Biosynthesis of cytidine diphosphate diglyceride by human platelets.

Homogenates of human platelets contain an enzyme which catalyzes the formation of cytidine diphosphate diglyceride from cytidine triphosphate and phosphatidic acid. The enzymatic activity could not be dissociated from platelet particles and the greatest specific activity was found in the membrane fraction. The K(m) for cytidine triphosphate was 0.16 mmole/liter and the apparent K(m) for phosphatidic acid was 6.2 mmoles/liter. The pH optimum was 7.0 and the most effective buffers were triethanolamine-HCl and Tris-HCl. The reaction was dependent on the presence of divalent cations, magnesium being the most effective of those investigated. Monovalent cations did not alter the reaction rate. Evidence is presented that the cytidine diphosphate diglyceride produced can serve as a precursor for the synthesis of phosphatidylinositol. No difference was found in the enzymatic activity in platelets from normal subjects and from patients with diseases known to interfere with platelet thromboplastic function.