CLONING AND EXPRESSION OF A RAT-KIDNEY CYTOSOLIC GLUTAMINE TRANSAMINASE-K THAT HAS STRONG SEQUENCE HOMOLOGY TO KYNURENINE PYRUVATE AMINOTRANSFERASE

An alpha-keto acid-dependent cysteine S-conjugate beta-lyase of rat ki dney was identified as cytosolic glutamine transaminase K by Stevens e t al. (J. Biol. Chem. 261, 15529-15537, 1986), Subsequently, a rat kid ney protein with both glutamine transaminase K- and cysteine S-conjuga te beta-lyase activities was cloned and sequenced by Ferry et al. (Mol . Pharmacol, 43, 660-665, 1993). This protein was later shown to be id entical with kynurenine pyruvate aminotransferase (Mosca et al., FEES Lett, 353, 21-24, 1994), Thus, kynurenine pyruvate aminotransferase po ssesses both glutamine transaminase K- and cysteine S-conjugate beta-l yase-type activities, We have also cloned a cytosolic glutamine transa minase K from a rat kidney cDNA library and expressed the full-length clone in COS1 cells, The transfected cells exhibit marked increases in activities of both glutamine transaminase K and cysteine S-conjugate beta-lyase. The enzyme cloned in the present work is a homodimer, Each subunit has a molecular mass of 45.8 kDa and contains 426 amino acid residues, The sequence of cytosolic glutamine transaminase K obtained in the present work has strong similarities to other aminotransferases , including >90% identity with kynurenine pyruvate aminotransferase. O ur preparation of purified rat kidney cytosolic glutamine transaminase K possesses some kynurenine pyruvate aminotransferase activity. Thus, rat kidney cytosol possesses at least two distinct enzymes that catal yze glutamine transaminase K/cysteine S-conjugate beta-lyase/kynurenin e pyruvate aminotransferase reactions, The present work underscores th e difficulties associated with characterizing aminotransferases with o verlapping specificities. (C) 1996 Academic Press, Inc.