PLANT PHYSIOLOGY , Vol 112, Issue 1 249-258, Copyright © 1996 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
NAD(P)H:(Quinone-Acceptor) Oxidoreductase of Tobacco Leaves Is a Flavin Mononucleotide-Containing Flavoenzyme
F. Sparla, G. Tedeschi and P. Trost
Dipartimento di Biologia, Universita di Bologna, Bologna, Italy (F.S., P.T.)
The soluble NAD(P)H:(quinone-acceptor) oxidoreductase [NAD(P)H-QR, EC
1.6.99.2] of Nicotiana tabacum L. leaves and roots has been purified.
NAD(P)H-QR contains noncovalently bound flavin mononucleotide. Pairs of
subunits of 21.4 kD are linked together by disulfide bridges, but the
active enzyme is a homotetramer of 94 to 100 kD showing an isoelectric
point of 5.1. NAD(P)H-QR is a B-stereospecific dehydrogenase. NADH and
NADPH are electron donors of similar efficiency with Kcat:Km ratios (with
duroquinone) of 6.2 x 107 and 8.0 x 107 m-1 s-1, respectively. Hydrophilic
quinones are good electron acceptors, although ferricyanide and
dichlorophenolindophenol are also reduced. The quinones are converted to
hydroquinones by an obligatory two-electron transfer. No spectral evidence
for a flavin semiquinone was detected following anaerobic photoreduction.
Cibacron blue and 7-iodo-acridone-4-carboxylic acid are inhibitory. Tobacco
NAD(P)H-QR resembles animal DT-diaphorase in some respects (identical
reaction mechanism with a two-electron transfer to quinones, unusually high
catalytic capability, and donor and acceptor substrate specificity), but it
differs from DT-diaphorase in molecular structure, flavin cofactor,
stereospecificity, and sensitivity to inhibitors. As in the case with
DT-diaphorase in animals, the main NAD(P)H-QR function in plant cells may
be the reduction of quinones to quinols, which prevents the production of
semiquinones and oxygen radicals. The enzyme appears to belong to a
widespread group of plant and fungal flavoproteins found in different cell
compartments that are able to reduce quinones.
