Critique of the Foreman method for the estimation of the dicarboxylic acids in protein hydrolysates

Abstract

The lime-ethanol method of Foreman for the detn. of aspartic and glutamic acids in protein hydrolysates, and the various modifications of it introduced by other workers, were investigated. During treatment with the lime the cystine present in the hydrolysate undergoes partial dis-mutation to the sulphinic or sulphonic acids; these are precipitated with the Ca dicarboxylates by the ethanol, together with small amts. of cystine itself, tyrosine, serine, and bases. The Cu salts of these cystine dismutation products are very insoluble and interfere with the estimation of aspartic acid as Cu aspartate. To prevent their formation it has been found advantageous to remove all cystine as the cuprous mercaptide of cysteine before the hydrolysate is made alkaline with lime. The solubility of the Ca dicarboxylates under the conditions of the modified Foreman procedure was investigated by clarifying the lime-ethanol filtrate, removing the bases and a major part of the monoamino-acids, and repeating the lime-ethanol treatment on the final mother liquor. Significant amts. of both aspartic and glutamic acids were isolated. Based on these findings a new extended procedure was elaborated which has given values for the aspartic acid and glutamic acid contents of gliadin, edestin and egg albumin that are accurate to about 2%. The possibility of applying solubility corrections to the dicarboxylic acid results obtained in an analysis covering only 1 complete lime-ethanol treatment was investigated. Results for casein, horse Hb and cattle Hb obtained in this way are higher than those hitherto recorded, and it is probable that those for the 2 latter proteins are still too low. Reasons are given for the belief that the "hydroxyglutamic acid" fractions obtained by earlier workers are complex mixtures containing inter alia aspartic acid and dismutation products of cystine.