C4b-binding protein protects coagulation factor Va from inactivation by activated protein C

We investigated the effect of C4BP on APC-mediated inactivation of factor V a (FVa) in the absence and presence of protein S. FVa inactivation was biph asic (k(506) = 4.4 x 10(8) M-1 s(-1), k(306) = 2.7 x 10(7) M-1 s(-1)), and protein S accelerated Arg(306) cleavage approximately 10-fold. Preincubatio n of protein S with C4BP resulted in a total abrogation of protein S cofact or activity. C4BP also protected FVa from inactivation by APC in the absenc e of protein S. Control experiments with CLB-PS13, a monoclonal anti-protei n S antibody, indicated that inhibition of FVa inactivation by C4BP was not mediated through contaminating traces of protein S in our reaction systems . Protection of FVa was prevented by a monoclonal antibody directed against the C4BP alpha -chain. Recombinant rC4BP alpha comprised of only alpha -ch ains also protected FVa, but in the presence of protein S, the level of pro tection was decreased, since rC4BP alpha lacks the beta -chain responsible for C4BP binding to protein S. A truncated C4BP beta -chain (SCR-1+2) inhib ited protein S cofactor activity, but had no effect on FVa inactivation by APC in the absence of protein S. In conclusion, C4BP protects FVa from APC- catalyzed cleavage in a protein S-independent way through direct interactio ns of the alpha -chaims of C4BP with FVa and/or APC.