Endogenous inhibitors of lysosomal proteinases.

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Abstract
Specific inhibitors of 3 lysosomal proteinases are present in the cytosolic and lysosomal compartments of rabbit liver. The cytosolic inhibitors, purified by chromatography on DEAE-Trisacryl and Sephadex G-75, show specificities toward cathepsin M, cathepsins B and L, and fructose 1,6-bisphosphatase converting enzyme (CE), respectively, and are designated IM, IB/L, and ICE. Inhibitors with similar specificities were isolated from the intralysosomal compartment. Two of these inhibitors, IM and ICE, are also present in the lysosomal membranes. The lysosomal distribution parallels that of the respective proteinases. The inhibitors are polypeptides with MW of 5,000-10,000 for the 2 forms of IB/L, 12,500 for IM, and 10,000-40,000 for the ICE species.