Cathepsin B, the lysosomal thiol proteinase of calf liver

Cathepsin B from calf liver was obtained by a method involving preparation of a lysosomal-mitochondrial pellet and treatment of this pellet with acetone. The material was extracted with an acid buffer, pH4.0, and then precipitated from the extract with acetone. The precipitate was dissolved in phosphate buffer, pH7.4, and subjected to gel filtration on Sephadex G-200 and G-100. The cathepsin B emerged in a range of molecular weight much lower than 50000 as a well-defined component. The purity of this material was checked by electrophoresis. To obtain maximum activity the enzyme had to be activated with a chelating agent and a reducing agent (i.e. EDTA and cysteine). A number of different substrates were used. The enzyme was active for the hydrolysis of both peptide bonds and ester bonds and had approximately equal reactivity in the two cases. The pH-dependence of the hydrolysis was the same with both substrates. The binding of the substrates was half-maximal at pH4.5 and at pH6.8. A thiol group occurred in the active centre but this group ought to have a much higher pK than that found in this enzyme.