MULTIPLE BINDING WITH IDENTICAL LINKAGE - A MECHANISM THAT EXPLAINS THE EFFECT OF LIPOPROTEIN(A) ON FIBRINOLYSIS

We have previously shown that both recombinant apo(a) and native Lp(a) inhibit the binding of Glu-plasminogen to fibrin surfaces [Fleury & A ngles-Cano (1991) Biochemistry 30, 7630-7638; Rouy et al. (1992) Bioch emistry 31, 6332-6339]. The aim of the present study was to characteri ze the mechanism of this inhibition and to define the parameters gover ning binding when two different Lp(a) species compete with plasminogen for fibrin, a situation that may be found in vivo in subjects heteroz ygous for the apo(a) trait. The K-d for the binding of plasminogen to fibrin was 660 nM whereas the affinity of Lp(a) was inversely related to apo(a) size (K-d range: 50 to >500 nM). To determine the effect of plasminogen on Lp(a) binding and reciprocally, competition experiments were performed. The K-d of either Lp(a) or plasminogen for fibrin rem ained unchanged in the presence of the other competitor whereas B-max, the maximal amount bound, was importantly decreased. In a similar fas hion, competition for fibrin binding among Lp(a) isoforms was shown wi th the use of Lp(a) density fractions containing varying proportions o f isoforms B (similar to 460 kDa) and S3 (similar to 640 kDa); variati ons in K-d values (from 141 nM to 460 nM) as a function of the relativ e content in isoform S3 were observed. Altogether, these results are i ndicative of multiple binding by ligands that bind with different affi nities to equivalent but independent sites. Thus, in plasma from heter ozygous subjects, the influence of each Lp(a) isoform on fibrinolysis will depend on their affinity for fibrin and on their concentration re lative to each other and to plasminogen.