Thermostable Endogenous Inhibitors of Cathepsins B and H

Abstract

Separate thermostable inhibitors of cathepsins B and H (I B and I H ) were found in every rat and human tissue that was analyzed. The inhibitors were separated from the cathepsins by heating at 80–90°C at pH 3. I B and I H from rat lung were purified and found to occur in multiple molecular forms. Rat lung I B and I H were very similar proteins with molecular weights of 14000. I B and I H from hog kidney were also purified and I H was obtained free of I B . These inhibitors also displayed microheterogeneity and had molecular weights of 11000. Hog kidney I H inhibited chymopapain, but did not inhibit bromelain, chymotrypsin or cathepsin D. Rat and human serum contained thermostable inhibitors of cathepsins B and H, but these molecules had relatively high molecular weights. Therefore the low‐molecular‐weight I B and I H from other tissues appear to be intracellular in origin. When extracts of kidney or liver were autolyzed at pH 3–6, there was a large increase in cathepsin B or H activity with a concomitant decrease in I B and I H . Inhibitor destruction was apparently catalyzed by a proteinase other than cathepsin D or E. Rat kidney, liver, and spleen were rich sources of cathepsins B and H, the kidney yielding three times as much activity per gram as liver or spleen. Hog kidney was rich in cathepsin H, but yielded little or no cathepsin B. I B and I H were also present in protozoa, luna fish, chicken and toad. The inhibitors from Tetrahymena pyriformis inhibited the thiol proteinase from this protozoan.