INVESTIGATIONS OF AMINO ACIDS, PEPTIDES, AND PROTEINS

Abstract

The microbiological determination of a&nine, isoleucine, leucine, tryptophane, and valine in the hydrolysates of casein and other proteins has been described recently by McMahan and Snell(2), Kuiken et al. ( 3), Greene and Black (4), and Hegsted (5).The microorganisms Lactobacillus arabinosus 17-5 and Lactubacillus casei and a turbidimetric or titrimetric method of analysis were employed.It was reported that negligible amounts of isoleucine, leucine, and valine were lost because of humin formation even in the presence of carbohydrates and that only the naturally occurring isomers of these amino acids were active.The values under varying hydrolytic conditions and at different levels of protein hydrolysate agreed closely, the recovery of amino acids added to protein hydrolysates was nearly theoretical, and the percentage of amino acids agreed satisfactorily with values selected from the literature.Data of comparable precision and accuracy have been obtained in analogous experiments performed in the writers' laborat0ry.lIt seems evident from these findings that amino acids in protein hydrolysates may be determined by microbiological assay conveniently and, for many purposes, with satisfactory accuracy.It could have been predicted that greater difficulties would be encountered in the determination of some amino acids than of others in protein hydrolysates because of the stimulatory or inhibitory influence exerted by amino acids or other hydrolytic products.It is of interest in this connection that Hutchings and Peterson (8) have shown that histidine and isoleucine were inhibitory, lysine and alanine were stimulatory, and that * For Paper XVI in this series see Schott et al. (1).This work was aided by