Characteristics of activation of cathepsin B by sodium salicylate and comparison of catalytic site properties of cathepsins B and H.

Attempts to gain greater understanding of characteristics of catalytic site properties of two homologous lysosomal cysteine proteinases, cathepsins B and H, were made by using sodium salicylate (SA) and several specific inhibitors, which extends the studies on the structure-activity relationship and kinetics for the activation of cathepsin B by SA in the previous paper (Yamamoto, K., Takeda, M. and Kato, Y.: Japan. J. Pharmacol. 38, 215-218, 1985). The half-maximal activation of cathepsin B by SA was observed at around a molar ratio of 10(4):1 (SA/cathepsin B). No preincubation time was needed for the SA-stimulated reaction, but the rate of activation was more rapid as pH values in the preincubation mixture decreased. The extent of inactivation of cathepsin B by leupeptin and E-64 significantly decreased in the presence of SA. Catalytic site properties of cathepsins B and H were also distinguished by differences in the extent of inhibition by cysteine proteinase inhibitors. Cathepsin B was more sensitive than cathepsin H to inhibition by antipain, chymostatin, iodoacetic acid and mercuric chloride as well as leupeptin and E-64. Despite the similarity in inhibitory effects of iodoacetic acid and E-64, the rat spleen cathepsin H was characterized by insensitivity to mercuric chloride that had a considerable inhibitory effect on the corresponding enzyme from rat liver and the rat spleen cathepsin B.