α2-Macroglobulin and C1-Inactivator are Plasma Inhibitors of Human Glandular Kallikrein

ABSTRACT: Human glandular kallikrein (hK2) is a possible new marker for prostate cancer that is homologous to prostate specific antigen. Purified hK2 added to serum or plasma reacted with endogenous protease inhibitors to form complexes of >350, 135, and 80 kDa, and some hK2 remained free, as judged by immunoblotting. The former two complexes could be removed by specific antibodies to α2-macroglobulin and to C1- inactivator, respectively, and they comigrated on SDS-PAGE with complexes formed between hK2 and purified α2-macroglobulin or C1-inactivator. hK2 complexes of 80 kDa could not be completely removed with any anti-serpin antibody used. Thus, these may consist of more than one type of hK2 complex. In contrast, essentially all hK2 complexes were removed from seminal plasma by antibody to protein C inhibitor, demonstrating that protein C inhibitor is the only significant inhibitor of hK2 in semen. hK2 reacted more rapidly with α2-macroglobulin than with any other inhibitor in plasma or serum. Divalent metal ions and heparin did not appreciably affect the rate of formation of any of the hK2 complexes in serum or plasma or with purified α2-macroglobulin or C1-inactivator. Measurement of one or more of the hK2 forms identified here may have diagnostic or prognostic potential for prostate cancer.