Cloning and Expression of a Rat Kidney Cytosolic Glutamine Transaminase K That Has Strong Sequence Homology to Kynurenine Pyruvate Aminotransferase

An α-keto acid-dependent cysteineS-conjugate β-lyase of rat kidney was identified as cytosolic glutamine transaminase K by Stevenset al.(J. Biol. Chem.261, 15529–15537, 1986). Subsequently, a rat kidney protein with both glutamine transaminase K- and cysteineS-conjugate β-lyase activities was cloned and sequenced by Perryet al.(Mol. Pharmacol.43, 660–665, 1993). This protein was later shown to be identical with kynurenine pyruvate aminotransferase (Moscaet al., FEBS Lett.353, 21–24, 1994). Thus, kynurenine pyruvate aminotransferase possesses both glutamine transaminase K- and cysteineS-conjugate β-lyase-type activities. We have also cloned a cytosolic glutamine transaminase 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 cysteineS-conjugate β-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. Our 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 catalyze glutamine transaminase K/cysteineS-conjugate β-lyase/kynurenine py- ruvate aminotransferase reactions. The present work underscores the difficulties associated with characterizing aminotransferases with overlapping specificities.