A new thiol proteinase from rat liver.

An enzyme recently purified from rat liver (Gohda, and H. C. (1980) J Biol. press), which catalyzes the conversion of multiple forms of tyrosine aminotransferase, has been further characterized. The purified enzyme, termed a convertase, hydrolyzed oxi- dized ribonuclease A and acid-denatured hemoglobin in addition to Form I of tyrosine aminotransferase and azocasein. The enzyme exhibited little activity toward albumin, native ribonuclease A, cytochrome C, and oxidized insulin B chain. During the early stages of diges- tion of oxidized ribonuclease A with the purified convertase, two major cleavage products were detected. Oxidized ribonuclease A inhibited in a dose-dependent manner the conversion of Form I of tyrosine aminotransferase to Forms II and 111. Hydrolysis by the con- vertase of several synthetic substrates for exopeptidases was not detectable. During the purification of the convertase from rat liver, the activities of cathepsins B,, H, and L were eliminated from the convertase frac- tions, and the final preparation of the convertase showed no detectable activity of these cathepsins. Cathepsin D activity, measured as acid hemoglobin-hy- drolyzing activity, was still detectable in the purified convertase fraction, but its yield and purification were much less than that of the convertase (about 2% of the convertase). The purified convertase was activated by sulfhydryl compounds and EDTA, and inhibited by sulfhydryl-reactive reagents. The convertase was also sensitive to leupeptin, antipain,