Kininogenase Activity of Prostate‐Derived Human Glandular Kallikrein (hK2) Purified From Seminal Fluid

Abstract

ABSTRACT: Prostate‐specific human glandular kallikrein (hK2) is an active enzyme in human seminal fluid. It is one of three serine proteases in the human kallikrein gene family, which includes hK1 (tissue kallikrein) and hK3 (prostate‐specific antigen [PSA]). In order to examine kininogenase activity (i.e., production of kinin by these enzymes), we tested for bradykinin and/or Lys‐bradykinin release upon incubation of hK2 and for other kallikreins with high‐molecular weight kininogen (HMWK), which contains the nonapeptide bradykinin. Kinins are important regulatory peptides (especially for vascular permeability), and they may have a role in enhancing sperm motility. High‐molecular weight kininogen is the substrate for plasma kallikrein (PK— a potent kinin‐generating enzyme circulating in blood, not of the same gene family) and for hK1. Glandular kallikrein and protein‐C inhibitor (PCI)‐hK2 complex, a serpin protease inhibitor that binds hK2, were purified to homogeneity by affinity and size‐exclusion chromatography. About one‐half of the hK2 is found in complex with PCI. The kallikrein enzymes were incubated with HMWK, and the resulting cleavage products were analyzed for kinin activity using enzyme immunoassay, high‐performance liquid chromatography and mass spectrometry, and in vitro bioassay. Our results show that hK2 cleaves HMWK to produce bradykinin, not Lys‐bradykinin (like hK1), and the resultant heavy (56‐kDa) and light (42‐kDa) chains of HMWK show similar electrophoretic mobility to those cleaved by PK. Prostate‐specific antigen (hK3) had no kinin‐generating activity. We also identified three other internal cleavage sites for hK2 in HMWK (Arg427, Arg427, and Arg457) that yielded two peptides, one of which is identical to a PK‐cleaved peptide. Glandular kallikrein is about 500‐fold less active than is PK or tissue kallikrein, but it may play a physiologically important role in bradykinin release in seminal fluid.