Endogenous thiol proteinase inhibitor from rat liver.

A thiol proteinase inhibitor was purified from rat liver by a rapid procedure involving heat treatment of the post-lysosomal fraction, affinity chromatography on papain-Sepharose 4B and Sephadex G-75. The purified inhibitor appeared homogeneous on sodium dodecyl sulfate electrophoresis. The inhibitor had a molecular weight of about 11000 and consists of three forms (pI 4.9, 5.2 and 5.6). The preparation inhibited thiol proteinases but not serine proteinase or aspartate proteinase. Kinetic studies of inhibition of papain by the inhibitor indicate that the inhibition was noncompetitive and pseudo-irreversible. Subcellular fractionations of rat liver indicate that most of all the inhibitor is localized in the cytosol fraction. The physiological role of the inhibitor is discussed.