THE LINKAGE OF PHOSPHATE ‘I’0 PROTEIN IN PEPSIN AND OVAT,lXJMIN

The nature of the linkage between phosphorus and protein in phosphoproteins not associated with lipide or nucleic acid came under investigation some 20 to 25 years ago with the study of non-homogeneous protein fractions from milk and eggs containing many phosphorus atoms per molecule of protein, Phosphoserine was isolated from acid digests of vitellinic acid (1, 2) and casein (3, 4), and, more recently, from digests of DIP-inhibited chymotrypsin (5) and cholinesterase (C).1 The dipcpticlc SerP.Gly has been obtained from the chymotrypsin derivative (8) a.nd, in addition, phosphothreonine has been isolated from casein (9). A peptide containing only phosphoserine and glutamic acid has also been identified in combined tryptic and acid hydrolysates (10) and in wholly enzymatic (11) digests of casein, and has been shown, after isolation from an acid digest, to have the sequence SerP.Glu (12). It has been noted (13) that in all of these invcstigations the possibility has not been ruled out that the phosphate may have been linked, in the intact protein, to the amino group of serine or thrconine, and that it may have undergone N to 0 acyl migration during the course of the acid hydrolysis (14, 15). Renewed interest in the functions and biosynthesis of phosphoproteins has resulted from studies of phosphocnzymes involved in phosphorus metabolism (10, 17) and of enzymes whose “active centers” can be artificially tagged with phosphate (5,6). The possibility that the mechanism of action of these enzymes might be elucidated by a knowledge of the amino acid configurations about their “active centers” has focused attention not only on the amino acids to which phosphate is linked, but on those which lie adjacent in the peptide chain.