A biochemical and ultrastructural study on the endogenous protein kinase activity of secretory granule membranes of prostatic origin in human seminal plasma

Abstract

A phosphorylation of endogenous acceptor proteins has been shown to occur in the membranes of secretory granules and vesicles of prostatic origin in human seminal plasma. The reaction was catalyzed by endogenous protein kinase in the presence of exogenous [ γ 32 P] ATP and Mg ions. Phosphoryl group transfer occurred into phosphoseryl and phosphothreonyl residues recovered from these organelles. The phosphoryl serine/phosphoryl threonine ratio was about 3.5 in the presence of Mg ions. Magnesium ions were preferred compared with Ca 2− and Zn 2+ for optimal reaction. Adenosine at 0.5 m M also had an inhibitory effect on the reaction. Dibutyryl cyclic AMP stimulated the endogenous protein kinase by about 20% and the phosphoryl serine/phosphoryl threonine ratio was maintained at about 3.5. Histone and phosvitin were excellent exogenous phosphoryl group acceptors and this effect was not further corroborated by the cyclic nucleotides. The organelle membranes phosphorylated under optimal conditions were significantly thicker than the controls and those phosphorylated in the presence of inhibitors to the protein kinase when assessed by electron microscopy. This change in configuration of the membranes might suggest drastic conformational alterations of the membranous proteins.