Four papain-inhibiting peaks, labeled F-I, F-II, F-III, and F-IV, were fractionated from a crude bovine muscle extract by gel filtration chromatography on Sephadex G100, and the F-III fraction was analyzed. From F-III, a cysteine proteinase inhibitor was purified by two successive anionic exchange chromatography steps on Q-Sepharose and Mono-Q columns. This inhibitor has a molecular weight of about 30 kDa. Regarding its specificity toward different proteinases, the purified 30 kDa inhibitor was inactive against serine (trypsin and chymotrypsin) and aspartyl (pepsin) families. In contrast, cathepsin L, H, B, and papain, four enzymes of the cysteine class were strongly inhibited suggesting that this inhibitor was specific to the cysteine proteinase group. However, no inhibitory activity was shown against calpains. Kinetic parameters, including inhibition constants (Ki), rate constant for association (kass) and time required for almost complete inhibition of proteinase in vivo were determined. The values are consistent with a possible physiological function for this inhibitor protein in controlling in vivo cathepsin L activity.
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