Phosphatidyl inositol inhibition of a sperm cyclic AMP-independent protein kinase

Abstract Phosphatidyl inositol has been found to inhibit strongly the activity of a cyclic AMP-independent protein kinase located on the external surface of goat epididymal intact spermatozoa. Phosphatidyl inositol at a concentration as low as 10 μg/ml inhibited nearly 50% of the ecto-kinase activity for the phosphorylation of the exogenous protein substrate: casein. Phosphatidyl ethanolamine at a relatively high concentration (125 μg/ml) inhibited slightly (approx 25%) the activity of the enzyme whereas other phospholipids: phosphatidyl serine and choline, diacyl glycerol, phosphatidic acid and myo-inositol-2-phosphate had no appreicable effect on the kinase activity. Phosphatidyl inositol has also served as a potent inhibitor of the phosphorylation of sperm ectophosphoproteins by the endogenous kinase activity of intact spermatozoa. By thin layer chromatography it has been shown that the observed inhibitory effect of the phospholipid was not due to any impurities or degraded products of phosphatidyl inositol. Phosphatidyl inositol inhibited the kinase activity noncompetitively with respect to casein and Mg 2+ but uncompetitively with respect to ATP. The results raised the possibility that phosphatidyl inositol-mediated high affinity inhibition of protein kinase(s), may constitute a novel mechanism for the regulatory actions of the phospholipid in mammalian cells.