Three forms of thiol proteinase inhibitor from rat liver formed depending on the oxidation-reduction state of a sulfhydryl group.

Three forms of a thiol proteinase inhibitor were isolated from rat liver cytosol. The monomeric inhibitor (pI 5.2) (TPI-1) formed a complex with cathepsin H even in the absence of reducing agents. The inhibitor with pI 5.0 (TPI-2) was inactive in the absence of reducing agents but was converted to an active inhibitor on addition of reducing agents such as dithiothreitol, GSH, cysteine, or 2-mercaptoethanol. The dimeric inhibitor (TPI-D) with an intermolecular disulfide bridge was also inactive and was converted to the active monomeric inhibitor on addition of dithiothreitol. TPI-2 is most likely a mixed disulfide with glutathione. One (Cys-3) of two cysteine residues exposed on the surface of the molecule of TPI-2 is involved in the formation of a mixed disulfide, and the other cysteine residue (Cys-64) is buried in the molecule. The activity of rat liver thiol proteinase inhibitor may possibly be regulated by formation of a protein mixed disulfide or by reduction of the mixed disulfide.