Chiba: Advanced Nonruminant Nutrition Protein - Quiz (Key) Page 1 Protein and Amino Acids (Lee I. Chiba, Auburn University) Quiz - Answer Key [Not expecting all those points to be addressed in your response/answer! Also, expressing your own ideas and(or) unique perspectives on some topic would be certainly appropriate! (. . . Sorry about some typos on our questions, and also not proofreading the answer key!)] 1. Contrary to a classic view, i.e., "protein is completely hydrolyzed to amino acids before absorption," it is clear now that some peptides can be absorbed. What would you think the importance or significance of "peptide absorption?" Also, how would you describe "brush border and mucosal digestion" of protein? Please discuss briefly. Importance of peptide absorption?  Can be beneficial when having some problems in absorbing AA? For instance, some congenital AA absorption defects such as "cyntinurea & Hartnup disease." Cyntinurea - Impaired absorption of some basic AA, which can lead to, among others, Cys calculi (. . . because of the increased excretion of Cys). Hartnup disease - Impaired absorption/metabolism of, e.g., Trp (& others), which may result in, e.g., a "pellagra-like" disorder because of the reduced conversion of Trp to niacin!?  Only 1/3 of AA may exist in the intestinal lumen as free AA, implying that about 2/3 of total AA may be absorbed as peptides!? Thus, important in terms of the quantity of AA absorbed!?  A lot of variations in the absorption of individual AA. Perhaps, supplying slow-absorbing AA in the form of peptides can increase absorption of those AA!? Perhaps, possible because of the lack of competition between AA and peptides absorption/transport mechanisms? Why is this important? For the translation process or protein synthesis, need all AA in adequate amounts simultaneously! Brush border & mucosal digestion?  The final stage of digestion at the external membrane of intestinal mucosa/brush border and within the small intestinal cell. In the intestinal mucosa, enzymes are all amino peptidases, and they can remove 1 AA at a time from the N-terminal end of a peptide.  About 10 to 20% of aminopeptidase activities at the brush border, and larger peptides are broken down to smaller peptides or free AA. Free AA. dipeptides, and tripeptides can be absorbed or taken into the mucosal cells. Based on some data, tetrapeptides may not be absorbed, which may not be entirely true!?  About 80% of amino peptidase activities within the mucosal cell. Small peptides are broken down to AA, and might be some synthesis/re-synthesis of peptides and proteins!?  In the circulation, see mostly free AA, i.e., no peptides from dietary source! The bottom line? Proteins would be broken down completely to AA before being used, which may have some implications. Advanced Nonruminant Nutrition Fall Semester, 2014Chiba: Advanced Nonruminant Nutrition Protein - Quiz (Key) Page 2 2. By now, hopefully, you know something about amino acid "disproportions" such as "deficiency, imbalance, toxicity, and antagonism." Well, how would you distinguish those terms, and also why should we be concerned about those amino acid disproportions? Please discuss briefly. Disproportions?  "Deficiency" - Simply, a protein or diet that is deficient in one or more indispensable AA. "Imbalance" - Created by adding one or more AA without adding the most limiting AA to a protein or diet that is deficient in some/many AA. Thus, can be termed/referred to as "natural or man-made" unbalanced or disproportioned protein/diet, respectively? Usually, see a prompt reduction of feed intake in both situations, and can be alleviated/corrected by supplementing the protein/diet with the most limiting AA.  "Toxicity" - Simply, caused by the addition of a large amount of AA, and may be accompany by specific gross or histopathological lesions. Some differences in the toxicity among AA, with Met being the most toxic and Thr being the least toxic. Can be alleviated/corrected by reducing or removing the "toxic" AA.  "Antagonism" - Simply, 1 AA can affect the requirement/metabolism of other AA. Can happen among structurally similar AA (e.g., Lys & Arg or Val, Ile, & Leu). Can be alleviated/corrected by supplementing with a structurally similar AA/structurally similar AA!? Why should we be concerned?  Especially concerned about this because of the increased use/availability of synthetic AA in recent years, which can increase a chance of formulating a diet with AA disproportion!? Also, for humans? Perhaps, increasing a chance of making some mistakes in the "parental nutrition or enteral nutrition" program!? 3. We tried to determine the "lysine (Lys) requirement" of young rats using "plasma Lys." Our data are presented in the figure. Assuming that Lys is the first limiting amino acid: a) what are some sources of plasma Lys, b) how should we explain changes in plasma Lys as the dietary Lys increased from 0.30 to 1.20%, c) what would you think the Lys requirement of those rats and why? Finally, d) are there any problems in using so called an "empirical method" in determining the energy or nutrient requirement? Please respond to each of these questions briefly. a) Sources of plasma Lys (& AA)?  A combination of those absorbed from the diet, released from the body protein, and released from the synthesized peptides/proteins? b) Responses?  Plasma/circulatory Lys would be low until meeting the requirement because "all or available" Lys would be used for protein synthesis or incorporated into proteins.  Once the requirement is met, "excess" Lys is no longer used for protein synthesis or incorporated into proteins, thus, plasma Lys would start to increase.  After a certain point, "excess" Lys (i.e., those not used for protein synthesis & others) would be deaminated and used for other purposes or eliminated from the body, thus, stabilizing plasma Lys!? c) Requirement?  Somewhere between 0.60 and 1.00, and more likely 0.70 to 80%? Perhaps, the requirement would be the intersection of "plateau & increase?"Chiba: Advanced Nonruminant Nutrition Protein - Quiz (Key) Page 3 d) Difficulties/problems?  Difficult to define a point that the maximum response is reached. Can be done easily mathematically or statistically . . . but biologically?  Response to incremental, e.g., Lys can be dependent on a