Properties of human erythrocyte phosphatidylinositol kinase and inhibition by adenosine, ADP and related compounds.

1. An improved assay for the enzyme phosphatidylinositol kinase is described. The kinase activity is increased more than 10-fold by the addition of mercaptoethanol and exogenous phosphatidylinositol in the presence of Triton X-100. The enzyme is solubilized by non-ionic detergents. 2. Phosphatidylinositol kinase is inhibited by physiological concentrations of ADP and by similar levels of adenosine. Cyclic AMP, AMP and theophylline inhibit at higher concentrations. Caffeine is much less effective than theophylline as an inhibitor. Guanine nucleotides do not appreciably inhibit the kinase. All the inhibitors tested seemed to compete with ATP. Theophylline also reduced the rate of polyphosphoinositide synthesis in intact cells. 3. Possible roles of polyphosphoinositides in energy charge maintenance and inversion and resealing are discussed.