The Effect of Heat on the Availability of Pork Protein in Vivo and in Vitro

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 ...

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

Bender¹

1954

The value of certain agricultural, marine and industrial products and by‐products in livestock feeding. II. —Non‐ruminants

Duckworth¹

1955

FEW new energy sources have proved to be worth exploiting for the feeding of non-ruminants.Cereals and root crops remain the main sources of energy, with fibrous materials, whether in their native or processed state, playing a very small part. However, American feeders have recently begun to use low-grade fats, inedible by man, in poultry feeding.With protein concentrates more attention is being given to improving the methods of processing to obtain materials of higher nutritive value than to developing new products. Since many materials at the onset of processing have a high intrinsic nutritive value, the problem is to devise methods of processing that will minimize loss of this value in the by-product destined for animal feeding. In some cases, as in the production of white iish meal and soya-bean meal, processing requirements are well understood. In other cases little is known, or obstacles stand in the way of adopting superior processing methods.One of the commonest and most informative is the biological value (BV), essentially a measure of the nutritive value of absorbed nitrogen. Another is the gross protein value (GPV), which is a measure of the value of the protein in a concentrate for supplementing the proteins in mixtures of cereals and miller's offals.In Table I1 (p. 243) are set out the GPV ratings of a number of concentrates, most of which * Part I:

Availability of essential amino acids from proteins. I.—beef serum albumin

Dvořák¹

1968

Available essential amino acids from beef serum albumin were determined microbiologically, after total enzymic hydrolysis of the protein by papain, leucine aminopeptidase and prolidase. Samples of pu: e beef serum albumin of different moisture content were heat-treated with glucose at 90" or without it at 121 .In the enzymic digest of the protein, available essential amino acids were determined using Streptococcus faecalis, S. zymogenes and Lactobacillus arabinosus. The content of essential available amino acids was correlated with the degree of in virro digestibility of heat-treated albumin.The amount of lysine determined by Carpenter's method with ff uordinitrobenzene did not correspond to the values found for lysine by microbiological analysis with S. faecalis. Lysine reactive with fluordinitrobenzene was found in the non-digestible residue of the protein, so that the values obtained by the chemical method are probably higher than the actual amount of available lysine. The results for the trypsin digest of the albumin were similar by both methods. The availability of lysine as well as that of amino asids in general depended on the enzymic digestibility.