Purification and characterization of thiol proteinase inhibitor from rat liver

A thiol proteinase inhibitor was purified from rat liver by essentially the same procedure as reported previously (Kominami, E., Wakamatsu, N., and Katunuma, N. (1981) Biochem. Biophys. Res. Commun. 99, 568-575), but without heat treatment. The purified inhibitor appears homogeneous on polyacrylamide gel electrophoresis with and without sodium dodecyl sulfate and displayed no multiple forms. The inhibitor has Mr = 12,500 and contains 50.5% of polar amino acid residues, 9.3% aromatic amino acids, and no tryptophan. The presence of 2 half-cystines/molecule and the absence of free thiol groups indicate that the inhibitor possesses one disulfide bridges. The inhibitor inhibits cathepsin H by forming an enzyme-inhibitor complex in a molar ratio of 1:1. It inhibits most thiol proteinases such as cathepsin H, L, B, and C, papain, and ficin, but not calcium-activated neutral proteinase or serine proteinases or carboxyl proteinases. The inhibitor was found in various rat tissues. Immunological diffusion analysis with anti-liver thiol proteinase inhibitor serum indicated that the rat liver inhibitor is immunologically identical with the inhibitors from other rat tissues. On subcellular fractionation of rat liver, the thiol proteinase inhibitor was recovered in the cytosol fraction.