39-KD PROTEIN INHIBITS TISSUE-TYPE PLASMINOGEN-ACTIVATOR CLEARANCE IN-VIVO

Tissue-type plasminogen activator (t-PA) is a plasma serine protease t hat catalyzes the initial and rate-limiting step in the fibrinolytic c ascade. t-PA is widely used as a thrombolytic agent in the treatment o f acute myocardial infarction. However, its use has been impaired by i ts rapid hepatic clearance from the circulation following intravenous administration. Studies with both rat hepatoma MH1C1 cells (G. Bu, S. Williams, D. K. Strickland, and A. L. Schwartz. 1992. Proc. Natl. Acad . Sci. USA. 89:7427-7431) and human hepatoma HepG2 cells (G. Bu, E. A. Maksymovitch, and A. L. Schwartz. 1993. J. Biol. Chem. 28:13002-13009 ) have shown that binding of t-PA to its clearance receptor, the low d ensity lipoprotein receptor-related protein/alpha2-macroglobulin recep tor, is inhibited by a 39-kD protein that copurifies with this recepto r. Herein we investigated whether administration of purified recombina nt 39-kD protein would alter t-PA clearance in vivo. We found that int ravenous administration of purified 39-kD protein to rats prolonged th e plasma half_life of I-125-t-PA from 1 min to approximately 5-6 min. The plasma half-life of t-PA enzymatic activity was similarly prolonge d following intravenous administration of purified 39-kD protein. In a ddition we found that the 39-kD protein itself was rapidly cleared fro m the circulation in vivo. Clearance of I-125-39-kD protein was a biph asic process with half-lives of 30 s and 9 min and the liver was the p rimary organ of clearance. Preadministration of excess unlabeled 39-kD protein slowed I-125-39-kD protein clearance in rats in a dose-depend ent manner, suggesting that specific clearance receptors were responsi ble for this process Administration of increasing doses of unlabeled 3 9-kD protein along with labeled 39-kD protein resulted in a decrease i n the amount of labeled 39-kD protein associating with the liver and a concomitant increase in the amount of labeled 39-kD protein associati ng with the kidneys, indicating two clearance mechanisms exist for the 39-kD protein.