Autocatalytic processing of recombinant human procathepsin B is a bimolecular process11Dedicated to Prof. Pavao Mildner on the occasion of his 80th birthday.

AbstractCathepsin B and other lysosomal cysteine proteinases are synthesized as inactive zymogens, which are converted to their mature forms by other proteases or by autocatalytic processing. Procathepsin B autoactivation was shown in vitro at pH 4.5 to be a bimolecular process with Ks and kcat values of 2.1±0.9 μM 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 after the cleavage. Procathepsin B autoactivation is pH-dependent with a pH optimum at 4.5 and with no processing observed at pH>6.0. However, in the presence of 0.5 μg/ml of dextran sulfate, relatively rapid processing is observed even at pH 6.5 (t1/2∼90 min), suggesting that glycosaminoglycans are involved in in vivo processing of lysosomal cysteine proteases