Presence of a thiol protease in regenerating rat-liver nuclei. Partial purification and some properties

1. Nuclei of regenerating rat liver washed with Triton X-100 were found to contain a new protease. Since the enzymatic activity for degrading ribosomal proteins was inhibited in vivo by administration of E-64, a thiol protease inhibitor, the enzyme may participate in the degradation of newly synthesized ribosomal proteins and histones in regenerating rat liver nuclei as reported previously by us [Biochem. Biophys. Res. Commun. 75, 525-531 (1077)]. The optimum pH was 5.5. 2. The enzyme was extracted from washed nuclei and partially purified by gel filtration through Sepharose 6B. Its molecular weight was about 40 000. A maximal activity of partially purified enzyme was observed in the presence of 1 mM EDTA and 2 mM dithiothreitol at pH 5.5 It was inhibited by thio reagents, E-64, leupeptin and hevy metal ions. The enzyme degraded ribosomal proteins endoproteolytically and degraded most proteins tested as substrates, although liver cell sap proteins and serum albumin were less degraded than ribosomal proteins and histones, alpha-N-Benzoylarginine-beta-naphthylamide and benzoylarginine amide were not hydrolyzed.