Monoclonal antibodies selective for activated Factor V. Immunochemical probes for structural transitions occurring during the thrombin-catalyzed activation of the procofactor.

Monoclonal antibodies have been used to probe for structural transitions which potentially occur during the activation of bovine Factor V by thrombin. One of these antibodies (alpha 2D) is reactive with an epitope on the NH2-terminal segment of Factor V (i.e. the Mr = 94,000 component D), while a second antibody (alpha 2E) recognizes an epitope on the carboxyl-terminal segment of Factor V (the Mr = 74,000 component E). Neither antibody reacts with native unactivated Factor V. The alpha 2D-reactive epitope requires at least two proteolytic events for expression, and the alpha 2E-reactive epitope is expressed following the initial cleavage catalyzed by thrombin. In addition, the alpha 2E-reactive epitope is expressed upon the addition of chelators to Factor V. However, the alpha 2D epitope is not influenced by the removal of calcium ions. The interactions of these antibodies with Factor V and Factor V-derived peptides suggest that conformational changes occur in both the NH2-terminal and carboxyl-terminal regions of Factor V concomitant with activation by thrombin, which give rise to the antibody recognition sites.