The dual role of factor VII in blood coagulation. Initiation and inhibition of a proteolytic system by a zymogen.

Abstract

Factor W, a single-chain zymogen, is known to be unusually active with respect to ester hydrolysis and inhibition by diisopropylfluorophosphate (DFP).Assays of coagulant activity have shown the zymogen to contain 1-2% of the activity of its two-chain derivative enzyme, Factor VII,.This activity could be inherent in the zymogen or due to contamination with Factor VII.. To distinguish between these possibilities and to confirm the activity of the zymogen, we show the following: 1) Factor W inhibits the activation of 3H-labeled Factor M by Factor W, when tissue factor is limiting, indicating that enzyme and zymogen compete for the cofactor.In contrast, when tissue factor is in excess, the activities are additive.2) Diisopropylphosphoryl derivatives of Factor W and Factor VII, both inhibit the radioassay for Factor W,-tissue factor, the KlIz of inhibition being 2.8 k 0.4 and 2.0 k0.2 m, respectively.3) The rate of incorporation of r3H]DFP by these proteins is insensitive to trace contamination; pseudo-first order rate constants were calculated for the incorporation of 2 ~l l ~ DFP into Factor VI1 and Factor VII,.These were 0.032 k 0.001 min" and 0.130 k 0.003 min", respectively.Inhibition rates of the coagulant activity of the two proteins were also determined in 2 m M DFP.The inhibition kinetics and the rate constants of incorporation were used to calculate the intrinsic coagulant activity of the zymogen.It was nearly 0.8% that of the enzyme.4) Factor W was rendered virtually free of Factor VII. by incubation with 2 m DFP for over 6 half-lives of Factor VII,.At this point, Factor W had approximately 0.8% the activity of the enzyme.Further, the coagulant activity decayed with the rate constant of the zymogen, 0.033 2 0.003 min-'.We also explored the recent proposal that a ternary complex of tissue factor with Factor VI1 and Factor X, (Morrison-Silverberg, S.