Accumulation of
lipoprotein(a) (Lp(a)) in
atherosclerotic plaques is mediated through interaction of
fibrin-(
ogen) deposits with the
apolipoprotein(a) (
apo(a)) moiety of Lp(a). It was suggested that because
apo(a) competes with
plasminogen for binding to
fibrin, causing inhibition of fibrinolysis, it could also promote
atherothrombosis. Because the
fibrin(
ogen) alphaC-domains bind
plasminogen and
tissue-type plasminogen activator with high affinity in a Lys-dependent manner, we hypothesized that they could also bind
apo(a). To test this hypothesis, we studied the interaction between the recombinant
apo(a)
A10 isoform and the recombinant alphaC-fragment (Aalpha-(221-610)) corresponding to the alphaC-domain by
enzyme-linked
immunosorbent assay and surface plasmon resonance. Both methods revealed a high affinity interaction (Kd = 19-21 nm) between the immobilized alphaC-fragment and
apo(a), indicating that the former contains an
apo(a)-binding site. This affinity was comparable to that of
apo(a) for
fibrin. At the same time, no interaction was observed between soluble
fibrinogen and immobilized
apo(a), suggesting that, in the former, this and other
apo(a)-binding sites are cryptic. Further experiments with truncated recombinant variants of the alphaC-fragment allowed localization of the
apo(a)-binding site to the Aalpha-(392-610) region. The presence of
epsilon-aminocaproic acid only slightly inhibited binding of
apo(a) to the alphaC-fragment, indicating the Lys-independent nature of their interaction. In agreement, the influence of
plasminogen or
tissue-type plasminogen activator on binding of
apo(a) to the alphaC-fragment was minimal. These results indicate that the alphaC-domains contain novel high affinity
apo(a)-binding sites that may provide a Lys-independent mechanism for bringing Lp(a) to places of
fibrin deposition such as injured vessels or atherosclerotic lesions.