THE INHIBITION OF CATHEPSIN-S BY ITS PROPEPTIDE - SPECIFICITY AND MECHANISM OF ACTION

The interaction of human recombinant full-length cathepsin S propeptid e (amino acids 16-114) with mature cysteine proteinases was studied wi th respect to selectivity and pH dependence. The inhibitory capacity w as tested towards mature human recombinant cathepsin S, purified cathe psin L from rat and Paramecium tetraurelia, rat cathepsin B, human cat hepsin Fl. and papain. The propeptide of cathepsin S strongly inhibite d cathepsin S (K-i = 0.27 nM) and the two cathepsin L species (K-i = 0 .36 nM) at neutral pH. Papain, and to a minor extent cathepsin H, hydr olyzed the propeptide of cathepsin S, leading to competition with the hydrolysis of the fluorogenic substrates in the respective assays. Cat hepsin B activity was nearly unaffected up to micromolar propeptide co ncentrations in the assay. The inhibition of cathepsin-l-like peptidas es was diminished with decreasing pH, probably due to dramatic changes in the conformation of the propeptide. This assumption was supported by far-ultraviolet CD spectroscopy and by the finding of rapid hydroly sis of the cathepsin S propeptide by cathepsin L at pH values less tha n 5.5.