Human dioxin-inducible cytosolic NAD(P)H:menadione oxidoreductase. cDNA sequence and localization of gene to chromosome 16.

Abstract

NAD(P)H:menadione oxidoreductase (NMOR1) is a flavoprotein that catalyzes the two-electron reduction of various redox dyes and quinones. It has been proposed that this enzyme may have a protective effect against cancer caused by quinones and their metabolic precursors. We show that tetrachlorodibenzo-p-dioxin (TCDD) treatment of the human hepatoblastoma cell line Hep-G2 produces a 5-fold induction of NMOR activity. Several overlapping human NMOR1 cDNAs were isolated from a human liver lambda gt 11 expression library, and their composite sequence corresponds to an mRNA of 2448 nucleotides containing a continuous open reading frame encoding a protein of 274 residues (molecular weight, 30,880). The corresponding human NMOR1 mRNA has an unusually long 3'-untranslated region (1679 base pairs) with four potential polyadenylation signals (I-IV) at positions 986, 1460, 1838, and 2419 and a single copy of human Alu repetitive sequence between polyadenylation sites II and III. Southern blot analysis of human genomic DNA suggests the presence of a single NMOR1 gene approximately 10 kilobases (KB) in length. The use of three of the aforementioned polyadenylation signals is likely to account for the three different species (2.7, 1.7, and 1.2 kb) of mRNA hybridizing to NMOR1 cDNA in Hep-G2 cells. Indeed several partial cDNA clones were isolated that corresponded to the mRNA derived by use of the proximal polyadenylation signal. Interestingly, the longest (2.7 kb) mRNA species was induced severalfold by TCDD, whereas the other two mRNAs (1.7 and 1.2 kb) were induced to a much lesser extent by TCDD treatment. The human NMOR1 cDNA and protein are 83 and 85% similar to rat liver cytosolic NMOR1 cDNA and protein, respectively. Southern analysis of DNA from 54 human x mouse and 39 human x hamster somatic cell hybrids shows that the NMOR1 gene resides on human chromosome 16.