Existence of two kinds of thiol protease inhibitors in rat serum

In rat serum two kinds of thiol protease inhibitors were found. One had molecular weight about 103,000 and the other about 16,000. Though both inhibitors inhibited all thiol proteases examined, the high molecular weight inhibitor showed stronger inhibition on papain than cathepsin H. But the low molecular weight inhibitor equally inhibited papain and cathepsin H. The isoelectric point of low molecular weight inhibitor was pH 9.16 and that of high molecular one was pH 4.76.