Summary

Lp(a) binds directly to fibrin and competes for the interaction ofplasminogen with this substrate. This competition may play a role inthe proatherothrombogenic consequences of high Lp(a) levels.Previous studies by us and others showed that apo(a) Kringle IV-10 competes for the interaction of Lp(a) with plasmin-treated fibrinogen.However, kringle IV-10 cannot account for the entire high affinityinteraction of Lp(a) with fibrinogen. Therefore, we tested the hypothesisthat the apo(a) kringle V protease-like domain (KV-PD) couldinteract with plasmin-treated fibrinogen. We cloned the apo(a) KV-PDregion from a human liver cDNA library. Fusion apo(a) KV-PD wasexpressed in COS 7 cells and purified from the conditioned media.Western blotting of the apo(a) KV-PD protein revealed two bandsmigrating with apparent molecular weights of 45K and 48K. Whenfusion apo(a) KV-PD was treated with O-glycosidase and neuraminidase,the higher molecular weight band disappeared suggesting that theapo(a) KV-PD was O-glycosylated. Apo(a) KV-PD bound to plasmintreatedfibrinogen in a dose-dependent fashion. An EC₅₀ of 3.9 ±0.2 µM was determined for this interaction. Treatment of the apo(a)KV-PD with O-glycosidase did not significantly affect its ability tobind to plasmin-treated fibrinogen. In addition, apo(a) KV-PD competedfor the binding of 125I-Lp(a) to plasmin-treated fibrinogen. AnIC₅₀ of 7.90 ± 0.95 µM was obtained. Our data suggest that the KV-PDof apo(a) shares binding sites on plasmin-treated fibrinogen with Lp(a)and also may participate in the interaction of the Lp(a) particle withplasmin-treated fibrinogen.