves to mix and churn the chyme with digestive secretions in the small intestine. Peristaltic waves, or progressive contractions, move the chyme distally along the intestinal mucosa toward the ileocecal valve.

  1. Where does digestion of protein begin? What are the sources of enzymes that digest protein? Digestion of protein begins in the stomach, where pepsin functions as a protease -
    • that is, an enzyme that hydrolyzes proteins. Hydrochloric acid converts or activates the zymogen pepsinogen to form pepsin. Most protein digestion takes place in the deuodenum, however. The pancreatic proteolytic enzymes pancreatic trypsin, chymotrypsin, and carboxypolypeptidase break down intact protein and continue the breakdown started in the stomach until small polypeptides and amino acids are formed. Proteolytic peptidases located on the brush border also act on polypeptides, changing them to amino acids, dipeptides, and tripeptides. The final phase of protein digestion takes place in the brush border, where dipeptides and tripeptides are hydrolyzed to their constituent amino acids by peptide hydrolases.
  2. How is the acid chyme neutralized in the small intestine? What are the effects if the chyme is not effectively neutralized? Chyme, moving from the stomach into the duodenum, initially has a pH of about 2 because of its gastric acid content. Chyme is neutralized in the duodenum by secretions from the pancreas and Brunner's glands. If the chyme is not effectively neutralized, the duodenum is not protected from the gastric acidity.
  3. Where does digestion of most lipid begin? What enzyme is responsible? Enzymes necessary for lipid digestion are produced by the pancreas and secreted into the small intestine. Pancreatic lipase, the major fat-digesting enzyme, hydrolyzes triacylglycerols.
  4. Name two substances absorbed from the stomach. Water, some fat-soluble drugs (aspirin), and alcohol.
  5. Where are enterocytes found? The villi of the small intestine are fingerlike projections lined by hundreds of cells (enterocytes, also called absorptive epithelial cells).
  6. How frequently are cells in the small intestine renewed (replaced)? Intestinal cell turnover is rapid, approximately every 3 to 5 days.
  7. Where in the gastrointestinal tract are most nutrients absorbed? Most in duodenum and jejunum.
  8. If disease or drugs increase intestinal motility, what is the potential effect on absorption? Digestion and absorption of nutrients within the small intestine are rapid, with most of the carbohydrate, protein, and fat being absorbed within 30 minutes after chyme has reached the small intestine. About 90-95% of the water and sodium

entering the colon each day is absorbed. Increased intestinal motility could adversely impact the ability of the intestinal tract to adequately perform its job of absorbing nutrients and water.

  1. What is recirculation of compounds such as bile salts between the small intestine and the liver called? The circulation of bile is termed entero-hepatic circulation. The pool of bile is thought to recycle at least twice per meal.
  2. Where are bile salts secreted into the intestine? Where are they reabsorbed? Where are they excreted? Bile flow into the duodenum is regulated by the intraduodenal segment of the common hepatic bile duct and the sphincter of Oddi, located at the junction of the common hepatic bile duct and the duodenum. More than 90% ofthe bile acids and salts secreted into the duodenum are reabsorbed by active transport in the ileum. Small amounts of the bile may be passively reabsorbed in the jejunum and the colon. Bile that is absorbed in the ileum enters the portal vein and is transported attached to plasma protein albumin in the blood back to the liver. New bile acids are typically synthesized in amounts about equal to those that are lost (excreted) in the feces.
  3. What type of nutrients pass into the portal blood? Carbohydrates ( the monosaccharides: glucose, galactose, and fructose) pass through the mucosal cell and, via the capillary of the villus, into the bloodstream, where they are carried by the portal vein to the liver. Protein (peptides and amino acids) are transported to the liver via the portal vein for release into the general circulation.
  4. What type of nutrients pass into lacteals for transport by the lymphatic system? Lacteals are intestinal lymph carrying vessels, arising from the villi, that convey chyle (lymph and emulsified fat or free fatty acids) to the thoracic duct.
  5. What difference does the route of absorption make in the destination of nutrients or drugs? Fat soluble vitamins (A, D, E, K), fat, and cholesterol are carried by lacteals in the lymphatic system and enter the bloodstream at a slow rate. Entry of chylomicrons into the blood from the lymph continues for up to 14 hours after consumption of a meal rich in fat.
  6. What are the principle nutrients absorbed in the large intestine? The proximal colonic epithelia absorb sodium, chloride, and water.
  7. What effect does fiber in the ileum have on enterohepatic recirculation? Bile acids bound to fiber cannot be reabsorbed and recirculated. Fiber-bound bile acids are typically sent into the large intestine for either fecal excretion or colonic microflora degradation.
  8. The immediate risk of severe diarrhea is loss of which nutrients? Excessive loss of fluid and electrolytes, especially sodium and potassium.

