PARTIAL ACTIVATION OF THE FACTOR VIIIA FACTOR IXA ENZYME COMPLEX BY DIHEXANOIC PHOSPHATIDYLSERINE AT SUBMICELLAR CONCENTRATIONS

Phosphatidylserine (PS)-containing membranes increase the k(cat) of th e factor VIIIa-factor IXa enzyme complex by more than 1000-fold, While PS supports specific, high-affinity membrane binding of factor VIIIa and factor IXa, it is not known whether PS is the lipid that activates the membrane-bound complex. It is also not known whether PS or other activating lipids must reside in the two-dimensional membrane matrix f or efficacy, We have found that submicellar concentrations of dihexano ic phosphatidylserine (C6PS) increase the activity of the factor Vma-f actor IXa complex in a biphasic manner with half-maximal concentration s of 0.2 and 1.6 mM while the micelle-forming concentration is 4.0 mM, Increased cleavage of factor X at 0.25 mM C6PS was due to a 25-fold e nhancement of the k(cat) and a 30-fold increase in the affinity of fac tor Vma for factor IXa, C6 phosphatidylethanolamine and C6 phosphatidi c acid, but not C6 phosphatidylcholine, also accelerated the Xase comp lex, indicating that k(cat) enhancement has less structural specificit y than membrane binding. Submicellar C6PS enhanced activity of factor IXa in the absence of factor VIIIa, but the effect was due to a decrea sed K-M rather than an increased k(cat), These results suggest that ac tivation of the factor VIIIa-factor IXa complex can result from bindin g of individual C6PS molecules or small aggregates in the absence of a membrane bilayer. They provide a model system in which the phospholip id-induced activation may be distinguished from membrane-binding of th e enzyme complex.