Studies on PIP2-phosphomonoesterase activity in human platelets.

An occurrence of phosphatidylinositol 4,5-bisphosphate (PIP2) phosphomonoesterase in human platelets was demonstrated by analyzing phosphoinositides metabolism. The activity of the enzyme was maximum at pH 7.0. It was active even in the absence of Ca2+ or Mg2+ but it was enhanced in the presence of Mg2+ or NaF. The activity was inhibited by pyrophosphate. The activity was not altered in the presence of Ca2+. Thereby, besides phosphodiesteric cleavage by phospholipase C, the amount of PIP2 in activated platelets may be reduced by the combined effect of PIP2-phosphomonoesterase and suppressed activity of PI-kinase by Ca2+.