ABSTRACT: Thrombin-stimulated (10 s) human platelets produce Ins(1,4,5)P3 and an additional inositol trisphosphate (InsP3), in approximately a 1:20 ratio. The major InsP3 co-migrates with Ins(1,3,4)P3 on strong-anion-exchange h.p.l.c. To identify this species unequivocally, we treated putative Ins(1,3,4)P3 obtained from thrombin-stimulated myo-[3H]inositol-labelled platelets with NaIO4/NaBH4 or 4-phosphomonoesterase. The products indicate that the major InsP3 is at least 90% D-Ins(1,3,4)P3. D-[3H]Ins(1,3,4)P3 added to saponin-permeabilized platelets is hydrolysed to an InsP2 (7.8%) and phosphorylated by a kinase to yield an inositol polyphosphate (0.9%) in 5 min. The phosphorylation product co-migrates with Ins(1,3,4,6)P4 on Partisphere WAX h.p.l.c. Under similar conditions, L-[3H]Ins(1,3,4)P3 is dephosphorylated but not phosphorylated. Relative phosphatase:kinase ratios are 8.7:1 (Vmax. values) and 0.86:1 (Km values) with respect to D-Ins(1,3,4)P3. The kinase activity is predominantly cytosolic (96.8% of total activity) in freeze-thaw-disrupted platelets, and the accumulation of its product is Ca2(+)-dependent. The activity is identified as a 6-kinase on the basis of its product's insensitivity to 5-phosphomonoesterase, resistance to periodate oxidation and co-migration with standard Ins(1,3,4,6)P4 on h.p.l.c. Incubation of platelets with beta-phorbol dibutyrate (beta-PDBu, 76 nM), causing activation of protein kinase C, results in a 57.5% inhibition (reversible by the protein kinase C inhibitor staurosporine) of Ins(1,3,4,6)P4 accumulation. alpha-PDBu, which does not stimulate protein kinase C, has no effect. Stimulation of intact platelets with thrombin results in the production of Ins(1,3,4,6)P4 (1.4-fold rise in 30 s) and Ins(1,3,4,5)P4, with the latter being the major InsP4 species. Accumulation of Ins(1,3,4,6)P4 is slightly delayed in comparison with Ins(1,3,4)P3 and is relatively small. We propose that the major route of Ins(1,3,4)P3 metabolism in stimulated human platelets is via phosphatase action.