hydrogen gas. Consumption of quantities greater than 12 g (the amount typically found in 240 mL of milk) may result in bloating, flatulence, cramps, and diarrhea.

  1. In the “usual U.S. diet”, how much of the digestible carbohydrate is starch and how much is sucrose? Roughly half of dietary carbohydrate is in the form of polysaccharides such as starches and dextrins, derived largely from cereal grains and vegetables. The remaining half is supplied as simple sugars, the most important of which include sucrose, lactose, and, to a lesser extent, maltose, glucose, and fructose. Sucrose, consisting of a glucose and a fructose residue, furnishes approximately one-third of total dietary carbohydrate in an average diet.
  2. How many grams of carbohydrate in the “usual U.S. diet”? 48% of 2000 kcal = 960 kcal from carbohydrate. 960 kcal / 4 kcal/g = 240 g. of carbohydrate in the "usual U.S. diet".
  3. Which carbohydrate is sweeter? Sucrose or fructose? Fructose is the sweetest of the sugars.
  4. Are amino acids transported via the portal blood or the lymphatic system? Amino acids are transported across the cell membrane into the surrounding fluid where they enter the capillaries on their way to the liver.
  5. What lipids make up the cell membrane? Membrane lipids consist primarily of phospholipids. Phosphoglycerides and phosphingolipids (phosphate-containing sphingolipids) comprise most of the membrane phospholipids. Of the phosphoglycerides, phosphatidylcholine and phosphatidylethanolamine are particularly abundant in higher animals. Another important membrane lipid is cholesterol.
  6. What effects does lack of bile acids have on lipid digestion and absorption? Bile salts are necessary to decrease the surface tension of the fat, thus permitting emulsification of the fat and enabling digestion (hydrolysis) of the triacylglycerol molecules to occur by pancreatic and intestinal lipases. Once hydrolyzed, bile acids and salts help in the absorption of these end products of lipid digestion. Bile acids and salts, along with phospholipids, help in the absorption of lipids by forming small, spherical, cylindrical, or disklike shaped complexes called micelles that permit solubility in the watery digestive fluids and transportation to the intestinal brush border for absorption. Without bile acids, digestion and absorption of lipids (fat) would not be possible.
  7. Are bile acids still secreted if the gall bladder is removed? Bile acids are still secreted, but cannot be stored and released in response to the presence of fat. Rather, the bile drips continuously into the duodenum.
  8. What is a micelle? A micelle is a small (<10 mm) spherical, cylindrical, or disklike shaped complex that can contain as many as 40 bile salt molecules. The hydrophobic steroid portion of bile salts and acids, which is mostly fat soluble, position themselves together and surround the monoacylglycerols and fatty acids that formed

following the action of lipases. Polar portions of the bile salts, bile acids, and phospholipids project outward from the lipid core of the micelle, thus permitting solubility in the watery digestive fluids and transportation to the intestinal brush border for absorption.

  1. What effect does chain length of a fatty acid have on the route of absorption? Some substances such as water and small lipid molecules cross membranes freely by diffusion. Other compounds cannot cross cell membranes without a specific carrier, such as in facilitated diffusion and active transport. Some large molecules are moved into the cell via pinocytosis, engulfment by the cell membrane.
  2. What are the principle components of the plasma membrane? Membrane lipids
    • phospholipids
    • cholesterol
    • hydrophobic) Membrane proteins -- pumps, receptors and enzymes Protects cellular components Allows exposure to the environment Contains some carbohydrate

Membranes are sheetlike structures composed primarily of lipids and proteins held together by noncovalent interactions. Membrane lipids consist primarily of phospholipids, which have both a hydrophobic and hydrophilic moiety. In water, they form lipid bilayers which retard the passage of many water-soluble compounds into and out of the cell. Membrane proteins serve as pumps, gates, receptors, energy transducers, and enzymes. The plasma membrane has a greater carbohydrate content owing to the presence of glycolipids and glycoproteins. The plasma membrane has a higher content of cholesterol which enhances the mechanical stability of the membrane and regulates its fluidity.

