Effect of plasminogen activators on human recombinant apolipoprotein(a) having the plasminogen activation cleavage site

The serine-proteinase domain in human apolipoprotein(a) [apo(a)] and plasmi nogen exhibit 89% sequence identity including the catalytic triad. Cleavage of the Arg(561)-Val(562) activation site in plasminogen by either tissue- or urokinase-type plasminogen activator results in formation of the fibrino lytic enzyme plasmin. Apo(a) does not contain measurable amidolytic activit y nor can it be activated. by plasminogen activators. It has been suggested that the latter finding might be explained by the substitution of the plas minogen Arg-Val activation site by Ser-Ile in apo(a). To investigate if int roduction of the Arg-Val activation site in apo(a) might result in sensitiv ity towards plasminogen activators, we expressed wild-type and Arg-Val muta nt recombinant apo(a) [r-apo(a)] in human embryonic kidney and hepatocyte c ell lines. Free r-apo(a) and lipoprotein-like particles [r-Lp(a)] were obta ined in the culture supernatants of transfected 293 and HepG2 cells, respec tively. Incubation of mutant r-apo(a)/r-Lp(a) with plasminogen activators p roduced neither plasmin-like activity nor cleavage at the Arg-Val activatio n site, even in the presence of various stimulators of plasminogen activati on. Our data suggest that the high selectivity of activators for plasminoge n activation requires interactions with regions in plasminogen distant from the activation disulfide loop which are not present in apo(a). (C) 1999 El sevier Science B.V. All rights reserved.