Reversible phosphate transfer between yolk phosphoprotein and adenosine triphosphate.

In recent years, attention has been focussed on the acceptor function of the serine hydroxyl in various enzymes. Acyl or phosphoryl residues appear to link to it in group transfer, and also in the initial phase of ester or peptide hydrolysis (l-4). This has led to a revival of interest in the classical phosphoproteins, vitellic acid and casein, which carry phosphate on the hydroxyl group of serine (5,6). Phosphoproteins of this type were thought to be limited to nutrient material for growing organisms, such as egg yolk or milk. However, the recent isolation of serine phosphate from a variety of tissues suggests their general occurrence (11-14). Moreover, serine-bound phosphate in tissue phosphoproteins has been found to turn over very rapidly (7-14). In some cases, this turnover has been correlated with tissue activity (15-17). For example, the rate of Pa2 incorporation into phosphoprotein was found to increase on electrical stimulation of brain slices (18, 19). The similarity between tissue phosphoprotein and ordinary phosphoprotein, such as casein, is more explicitly shown by the studies of Burnett and Kennedy (20) on the mitochondrial transfer of the terminal phosphate of ATP to casein; a soluble protein phosphokinase that was rather specific for casein was isolated from rat liver mitochondria. We will report here on the reactivity of egg yolk phosphoprotein with this type of enzyme. Egg yolk phosphoprotein was explored as a substrate because of its high phosphorus and serine content. In this protein, nearly every second amino acid is a serine. Serine and phosphate are present in approximately equal amounts. Therefore, serine must be the binding site of phosphate in the native protein. The only acceptors found so far that are specific for this enzyme are casein and egg yolk protein. This observation seems to bear out the presumption that in both these proteins the hydroxyl group of hydroxy amino acids is the natural site of phosphate binding. Doubts about this have been expressed recently by igren (21). We have explored the characteristics of protein-bound phosphoserine further by using a system consisting of egg yolk phosphoprotein and preparations of protein phosphokinase from yeast and brain. These enzymes are similar to the ones obtained by Burnett and Kennedy (20), and more recently by Sundararajan et al. (22). One of the more interesting results of this study was the observation of a relatively easy phosphate transfer from yolk protein to ADP.