  1. What are the key metabolic pathways in the cytoplasmic matrix? Glycolysis Pentose phosphate pathway Glycogenesis Glycogenolysis Fatty acid synthesis Production of nonessential, unsaturated fatty acids
  2. What are the key metabolic reactions that occur in mitochondria? Oxidative phosphorylation - production of most of ATP (via electron transport chain). Electron transport chain is exothermic. Release energy from food/couple to form ATP. Includes Krebs cycle and fatty acid oxidation. All cells except RBC have mitochondria.
  3. What organ secretes the majority of the digestive enzymes? The pancreas is the major source of most digestive enzymes. Enzymes from the pancreas digest 50% of all carbohydrates - alpha-amylase, 50% of all proteins -

The oxidation of 1 mol of acetyl CoA in the Krebs cycle yields a total of 12 ATPs. 12 ATPs x 2 mol of acetyl CoA per mole of glucose = 24 ATPs. Plus 6 ATPs from intramitochondrial ppyruvate dehydrogenase reaction = 38 ATPs..

  1. What does gluconeogenesis mean? Glucose synthesis from noncarbohydrate sources. (i.e., generation of new glucose.)
  2. What is a normal fasting serum glucose level? 70-105 mg/dL fasting..
  3. What is glycogen loading? Consuming carbohydrates over a period of several days prior to an athletic event in an attempt to store up glycogen which is a limiting factor for exercise at intensities requiring 70-85% VO2max.
  4. What effect does dietary fiber have on water in the stool? What effect does this have on constipation? The gastrointestinal effects of the ingestion of fibers that can hold water and create viscous solutions within the GI tract include: delayed (slowed) emptying of food from the stomach, reduced mixing of gastrointestinal contents with digestive enzymes, reduced enzyme function, ecreased nutrient diffusion rate and thus delayed nutrient absorption, and altered small intestine transit time. Fibers that are nonfermentable, especially cellulose and lignin, and fibers that are more slowly fermentable, such as some hemicelluloses, have been shown to be helpful in overcoming constipation, particularly constipation associated with symptomatic diverticular disease and/or irritable bowel syndrome.
  5. What effect do free sugars have on stool water? Free sugars would alter osmolarity, leading to a high osmotic pressure and an influx of water into the cells from the surroundings (the stool water).
  6. Name some specific chemical compounds produced by carbohydrate fermentation in the colon. The principal metabolites of fermentable fibers (including any starch that has passed into the cecum and been degraded by bacteria) are lactate and short-chain fatty acids (SCFAs). The short chain fatty acids include primarily acetic, butyric, and propionic acids. Other products of fiber fermentation are hydrogen, carbon dioxide, and methane gases that are excreted as flatus or are expired by the lungs.
  7. What difference does it make if glucose is joined by alpha-1-4 linkages or beta-1- linkages? The key enzyme in the digestion of dietary polysaccharides is α-amylase, a glycosidase that specifically hydrolyzes α-1,4 glycosidic linkages. Resistant to the action of the enzyme, therefore, are the β-1,4 bonds of cellulose.
  8. Give an example of a digestible polysaccharide and of a non-digestible polysaccharide? Starch is a digestible polysaccharide.

Cellulose and lignin are indigestible polysaccharides.

