Cleavage specificity and inhibition profile of proteasome isolated from the cytosol of Xenopus oocyte.

The specificity of action of the proteasome purified from the cytosol of Xenopus oocyte was investigated using oxidized insulin B chain as the substrate. HPLC analyses of the produced peptides followed by amino acid analyses showed that it cleaved four peptide bonds, Leu6-Cya7, Glu13-Ala14, Leu15-Tyr16, and Leu17-Val18, of the peptide. Cleavage at Leu6-Cya7 was found to be specific to the Xenopus enzyme. The enzyme did not cleave Gln4-His5 and Cya19-Gly20, which are commonly hydrolyzed by proteasomes from rat and mouse liver, and human erythrocyte. In contrast to previous results obtained with the mammalian proteasome, the cleavage by the Xenopus enzyme was inhibited selectively by chymostatin. These results demonstrate distinct species difference in cleavage specificity and inhibition profile among proteasomes of different origins.