Recent work on proteins, with special reference to peptide biosynthesis and nutritive value

Abstract

THE protein field is so immense that only two aspects can be dealt with in the limited space available : the biosynthesis of the peptide bond (with which is associated protein ' turn-over ' in the body) and the nutritive value of proteins. The first is of outstanding importance in the field of pure science, and the second is of great and increasing practical importance. Moreover, the two are closely related within the field of nutrition, which properly starts with a study of the soil in which food is grown and finishes with the cellular reactions in which that food, or its metabolites, participate. Dynamic equilibriumThe most profound change that has come over our thinking in the protein field, and indeed in the whole field of metabolism, is the concept of dynamic equilibrium. Our earlier views on nitrogen metabolism were crystallized half a century ago by Folin,l who distinguished two types of protein catabolism in the body-one variable, the other constant. The constant or endogenous metabolism produces the urinary end-products of creatinine, neutral sulphur, ethereal sulphur and uric acid. The distinguishing character is that it is constant, being related to the basal metabolism of the body and continuing even when the diet is completely deficient in nitrogen. The other form of nitrogen metabolism is variable, reflects directly dietary changes, and gives rise in the urine to urea and inorganic sulphur. This is the exogenous metabolism.It was assumed by Folin that protein synthesis came to an end when growth ceased in the adult. Borsook & Keighley2 in 1935 suggested that protein synthesis continues even in the adult, and it is the confirmation of this ' continuing ' metabolism that has given us a new outlook on the situation.The possibility of ' labelling ' the substances administered to the body so that their fate could be followed directly, despite their admixture with similar substances already present in the body, opened up completely new fields that could not otherwise have been explored. The technique had been used by Knoop 50 years ago, when he labelled his fatty acids with the phenyl group (which is difficult to metabolize), so that they could be identified in the urine, but this had obvious limitations.with isotopically labelled amino-acids showed immediately and conclusively that our views of the growth and stability of the adult organism were quite wrong. By the use of isotopically labelled N-15 (15N) aminoacids they showed that the exogenous metabolism of Folin is not static but dynamic. The nitrogenous groupings of the tissue proteins are constantly involved in chemical reactions, peptide links open, amino-acids are liberated, mix with others from the diet and tissues, and re-enter peptide links. All these reactions are balanced by an unknown regulator, so that the total amount of body material and composition do not change. The equilibrium appears to affect only the exogenous catabolism, for Schoenheimer showed that muscle creatine is dehydrated to creatinine at a constant rate and is not involved ...

Evaluation of protein quality of poultry feedingstuffs. 3. Expeller-processed oilseed cakes

“…The estimation of both pepsin and pepsin-trypsin digestibility of different oilseed cakes failed to reveal any correlation with GPV (P > 0-05). These findings are in accordance with the work of others (Melnick & Oser, 1949;Bender, 1954;Wheeler & Morgan, 1958;Miller, Kifer & Ambrose, 1970). All the digestible proteins are not biologically available owing to differences in the rate of liberation and subsequent absorption in the gut causing a disproportionate profile of the amino acid pool for protein synthesis.…”

The balancing of amino acid mixtures and proteins

“…For egg the BV was 99, NPR 6.1, PRE 98 and PER 4.0. T h e low PER of the amino acid mixture compared with that of egg was probably due to the lower food intake with the amino acid mixtures (Bender, 1956), namely 9.5 g/Ioo g body-weight daily, compared with 13 g of the egg diet.…”

“…Apart from the question of the accuracy of the analysis (Bender, 1954) errors could arise if egg protein contains one or more essential amino acids in quantities greater than required by the rat. Such a surplus would not be revealed by a biological assay but would cause an error in estimating chemical score.…”

Abstracts of Communication

“…The amino-acids present are compared with those of egg protein and the amount of the limiting amino-acid, expressed as a percentage of the same amino-acid in egg protein is the C.S. (Block & Mitchell, 1946-7;Bender, 1954).…”