Phosphorus Linkages in α-Casein

Abstract

RECENTLY, an investigation on the nature of phosphorus bonds in proteins has been initiated in this laboratory. This was stimulated by the observation that a variety of phosphatases will dephosphorylate phosphorus-containing proteins without proteolysis occurring simultaneously. Dephosphorylation, however, takes place only if the phosphate groups are present as monoesters with two ionizable hydroxyls^1,2. In addition, the occurrence, in phosphoproteins, of linkages such as those of pyrophosphates, —O—P—O—P—O—, phosphodiesters, —O—P—O— or —N—P—O—, and the presence of these bonds in cyclic arrangements, had to be anticipated. Thus, it should be possible to demonstrate such structural units with the aid of specific enzymes which bring about their transformation into monoesters that are then readily hydrolysed by the action of phosphomonoesterases. The work described here was designed to test these assumptions, and supplies the first experimental evidence for the existence of pyrophosphate bonds in a protein. Moreover, it is shown that α-casein, in addition to phosphomonoesters, also contains some of its phosphorus as pyrophosphate and phosphodiesters.