  1. What causes the sensation of “heartburn”? On swallowing, the LES pressure drops. This drop in LES pressure relaxes the sphincter so that food may pass from the esophagus into the stomach. Closure of the LES sphincter is important because it prevents gastroesophageal reflux, the movement of chyme from the stomach back into the esophagus. The gastric acid in the chyme when present in the esophagus is an irritant to the esophageal mucosa. The individual experiencing reflux feels a burning sensation in the midchest region, a condition referred to as heartburn.
  2. Think about nutrients in a meal. For each of these, where would most digestion and absorption occur? Nutrient Digestion Absorption Protein Stomach, Small Intestine Small Intestine Carbohydrate Small Intestine Small Intestine Lipid Small Intestine Small Intestine
  3. What types of foods provide fiber? Dietary fiber is derived from plant cells. The consumption of plant foods provides fiber in the diet. Cereal bran such as wheat bran provides primarily hemicellulose as well as lignin. Psyllium provides primarily mucilages but also some nonpolysaccharides. Consumption of fruits and vegetables provides almost equal quantities of cellulose and pectin.
  4. What makes a carbohydrate “a fiber” for humans? Probably the most widely accepted definition for dietary fiber is "plant polysaccharides and lignin which are resistant to hydrolysis by the digestive enzymes of man."
  5. What are the health benefits of insoluble fiber? Insoluble fibers decrease (speed up) intestinal transit time and increase fecal bulk. A shortened fecal transit time decreases the time during which toxins can be synthesized and in which they are in contact with the colon. Insoluble fibers such as lignin that resist degradation bind carcinogens, thereby minimizing the chances of interactions with colonic mucosal cells.
  6. What are the health benefits of soluble (viscous) fiber? Generally, soluble fibers delay gastric emptying, increase transit time (slower movement) through the intestine. This effect creates a feeling of postprandial (after eating) satiety (fullness) as well as slows down the digestion process. Wheat bran is one of the most effective fiber laxatives because it can absorb three times its weight of water, thereby producing a bulky stool. Fibers that increase fecal bulk decrease the intraluminal concentrations of carcinogens and thereby reduce the likelihood of interactions with colonic mucosal cells.
  7. What are the effects of fiber on stomach emptying? What type of fiber is involved? When fibers form viscous gels or hydrate within the stomach (i.e. soluble fiber), the release of the chyme from the stomach into the duodenum (proximal small intestine) is delayed (slowed). Thus, nutrients remain in the stomach longer with

The recommended intake of fiber for the general population ranges from 20 to 40 g/day. Another recommendation is 10 to 13 g dietary fiber intake per 1000 kcal. So, for a 2000 kcal/day diet, that would be 20 to 26 g/day.

  1. What are the indispensable (essential) amino acids? PVT TIM HALL Phenylananine Valine Tryptophan

Threonine Isoleucine Methionine

Histidine (Arginine) Leucine Lysine

  1. Where does protein digestion begin? What initiates digestion? Protein digestion begins in the stomach. There is no protein digestion in the mouth and esophagus. Protein digestion is initiated by the release of HCL, stimulated by gastrin, GRP (glucokinase regulatory protein), acetylcholine, and histamine.
  2. What are enzymes involved in digestion of protein? Gastrin, GRP, acetylcholine, and histamine, pepsin, pepsinogen, secretin, CCK, bicarbonate, trypsinogen, chymotripssinogen, collage-nase, proelastase, procarboxypeptiases, trypsin, additional peptidases.
  3. What are the sources of the enzymes that digestion protein? The release of HCL is stimulated by gastrin, GRP, acetylcholine, and histamine, and denatures 4º, 3º, and 2º structure. Pepsin is activated by pepsinogen and yields large polypeptides by breaking long protein chains. Acid chyme in the intestine lowers the pH and stimulates secretin and CCK. The pancreas secretes bicarbonate. Digestive enzymes from the pancreas include trypsinogen, chymotripssinogen, collage-nase, proelastase, procarboxypeptiases. Trypsin is formed and activates others. Additional peptidases in brush border.
  4. Where are most amino acids absorbed? Most a.a. absorption is in the proximal small intestine (first few feet).
  5. What is the RDA for protein? Calculate the protein recommendation for someone weighing 154 lbs. The RDA for protein for adults is 0.8 g/kg of body weight. 154 lbs. / 2.2 kg/lb = 70 kg. 70 kg x 0.8 g/kg = 56 g. protein required/day.
  6. When amino acids are degraded, what compound is formed in the largest amounts from the amino group? Ammonia is formed in the body from chemical reactions such as deamination.

The urea cycle, in the liver, is the body's way of removing ammonia.

