Summary
Lipoprotein(a) [Lp(a)] is associated with atherosclerosis and with disease processes
involving thrombosis. Lp(a) contains apoprotein (a) [apo(a)], which has a sequence
highly homologous to plasminogen. Hence, Lp(a) binds directly to extracellular matrix,
cellular plasminogen receptors and fibrin(ogen) and competes for the binding of plasminogen
to these regulatory surfaces. These interactions may contribute to the proatherothrombogenic
consequences of high Lp(a) levels. These interactions are mediated by lysine binding
sites (LBS). Therefore, we examined the role of apo(a) kringle IV-10 [the only apo(a)
kringle demonstrated to exhibit lysine binding activity in the intact lipoprotein]
in the interaction of Lp(a) with these regulatory molecules. We have compared directly
apo(a) KIV-10 with plasminogen K4 to examine whether these highly structurally homologous
kringle modules are also functionally homologous. Futhermore, because the plasminogen
K5-protease domain (K5-PD) binds directly to fibrin, we have also examined the ability
of this plasminogen fragment to inhibit the interaction of Lp(a) with these regulatory
molecules and with extracellular matrix. Apo(a) KIV-10 competed effectively for the
binding of 125I-Lp(a) to these surfaces but was less effective than either intact Lp(a), plasminogen
K4 or plasminogen. Plasminogen K5-PD was a better competitor than apo(a) KIV-10 for
125I-Lp(a) binding to the representative extra-cellular matrix, Matrigel, and to plasmin-treated
fibrinogen. In contrast, plasminogen K5-PD did not compete for the interaction of
Lp(a) with cells, although it effectively competed for plasminogen binding. These
results suggest that Lp(a) recognizes sites in all of the regulatory molecules that
are also recognized by apo(a) KIV-10 and that Lp(a) recognizes sites in extracellular
matrix and in plasmin-modified fibrinogen that also are recognized by plasminogen
K5-PD. Thus, the interaction of Lp(a) with cells is clearly distinct from that with
extracellular matrix and with plasmin-treated fibrinogen and the recognition sites
within Lp(a) and plasminogen for these regulatory molecules are not identical.
Portions of this manuscript were presented at the 69th Meeting of the American Heart
Association, New Orleans, LA 1996.