ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS Vol. 262, No. 1, April, pp. ill-117,1988 In Vivo Catabolism of Heparin Cofactor II and Its Complex with Thrombin: Evidence for a Common Receptor-Mediated Clearance Pathway for Three Serine Proteinase Inhibitors’ CHARLOTTE W. PRATT, FRANK C. CHURCH, AND SALVATORE V. PIZZ02 Departments of Biochemistry and Pathology, Duke University Medical Center, Durham, North Carolina 27710, and the Center for Thrombosis and Hemostasis, the University of North Carolina, Chapel Hill, North Carol&u 27’514 Received August 10,1987, and in revised form November 25.1987 The plasma clearance of ‘l-labeled human heparin cofactor II and its complex with thrombin was studied in mice to determine whether a specific mechanism exists for the catabolism of the inhibitor-proteinase complex. Initital studies demonstrated that mu- rine plasma contains a heparin cofactor II-like inhibitor as shown by the presence of a dermatan sulfate-sensitive thrombin inhibitor. Human heparin cofactor II cleared from the circulation of mice with an apparent half-life of 80 min while heparin cofactor II-thrombin complexes cleared with an apparent half-life of only 10 min. The specificity of the clearance mechanism was investigated by clearance competition studies involv- ing coinjection of excess unlabeled heparin cofactor II-cu-thrombin, antithrombin III-a-thrombin, or Lul-proteinase inhibitor-elastase, and by tissue distribution studies. The results demonstrated that the clearance of 1251-labeled heparin cofactor II-a- thrombin is a receptor-mediated process, and that the same hepatocyte receptor system recognizes complexes containing heparin cofactor II, antithrombin III, and al-protein- ase inhibitor. @ 1988 Academic Press. Inc. The regulation of proteolytic processes such as coagulation, fibrinolysis, and in- flammation depends in part on the activ- ity of plasma proteinase inhibitors includ- ing ar2-macroglobulin and inhibitors of the al-proteinase inhibitor family (the “ser- pins”) (1). In most cases, proteinase inhi- bition is characterized by the formation of an irreversible complex between the in- hibitor and the proteinase active site (2). Complex formation is accompanied by conformational changes in the inhibitor that can be detected by circular dichroic i This work was supported by research Grants HL-24066 (S.V.P.) and HL-32656 (F.C.C.) from the National Heart, Lung, and Blood Institute. *To whom correspondence should be addressed at: Box 3712, Duke University Medical Center, Durham, NC 27710. and ultraviolet absorbance spectroscopy (3-5). Such changes are essential for rec- ognition of inhibitor-proteinase com- plexes by cell-surface receptors that do not recognize unreacted inhibitors (6, 7). The plasma clearance of a number of inhibitor-proteinase complexes has been studied in mice. Murine clearance of a number of human proteinase inhibitors alone or in complex with proteinases has been compared to the behavior of the mu- rine equivalent proteins (6-10). The mouse model has consistently demonstrated com- parable behavior of murine and human proteins. Moreover, limited clearance studies of proteinase inhibitors alone or in complex with proteinases in humans have produced similar clearance behavior (6). These findings are consistent with the ex- tensive sequence homology observed 111 0003-9861/88 $3.00 Copyright Q 1988 by Academic Press, Inc. All righta of reproduction in any form reserved