  1. What are four possible fates of the carbon skeleton from amino acids? Once an amino group has been removed from an amino acid, the remaining molecule is referred to as a carbon skeleton or α-keto acid. Carbon skeletons of amino acids can be further metabolized with the potential for multiple uses in the cell. An amino acid's carbon skeleton, for example, can be used for the production of

  2. Energy Energy, CO2, NH4+ and H2O

  3. Glucose Conversion of a.a. to glucose increased by high glucagon: insulin & cortisol

  4. Ketone bodies

  5. Cholesterol Leucine generates HMG CoA; others generate acetyl CoA

  6. Fatty acids

  7. What is 3-methylhistidine? Why would it be measured? 3-Methlyhistidine is an index of protein degradation for tissues in the body. It's an indicator of muscle mass/catabolism.

  8. What does a “post-translational” modification mean? Translation is the process by which genetic information in an mRNA molecule specifies the sequence of amino acids in the protein product. The completed protein dissociates from the mRNA in active form, although some post- translational, chemical modification of the protein is often necessary.

  9. Approximately what % of basal energy need is associated with protein turnover? Protein turnover accounts for 10-25% of resting energy expenditure.

Amino Acid Metabolism? ~ 20% for protein/N compound synthesis (14% remains in liver for protein synthesis, 6% plasma proteins - synthesized in liver and secreted into bloodstream) ~ 57% catabolized in liver (assuming adequate a.a. intake). ~ 23% released to systemic circulation -- primarily branched a.a.

  1. What are two tissues that have a very high protein turnover rate? What are two tissues that have low turnover rates? Rapid turnover: plasma protein, visceral protein Low turnover: muscle protein, bone? nerves?
  2. How can a habitually high intake of amino acids affect the mRNA for enzymes that catabolize amino acids? Protein synthesis is affected by the amount of mRNA, ribosomes, availability of a.a for tRNA and hormonal environment. Amino acid oxidation increases if a.a are in surplus or if an essential a.a is missing. Therefore, a habitually high intake of a.a would induce the production of more enzymes to catabolize a.a.
  3. What are some conditions that increase proteolysis of muscle tissue? Why does each cause these increases?

During illness, starvation, malnutrition, protein synthesis and degradation are not in balance. In malnutrition, protein synthesis decreases. In starvation, protein catabolism is decreaseed (begin to use ketones). In sepsis, ketone formation is reduced so the body has to degrade body protein for glucose synthesis.

  1. What determines if an amino acid is indispensable or dispensable? Essential or indispensable a.a's must be provided in the diet. We can't make them at all or in enough quantity. The structure of the carbon chain makes a.a. essential or indispensable, i.e., we can't make a ring structure or branch chains; we have no enzymes to do that. Newer categories added to the essential/indispensable and nonessential/dispensable categories include conditionally or acquired indispensable a.a.'s. A dispensable amino acid may become indispensable should an organ fail to function properly as in the case of infants born prematurely or in the case of disease associated organ malfunction. For example, neonates born prematurely often have immature organ function and are unable to synthesize many nonessential amino acids such as cysteine and proline. Immature liver function or liver malfunction due to cirrhosis, for example, impairs phenylalanine and methionine metabolism, which occurs primarily in the liver. Consequently, the a.a's tyrosine and cysteine normally synthesized from phenylalanine and methionine catabolism, respectively become indispensable until normal organ function is established. In some kidney diseases, serine becomes indispensable because it cannot be synthesized in sufficient quantity by the diseased kidneys. Inborn errors of amino acid metabolism resulting from genetic disorders in which key enzymes in amino acid metabolism lack sufficient enzymatic activity also illustrate a situation in which dispensable amino acids become indispensable. Individuals with classical phenylketonuria (PKU) exhibit little to no phenylalanine hydroxylase activity. This enzyme converts phenylalanine to tyrosine. Without hydroxylase activity, tyrosine is not synthesized in the body and must be provided completely by diet; it is indispensable. In other inborn errors of metabolism, amino acids such as cysteine become indispensable. Thus, a.a's that are normally dispensable may become indispensable under certain physiological conditions.
  2. How does the usual amount of dietary protein compare with the amount of protein that is turned over in the body? RDA for protein is calculated on the basis of 0.8 g protein per kg body weight. The average American eats 80-100 g protein per day. Protein turnover is about 4.6 g/kg body weight. (For a 180 lb. person, it's 376 g.) (e.g., for a 70 kg male, protein turnover would be approx. 320 g daily.)
  3. Do amino acids compete with each other for transport? Explain. Competition between a.a for transport by a common carrier has been documented. Multiple energy-dependent transport systems with overlapping specificity for a.a. have been demonstrated in the intestinal brush border. Both sodium dependent and sodium-independent transport systems exist. Amino acids using the same carrier system compete with each other for absorption.
  4. What is transamination? Transamination reactions involve the transfer of an amino group from one amino acid to an amino acid carbon skeleton of α-keto acid (an amino acid without an

