Cathepsin K: A cysteine protease with unique kinin-degrading properties

Taking into account a previous report of an unidentified enzyme from macrophages acting as a kininase, the ability of cysteine proteases to degrade kinins has been investigated. Wild-type fibroblast lysates from mice, by contrast with cathepsin K-deficient lysates, hydrolysed BK (bradykinin), and released two metabolites, BK-(1-4) and BK-(5-9). Cathepsin K, but not cathepsins B, H, L and S, cleaved kinins at the Gly⁴-Phe⁵ bond and the bradykinin-mimicking substrate Abz (o-aminobenzoic acid)-RP-PGFSPFR-3- NO₂-Tyr (3-nitrotyrosine) more efficiently (pH 6.0: k _cat/K_m = 12 500 mM^-1·s^-1; pH 7.4: k_cat/K_m = 6930 mM^-1·s^-1) than angiotensin-converting enzyme hydrolysed BK. Conversely Abz-RPPGFSPFR-3-NO₂-Tyr was not cleaved by the Y67L (Tyr⁶⁷ → Leu)/L205A (Leu²⁰⁵ → Ala) cathepsin K mutant, indicating that kinin degradation mostly depends on the S2 substrate specificity. Kininase activity was further evaluated on bronchial smooth muscles. BK, but not its metabolites BK(1-4) and BK(5-9), induced a dose-dependent contraction, which was abolished by Hoe140, a B2-type receptor antagonist. Cathepsin K impaired BK-dependent contraction of normal and chronic hypoxic rats, whereas cathepsins B and L did not. Taking together vasoactive properties of kinins and the potency of cathepsin K to modulate BK-dependent contraction of smooth muscles, the present data support the notion that cathepsin K may act as a kininase, a unique property among mammalian cy steine proteases.