Molecular characterization of NAD(P)H :quinone oxidoreductase of tobacco leaves

The NAD(P)H:quinone oxidoreductase activity of tobacco leaves is catalyzed by a soluble flavoprotein [NAD(P)H-QR] and membrane-bound forms of the same enzyme. In particular, the activity associated with the plasma membrane cannot be released by hypoosmotic and salt washing of the vesicles, suggesting a specific binding. The products of the plasma-membrane-bound quinone reductase activity are fully reduced hydroquinones rather than semiquinone radicals. This peculiar kinetic property is common with soluble NAD(P)H-QR, plasma-membrane-bound NAD(P)H:quinone reductase purified from onion roots, and animal DT-diaphorase. These and previous results demonstrate that soluble and plasma-membrane-bound NAD(P)H:quinone reductases are strictly related flavo-dehydrogenases which seem to replace DT-diaphorase in plant tissues. Following purification to homogeneity, the soluble NAD(P)H-QR from tobacco leaves was digested. Nine peptides were sequenced, accounting for about 50% of NAD(P)H-QR amino acid sequence. Although one peptide was found homologous to animal DT-diaphorase and another one to plant monodehydroascorbate reductase, native NAD(P)H-QR does not seem to be structurally similar to any known flavoprotein.