Interaction of rabbit liver cathepsin M and fructose 1,6-bisphosphatase converting enzyme with their endogenous inhibitors.

The stoichiometry of complex formation between two lysosomal proteinases from rabbit liver, cathepsin M and fructose 1,6-bisphosphatase converting enzyme (CE), and their respective endogenous inhibitors was studied by the equilibrium gel penetration method. In each case the molecular weight of the complex was found to be the sum of the molecular weights of the proteinase and its inhibitor, indicating the formation of 1:1 complexes. From the reappearance of proteinase activity on dilution, it is concluded that complex formation is reversible. Localization of the proteinase activities on the outer surface of the lysosomes was confirmed in these experiments by the inhibition of this proteinase activity on addition of inhibitors to intact lysosomes. The digestion by subtilisin of rabbit liver aldolase and rabbit liver fructose 1,6-bisphosphatase, the endogenous substrates for the lysosomal proteinases, was unaffected by the inhibitors.