Two-electron reduction of quinones by rat liver NAD(P)H:quinone oxidoreductase: quantitative structure–activity relationships

Mammalian NAD(P)H:quinone oxidoreductase (NQO1, DT-diaphorase, EC 1.6.99.2) catalyzes the two-electron reduction of quinones and plays one of the main roles in the bioactivation of quinoidal drugs. In order to understand the enzyme substrate specificity, we have examined the reactions of rat NQO1 with a number of quinones with available potentials of single-electron (E17) reduction and pKa of their semiquinones. The hydride transfer potentials (E7(H−)) were calculated from the midpoint potentials of quinones and pKa of hydroquinones. Our findings imply that benzo- and naphthoquinones with a van der Waals volume (VdWvol) ⩽200 Å3 are much more reactive than glutathionyl-substituted naphthoquinones, polycyclic quinones, and FMN (VdWvol > 200 Å3) with the same reduction potentials. The entropies of activation (ΔS≠) in the reduction of “fast” oxidants are equal to −84 to −76 J mol−1 K−1, whereas in the reduction of “slow” oxidants ΔS≠=−36 to −11 J mol−1 K−1. The large negative ΔS≠ in the reduction of fast oxidants may be explained by their better electronic coupling with reduced FAD or the formation of charge-transfer complexes, since fast oxidants bind at the dicumarol binding site, whereas the binding of some slow oxidants outside it has been demonstrated. The reactivity of quinones may be equally well described in terms of the three-step (e−,H+,e−) hydride transfer, using E17, pKa(QH), and VdWvol as correlation parameters, or in terms of single-step (H−) hydride transfer, using E7(H−) and VdWvol in the correlation. The analysis of NQO1 reactions with single-electron acceptors and quinones using an “outer-sphere” electron transfer model points to the possibility of a three-step hydride transfer.