The Role of the Time Factor in Protein Synthesis

Abstract

A review and analysis of the contributions made to knowledge of protein synthesis and replacement in the living body by use of the technic of delayed supplementation is presented. In general, delayed supplementation of an amino acid-deficient protein diet with the deficient amino acid is relatively ineffective in promoting growth of young animals, and is also ineffective in preventing wt. loss of mature animals. Important implications of these and related findings are: (1) The protein anabolism of growth and that of maintenance do not differ markedly; (2) amino acids are not stored in the body as such nor as sub-protein building blocks which can be transformed into protein at a later time when deficient amino acids are supplied; (3) proteins can be synthesized only when all the amino acids used in their synthesis are present simultaneously and in the proper proportions; (4) in replacement of "worn out" protein molecules, they are replaced by whole protein molecules and not by restoration of the previous molecule by replacement of amino acid molecules in it; and (5) since amino acids are not stored when fed in an amino acid-deficient diet, and are not transformed into fat or carbohydrate because there is a loss and not a gain in wt., they must be eliminated from the body through pathways yet to be elucidated. There are some lines of evidence that the feeding of a biologically complete protein does not mean that a delayed supplementation effect may not develop due to time differences in liberation of amino acids by gastrointestinal enzymes; thus the relatively poor utilization of raw soybean meal is probably due to delayed liberation of the constituent methionine, which is thus unavailable for opt. use in protein synthesis. Practical applns. of these findings may be made in admn. of relief diets and other diets of marginal utility; in prepn. of amino acid mixtures for therapeutic uses, and in numerous other fields.