amino group). The carbon skeleton/α-keto acid that gains the amino group becomes an amino acid, and the amino acid that loses its amino group becomes an α-keto acid.

  1. Does catabolism of amino acids increase or decrease after a meal? Amino acids not used by the intestinal cell are transported across the basolateral membrane of the enterocyte into interstitial fluid, where they enter the capillaries of the villi and eventually the portal vein for transport to the liver. The liver is the primary site for the uptake of most of the amino acids following ingestion of a meal. The liver is thought to monitor the absorbed amino acids and to adjust the rate of their metabolism according to the needs of the body. Approximately 57% of amino acids taken up by the liver are typically catabolized in the liver. "That percentage goes way up if intake is a very big protein meal. Extra a.a are destroyed really rapidly." Therefore, catabolism of amino acids would increase after a meal.
  2. If energy is inadequate, are more or less amino acids catabolized than normal? Amino acids are used for energy in the body when diets are inadequate in energy (measured in kilocalories). Therefore, less amino acids would be catabolized than normal.
  3. What is elevated during stress that increases protein breakdown? With stress, including sepsis, trauma, surgery, and burns, glucocorticoids (primarily cortisol), catacholamines (e.g. epinephrine), insulin, and glucagon release increase. However, the glucagon:insulin ratio favors glucagon. Consequently, tissues become resistant to insulin action, and hyperglycemia (high blood glucose concentrations) persists. In addition, cortisol concentrations may remain elevated in the blood for prolonged periods following severe trauma or stress events. High blood cortisol promotes proteolysis and hyperglycemia.

The principal mechanism of adjustment to starvation is a change in hormone balance. In particular, there is a sharp decrease in insulin production. Decreased insulin activity, coupled with increased synthesis of counterregulatory hormones such as glucagon, promotes fatty acid mobilization from adipose tissue, production of ketones, and the availability of amino acids for gluconeogenesis.

  1. What does the half-life of serum or plasma proteins have to do with their effectiveness for measuring nutritional status? Because of albumin's relatively long half-life (~14-18 days), it is not as good or as sensitive an indicator of visceral protein status as some of the other plasma proteins. The half-life is the time that it takes for 50% of the amount of a protein such as albumin to be degraded. Transthyretin (pre-albumin) and retinol-binding proteins are used as indicators of visceral protein status. However, because these two proteins have relatively shorter half-lives (~2 days and 12 hours, respectively) than albumin, they are more sensitive indicators of changes in visceral protein status than albumin. Pre-albumin is a better indicator for short-term changes than is albumin because of the half-lives of the proteins.
  2. Presence of what enzymes is serum is taken as a measure of tissue damage? Alanine in serum means heart attack (leakage of alanine from damaged tissue to

Tyrosine hydroxylase, catalysis of tyrosine metabolism also yields in other cells, such as the skin, eye and hair cells, and melanin (a pigment that gives color to skin, eyes, and hair).

