Identification of Inorganic Pyrophosphate in Human Platelets and Its Release on Stimulation with Thrombin

Abstract

Serum inorganic pyrophosphate (PPi) levels were consistently two- to threefold higher than plasma PPi prepared from the same blood. PPi was found in platelets in amounts ranging from 1.4 to 3 nmol/10⁸ cells, using three different techniques for quantification. These levels are approximately 800 times higher than the mean PPi concentration in normal plasma and approximate the levels of ADP found in platelets by other workers. About 50% of platelet PPi was specifically released extracellularly after stimulation with thrombin. Timed release experiments showed a pattern of release that resembled that described for ADP and ATP. This pattern was clearly different from that shown by platelet calcium, serotonin, or β-glucuronidase. Platelet inorganic pyrophosphatase was not released into the supernate in detectable amounts. Platelets from patients with nucleotide storage pool deficiency showed greatly reduced levels of PPi as compared with control. There was no detectable release of PPi into extracellular medium after thrombin addition to a suspension of these platelets.