NAD(P)H:menadione oxidoreductase induction by polycyclic hydrocarbons is known to be governed by the aromatic hydrocarbon-responsive (Ah) locus. This cytosolic enzyme was isolated from 3-methylcholanthrene-treated rat liver by a rapid two-step procedure with the use of affinity gel purification and fast-protein liquid chromatography. Polyclonal antiserum to menadione reductase was raised in rabbits. On Western (immuno) blot analysis, large increases in this hepatic menadione reductase protein

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... MOR1) of 3-methylcholanthrene-treated C57BL/6N but not DBA/2N mice confirmed that induction of this enzyme by 3-methyl-cholanthrene is regulated by the Ah receptor. A cDNA expression library was constructed in lambda gt11 and screened with antiserum. Positive cDNA clones were plaque purified and further characterized by showing enhanced hybridization to 3-methylcholanthrene-induced poly(A+) RNA from rats; the longest cDNA clone (1,501 base pairs) has an open reading frame (bases 75-899) and a nucleotide sequence consistent with a new gene family. On Northern blot analysis, a single 3-methylcholanthrene-inducible rat liver mRNA (approximately 1.6 kilobases) hybridizes to the cDNA probe. On Southern blot analysis a total of 14-16 kilobases of rat genomic DNA fragments hybridize to the cDNA probe, indicating one or a small number of menadione reductase genes in this family. The amino acid sequence (274 residues) and Mr of 30,946 compare well with the size of the rat enzyme (32 kDa) estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The amino acid sequence of two internal fragments of the trypsin-digested purified NMOR1 protein is in complete agreement with that deduced from the cDNA nucleotide sequence. This study represents the first cloning and sequencing of a cDNA encoding a Phase II drug-metabolizing enzyme under control of the Ah locus.