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It has been reported that platelet aggregation or release of arachidonic. acid in stimulated platelets is inhibited by protease inhibitors or by synthetic substrates for proteases, suggesting possible involvement of unknown proteases in the platelet reaction. The present study was initiated to identify such proteases in platelets.  Subcellular fractions of bovine platelets were obtained according to the method of Barber and Jamieson and caseinolytic activity was studied in each fraction at pH 8.0 in the presence or absence of Ca2+, using fluorescamine method. A significant caseinolytic activity was detected in the soluble fraction but none in the fractions of plasma membrane or pellet. Then, this protease was partially purified from the soluble fraction by means of ammonium sulfate fractionation and subsequent column chromatographies on DEAE-Sepharose CL-6B and Sephadex G-150. The partially purified protease required Ca2+ strictly for its activity with the optimal concentration of 1 mM and pH optimum for the activity was 8.0, by which this enzyme was classified as a calcium-activated neutral protease. The molecular weight was estimated to be 135,000 by Andrew’s method. The activity of the protease was strongly inhibited by sulfhydryl reagents but no enhancement of the activity was observed in the presence of cysteine or reduced glutathione. The inhibition of the activity by PMSF or DFP was observed only when the protease was preincubated at 25°C for 1 hour in the presence of Ca2+.  From these observations, it may be suggested that the enzyme may possess calcium binding site independent of the catalytic site and that by changing the conformation of the enzyme Ca2+ may convert the enzyme from an inactive form to an active form. However, the physiological role of the enzyme in the platelet reaction has yet to be fully elucidated.

Studies On Calcium-Activated Neutral Protease In Platelet