  1. What neurotransmitter is synthesized from tryptophan? What neurotransmitter? Tryptophan is partially glucogenic as it is metabolized to form pyruvate; it is also partially ketogenic and forms acetyl CoA. It can be metabolized to produce nicotinamide, serotonin, and melatonin.
  2. What is melatonin? The hormone melatonin is derived in the brain from the a.a. tryptophan. Use of tryptophan supplements to promote sleep has been promoted, as has supplements of melatonin, which is also made from tryptophan in the pineal gland, which lies about in the center of the brain. Melatonin plays a role in the regulation of sleep. Yet, melatonin supplements of 2-500 mg for use as a sleep aid have yielded variable results. The LT use of melatonin and a.a, as well as effective dosing and administration, remain unknown.
  3. What is the metabolic role of carnitine? The oxidation of fatty acids is compartmentalized within the mitochondrion. Fatty acids and their CoA derivatives, however, are incapable of crossing the inner mitochondrial membrane, necessitating a membrane transport system. The carrier molecule for this system is carnitine which can be synthesized in humans from lysine and methionine, and which is found in high concentration in muscle. Carnitine is needed for the transport of long-chain fatty acids across the inenr mitochondrial membrane for oxidation. In muscle, carnitine also may serve as a buffer for free coenzyme (Co)A and may be involved in branched-chain amino acid metabolism. Carnitine is also thought to be involved with immune system function.
  4. In what tissues are branched chain amino acids metabolized? Muscle, as well as the heart, kidney, diaphragm, and other organs, possess BCAA transferases, located in both the cytosol and mitochondria. The enzyme complex needed for the next step is found in the mitochondria of many tissues, including liver, muscle, heart, kidney, intestine, and the brain.
  5. What are the products of complete catabolism of a simple amino acid? Energy, CO 2 , NH 4 +^ and H 2 O.
  6. Which tissue has the complete urea cycle? Are these enzymes sensitive to the amount of substrate? The urea cycle occurs in the liver. Activities of urea cycle enzymes fluctuate with diet and hormone concentrations. For example, with low-protein diets or acidosis, urea synthesis (the amount of mRNA for each of the enzymes) diminishes and urinary urea nitrogen excretion decreases significantly. In the healthy individual with a normal protein intake, blood urea nitrogen (BUN) concentrations range from 8 to 20 mg/dL, and urinary urea nitrogen represents about 80% of total urinary nitrogen. Glucocorticoids and glucagon typically increase mRNA for the urea cycle enzymes.
  1. How is urea removed from the body? Through the urinary system. With normal protein intakes, urea may be 80% of total urinary nitrogen.
  2. How is creatinine related to muscle mass? Urinary excretion of creatinine and 3-methylhistidine are used as indicators of the amount of existing muscle mass and the rate of muscle degradation, respectively. Urinary creatine excretion is considered to be a reflection of muscle mass because it is the degradation product of creatine, which makes up approximately 0.3% to 0.5% of muscle mass by weight. The creatinine excreted in the urine reflects about 1.7% of the total creatine pool per day. However, urinary creatinine excretion is not considered to be a completely accurate indicator of muscle mass because of the variation that occurs in muscle creatine content.
  3. What are key metabolic reactions that occur in the mitochondria? The mitochondria are the primary sites of oxygen use in the cell and are responsible for most of the metabolic energy (adenosine triphosphate, or ATP) produced in cells. The electron transport chain couples the energy released by nutrient oxidation to the formation of ATP. Among the metabolic enzyme systems functioning in the mitochondrial matrix are those catalyzing reactions of the Krebs cycle and fatty acid oxidation. Other enzymes are involved in the oxidative decarboxylation and carboxylation of ppyruvate and in certain reactions of amino acid metabolism. Mitochondrial genes (inherited only from the mother) code for proteins vital to the production of ATP.
  4. Can amino acids from muscle be metabolized for energy? Yes. BCAA + aspartate, asparagine, glutamate are catabolized in skeletal muscle. When insulin increases, BCAA move in. α-keto acids may be oxidized in muscle (mitochondria) or transported to other tissues. Creatine and CP cyclize to creatinine which is an indicator of the amount of existing muscle mass.
  5. When is a person in positive N balance? During growth, protein synthesis exceeds degradation, and nitrogen intake exceeds excretion, resulting in a positive nitrogen balance. Positive nitrogen balance means there is more coming in than going out. Growth, pregnancy. Excess energy intake fosters nitrogen retention.
  6. Describe a situation in which a person would be in negative nitrogen balance. Protein synthesis and protein degradation are under independent controls. Rates of synthesis can be quite high as with protein accretion during growth. Alternately, protein degradation can be quite high, as during fever. Negative balance means more is going out than coming in. Loss of LBM. Insufficient calories from carbohydrate and/or fat mandate the oxidation of some protein to supply energy needs.
  7. Give some examples of transport proteins. Albumin - transports a variety of nutrients such as calcium, zinc, and vitamin B 6. Transthyretin (formerly called prealbumin) - complexes with retinol-binding protein, for the transport of retinol (vitamin A).