Analysis of proteins

Abstract

The prep. of caseo-phosphopeptone, judging from its N and amino-N contents, is an octapeptide containing two H2PO4 groups, 2 mol. glutamic acid and 2 mol. serine. Its acidity indicates the presence of another mol. of a dicarboxylic acid, leaving 3 mol. of simple amino-acids. The deductions are confirmatory of the isolation of 3 mol. of isoleucine by Posternak, and by Damodaran and Ramachandran. The latter authors find caseophosphopeptone to be a decapeptide with 3 mol. glutamic acid, 3 or 4 mol. serine and 3 mol. H3PO4. Rimington''s phosphopeptone is a nona- or decapeptide. The isolation of a dipeptide of glutamic acid and phosphoserine by Levene and Hill points to the combination of these 2 amino-acids. In view of the rapid liberation of 1 mol. NH3 by the action of N/4 NaOH and a difference in the rate of separation of H3PO4 the authors believe that one phosphoserylglutamic group is at the end of the chain with a free amino group easily liberated, and the 2d group is in the middle of the chain, and picture the molecule of the octapeptide thus: phosphoserylglutamic-X-X-phosphoseryl-glutamic-X-X, where X is the unknown amino-acid, probably 3 mol. isoleucine and 1 mol. aspartic acid. The difference between the octapeptide and the decapeptide is 1 mol. of dipeptide of serine and glutamic acid, possibly at the other end of the chain from which it may be more easily split off: phosphoserylglutamic-X-X-phosphoserylglutamic-X-X-phosphoserylglutamic. There is no evidence whether the dipeptide is phosphoserylglutamic acid, or glutamyl-phosphoserine. Both isomers may be present, but the one at the left-hand end is phosphoserylglutamic acid. Phosphoserine may also be in combination with isoleucine or other amino-acid, if present.