Possible processing of mitochondria-bindable hexokinase to the nonbindable form by a lysosomal protease in rat liver

As a possible mechanism for the absence of mitochondria-bindable hexokinase in the liver, the presence of a protease similar in action to chymotrypsin, which specifically eliminates the binding ability of the bindable hexokinase without changing its catalytic properties, was investigated in rat liver. The lysosomal fraction prepared from the liver converted the bindable hexokinase prepared from rat brain to the nonbindable form with little change in catalytic activity. The activity of such a "processing protease" was much lower in rat brain, where the bindable form is predominant. The processing activity cosedimented with lysosomal marker enzyme activities in the subcellular fractionation of livers from normal and Triton WR-1339-injected rats. A fair portion of the activity was detected in the lysosomes without disruption. The activity was maximal at pH 6.0-7.0, inactivated almost completely by tosylphenylalanine chloromethyl ketone, tosyllysine chloromethyl ketone, leupeptin, antipain, and chymostatin, and dependent on dithiothreitol and mercaptoethanol. These results suggest that a protease, properties of which are fairly similar to those of cathepsin M, may be involved in the post-translational processing of original bindable hexokinase to the nonbindable form in rat liver.