The kringle V-protease domain is a fibrinogen binding region within Apo(a)

Lp(a) binds directly to fibrin and competes for the interaction of plasmino gen with this substrate. This competition may play a role in the proatherot hrombogenic consequences of high Lp(a) levels. Previous studies by us and o thers showed that apo(a) Kringle IV-10 competes for the interaction of Lp(a ) with plasmin-treated fibrinogen. However, kringle IV-10 cannot account fo r the entire high affinity interaction of Lp(a) with fibrinogen. Therefore, we tested the hypothesis that the apo(a) kringle V protease-like domain (K V-PD) could interact with plasmin-treated fibrinogen. We cloned the apo(a) KV-PD rec,ion from a human liver cDNA library. Fusion apo(a) KV-PD was expr essed in COS 7 cells and purified from the conditioned media, Western blott ing of the apo(a) KV-PD protein revealed two bands migrating with apparent molecular weights of 45K and 48K. When fusion apo(a) KV-PD was treated with O-glycosidase and neuraminidase, the higher molecular weight band disappea red suggesting that the apo(a) KV-PD was O-glycosylated. Apo(a) KV-PD bound to plasmin-treated fibrinogen in a dose-dependent fashion. An EC50 of 3.9 +/- 0.2 muM was determined for this interaction. Treatment of the apo(a) KV -PD with O-glycosidase did not significantly affect its ability to L bind t o plasm in-treated fibrinogen. In addition, apo(a) KV-PD competed for the b inding of I-125-Lp(a) to plasmin-treated fibrinogen. An C IC50 of 7.90 +/- 0.95 muM was obtained, Our data suggest that the KV-PD 0 of apo(a) shares b inding sites on plasmin-treated fibrinogen with Lp(a) and also may particip ate in the interaction of the Lp(a) particle with plasmin-treated fibrinoge n.