Presence and characterization of two protein kinase activities in human seminal fluid

The presence of two protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) activities has been demonstrated in human seminal fluid, utilizing partially dephosphorylated phosvitin and lysine-rich histones as model acceptor substrates. Both kinase activities were maximal in the presence of MgCl2 and a sulfhydryl-protecting agent such as dithiothreitol; however, the histone kinase was stimulated to a greater extent by the latter. The histone kinase displayed a broad shoulder of activity at pH values of 7.1 to 7.6 with optimal activity at pH 8.0, and was inhibited by increased ionic strength (53% at 160 mM NaCl) and by the cyclic AMP-dependent protein inhibitor from rabbit muscle. The kinase activity towards phosvitin exhibited a broad pH profile with maximal activity at pH 7.2, was slightly stimulated by NaCl (20% at 160 mM), and was unaffected by the cyclic AMP-dependent protein kinase inhibitor. Kinetic studies revealed more than one apparent Km for the protein substrates and ATP. These differences in enzymic properties of kinase activities towards phosvitin and lysine-rich histones strongly indicate the presence of multiple enzymes. It appears that the histone kinase activity is attributable to the free catalytic subunit of a cyclic AMP-dependent enzyme. The protein kinase activities of seminal fluids from vasectomized men were 12-20% of those found for seminal fluids of normal men. This suggests that sperm may be a major source of protein kinase activities in seminal fluid.