FACTOR-VIII-RELATED ANTIGEN IN HUMAN-BLOOD PLATELETS - LOCALIZATION AND RELEASE BY THROMBIN AND COLLAGEN

Abstract

Human platelets contain 0.61 .+-. 0.06 (SE) U [units] of F[factor]VIIIR:Ag[FVIII related antigen]/109 platelets or about 25% of the circulating antigen in whole blood. Assay of subcellular fractions obtained by sucrose density-gradient centrifugation of disrupted platelets indicates that over 85% of the FVIIIR:Ag was in the .alpha. granule fraction with other nonenzymatic proteins which could be released by treating platelets with collagen or thrombin. Seven percent or less was in the membrane fraction. After exposure of platelet suspensions to an optimal concentration of collagen, the supernatants contain, at most, 30% of the total amount of FVIIIR:Ag in the platelets but a higher percentage of other releasable substances. After exposure to thrombin, the supernatants contain virtually no FVIIIR:Ag. These low values were attributed to release of a limited amount of FVIIIR:Ag by collagen or thrombin rather than to adsorption or destruction of antigen, for several reasons. FVIIIR:Ag could be recovered in the platelet residue as effectively as in control samples. The antigen could not be detected on the surface of collagen- or thrombin-stimulated platelets. Relatively little released antigen was adsorbed by collagen or destroyed by thrombin. Release of FVIIIR:Ag by collagen was markedly reduced by aspirin, and release of the antigen as well as fibrinogen from platelets was much slower than release of 14C-serotonin and .beta.-TG [.beta.-thromboglobulin].