Biochemical properties of thiol proteinase inhibitor purified from psoriatic scales.

Epidermal thiol proteinase inhibitor (EPI) was extracted from normal and psoriatic cornified cells with 10 mM Tris-HCl, pH 8.0, and purified by papain-Sepharose affinity chromatography and gel filtration. Both EPIs showed a single band and the same mobility in gel electrophoresis with and without sodium dodecyl sulfate. Their immunological identity also was seen by agar diffusion. The inhibitor activity of EPIs to papain and rat liver lysosomal enzymes which caused a local inflammatory reaction after intradermal injection was determined on alpha-N-benzoyl-DL-arginine-2-naphthylamide and azocasein. Their activities to papain were the same at pH 8.0, but EPI of psoriasis showed only 50% of the activity of normal cells at pH 5.0 and 6.0. EPI of normal cells was heat stable, while that of psoriasis was reduced in activity after heating at 90 degrees C. Inhibitor activity of EPI from psoriatic cells toward the lysosomal enzymes, cathepsin B and/or cathepsin H and cathepsin L, was also inferior to EPI from normal cells at all pHs studied. We suggest the possibility that the inflammatory response associated with psoriasis seems in part to result from epidermal cells producing a less effective EPI, which may be a natural anti-inflammatory substance.