Binding of collagen alpha1 chains to human platelets.

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Abstract
We previously reported that purified alpha1 chains of type 1 chick skin collagen induce platelet aggregation. We now describe immunological and biochemical evidence that the peptide binds to intact platelets as an early event in the induction of platelet aggregation and the release reaction. Antibody against alpha1 (I) was obtained by immunizing rabbits with complete Freund's adjuvant mixed with purified alpha1. Immunofluorescence studies showed that alpha1(I)-treated platelets exhibited strong immunofluorescence. The intensity of fluorescence was markedly decreased by the pretreatment of platelets with alpha1-CB5 and glucosylgalactosylhydroxylysine. Dose-response curves of platelet aggregation induced by alpha1 and the binding of alpha1 by washed intact platelets are correlated. The biochemical studies showed that the binding of the alpha1 chain to washed intact platelets was platelet concentration and temperature dependent, and that it reached a maximum in 10 min. The process was reversible and specific, with an association constant of 1.7 muM. The inhibitor of alpha1-induced platelet aggregation, glucosylgalactosyl hydroxylysine, inhibited the alpha1 binding. These results suggest that alpha1(I) chains bind to specific receptor site(s) on platelet membranes to trigger aggregation and the release reaction.