Autocatalytic processing of recombinant human procathepsin B is a bimolecular process

Cathepsin B and other lysosomal cysteine proteinases are synthesized as ina ctive zymogens, which are converted to their mature forms by other protease s or by autocatalytic processing. Procathepsin B autoactivation was shown i n vitro at pH 4.5 to be a bimolecular process with K-s and k(cat) values of 2.1 +/- 0.9 mu M and 0.12 +/- 0.02 s(-1), respectively. Autoactivation is substantially accelerated in the presence of active cathepsin B molecules, indicating that mature cathepsin B is the catalytic species in the process, Proenzyme is cleaved without significant conformational changes as judged by circular dichroism, suggesting that propeptide unfolding occurs only aft er the cleavage, Procathepsin B autoactivation is pH-dependent with a pH op timum. at 4.5 and with no processing observed at pH > 6.0. However, in the presence of 0.5 mu g/ml of dextran sulfate, relatively rapid processing is observed even at pH 6.5 (t(1/2) similar to 90 min), suggesting that glycosa minoglycans are involved in in vivo processing of lysosomal cysteine protea ses, (C) 1999 Federation of European Biochemical Societies.