Cathepsin J has been partially purified [Liao, J. C. R. & Lenney, J. F. (1984) Biochem. Biophys. Res. Commun. 124, 909-916], but its detailed properties are still unknown. In this study, we have purified cathepsin J completely and characterized it. It was purified to homogeneity from the mitochondrial-lysosomal fraction of rat liver by acid treatment, followed by ammonium sulfate precipitation (20-65%), and chromatographies on S-Sepharose, ConA-Sepharose, Affi-gel 501, HPLC DEAE-5PW and HPLC TSK G3000SW. Cathepsin J was found to be a lysosomal high-molecular-mass cysteine protease of about 160 kDa consisted of two different subunits. One subunit (alpha subunit) was a glycoprotein with a molecular mass of 19-24 kDa which was reduced to 19 kDa by treatment with endoglycosidase F. It has the amino acid sequence LPESWDWRNVR at its N-terminus, which was very similar to those at the N-termini of rat cathepsins B, H and L. The other subunit (beta subunit) was a glycoprotein with a molecular mass of 17 kDa, which was reduced to 14 kDa by treatment with endoglycosidase F. It had DTPANETYPDLLG at its N-terminus, which had no similarity with the N-terminal sequences of other cathepsins. Cathepsin J showed strong affinity for synthetic substrates such as N-benzyloxycarbonyl-phenylalanyl-arginine 4-methyl-coumaryl-7-amide and glycyl-arginine beta-naphthylamide. It was activated by thiol reagents and chloride ion and was inhibited by cysteine protease inhibitors. However, its initial inhibition constant K(i(initial)) by N-(L-3-trans-carboxyoxirane-2-carbonyl)-L-leucine-3-methlbutylamide (E-64-c) was 1800 nM, which was 100-500 times those of cathepsins B and L. Many properties of cathepsin J were similar to those of cathepsin C (dipeptidylaminopeptidase I) reported as a lysosomal cysteine protease with dipeptidyl-aminopeptidase activity [McDonald, J. K. Reilly, T. J. & Ellis, S. (1964) Biochem. Biophys. Res. Commun. 16, 135 - 140]. Furthermore, antiserum against rat liver cathepsin C reacted with rat liver cathepsin J. These findings suggested that cathepsin J is identical with cathepsin C.
