Proteolysis of factor Va by factor Xa and activated protein C.

Bovine Factor Va, produced by selective proteolytic cleavage of Factor V by thrombin, consists of a heavy chain (D chain) of Mr = 94,000 and a light chain (E chain) of Mr = 74,000. These peptides are noncovalently associated in the presence of divalent metal ion(s). Each chain is susceptible to proteolysis by activated protein C and by Factor Xa. Sodium dodecyl sulfate electrophoretic analysis indicates that cleavage of the E chain by either activated protein C or Factor Xa yields two major fragments: Mr = 30,000 and Mr = 48,000. Amino acid sequence analysis indicates that the Mr = 30,000 fragments have identical NH2-terminal sequences and that this sequence corresponds to that of intact E chain. The Mr = 48,000 fragments also have identical NH2-terminal sequences, indicating that activated protein C and Factor Xa cleave the E chain at the same position. Sodium dodecyl sulfate electrophoretic analysis indicates that activated protein C cleavage of the D chain yields two products: Mr = 70,000 and Mr = 24,000. Amino acid sequence analysis indicates that the Mr = 70,000 fragment has the same NH2-terminal sequence as intact D chain, whereas the Mr = 24,000 fragment does not. Factor Xa cleavage of the D chain also yields two products: Mr = 56,000 and Mr = 45,000. The Mr = 56,000 fragment corresponds to the NH2-terminal end of the D chain and Factor V. Functional studies have shown that both chains of Factor Va may be entirely cleaved to products by Factor Xa without loss of activity, whereas activated protein C cleavage results in loss of activity. Since activated protein C and Factor Xa cleave the E chain at the same position, the cleavage of the D chain by activated protein C is responsible for the inactivation of Factor Va.