Inhibition of human liver cathepsin L by α‐thiol proteinase inhibitor

A protein inhibitor specific for thiol proteinases is present in human serum [1 ]. Two forms of this inhibitor have been purified [2,3] and named a-TPI. A thiol proteinase inhibitor has been demonstrated in human urine and this has been shown to be a degradation product of the plasma inhibitors [4]. The plasma protein strongly inhibits papain and ficin but inhibits cathepsin B, a typical lysosomal thiol proteinase, only weakly [1,3]. Here, we describe the inhibition of cathepsin L, another lysosomal thiol proteinase [5], by this plasma inhibitor. The results obtained suggest that one of the physiological functions of a-TPI is to inhibit cathepsin L. activity was assayed with the Boehringer UV system (Mannheim). The activity of papain was measured using BANA as substrate. For cathepsin L, the activity was tested using BANA, azocasein, aldolase, Arg-N-Nap and Leu-N-Nap as substrates. Azocasein was prepared by treating a-casein with diazotized sodium sulphanilate according to [6]. The inhibition of papain by a-TPI was assayed using BANA as substrate and for cathepsin L the inhibition by a-TPI was carried out with azocasein as substrate. The experimental conditions and the procedure for the activity measurements and the inhibition tests are described in [7]. Acrylamidegel electrofocusing was performed according to [8] and SDS acrylamide-gel electrophoresis according to [9].