Protein Inhibitors of Cysteine Proteinases. I. Isolation and Characterization of Stefin, a Cytosolic Protein Inhibitor of Cysteine Proteinases from Human Polymorphonuclear Granulocytes

Abstract
A protein inhibitor of cysteine proteinases, stefin, was purified from cytosol of human polymorphonuclear granulocytes. Affinity chromatography on carboxymethylated papain-Sepharose was used as the 1st step, followed by ion exchange chromatography on DEAE-Sephacel, which resolved 4 inhibitory peaks. The main peak, comprising .apprx. 80% of total inhibitory activity was characterized. It was found to be a homogenous protein with an apparent molecular mass slightly lower than that of cytochrome c and with an isoelectric point of 4.65. The inhibitor inhibits papain at a molar ratio of 1:1 as well as cathepsin B and H, but it does not inhibit serine and aspartic proteinases. It is stable at elevated temperature and in alkaline pH, but looses its activity in acid pH. Oxidized glutathione has no effect on it inhibitory activity.

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