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Vascular endothelium possesses multiple procoagulant properties, including synthesis and expression of Factor V. We studied the effects of homocysteine on the regulation of endothelial cell Factor V activity. Elevated levels of homocysteine are associated with the congenital thrombotic disorder homocystinuria. Treatment of cultured endothelial cells with 0.5-10 mM homocysteine had no effect on cell morphology, but did increase Factor V activity and prothrombin activation by Factor Xa. A radioimmunoassay for endothelial cell Factor V demonstrated that homocysteine treatment did not increase Factor V antigen levels. 125I-prothrombin was activated by treated endothelial cells and Factor Xa in the presence of thrombin inhibitors. Exogenous 125I-Factor V was cleaved by homocysteine-treated but not control endothelial cells. 125I-Factor V cleavage products distinct from those generated by thrombin and Factor Xa were identified. These data provide evidence for regulation of endothelial cell Factor V activity, and indicate that increased Factor V activity associated with homocysteine-treated vascular endothelium results primarily from induction of an activator of Factor V.