Insights into the redox cycle of human quinone reductase 2

Abstract

NRH:quinone oxidoreductase 2 (QR2) is a cytosolic enzyme that catalyzes the reduction of quinones, such as menadione and co-enzymes Q. With the aim of understanding better the mechanisms of action of QR2, we approached this enzyme catalysis via electron paramagnetic resonance (EPR) measurements of the by-products of the QR2 redox cycle. The variation in the production of oxidative species such as H(2)O(2), and subsequent hydroxyl radical generation, was measured during the course of QR2 activity under aerobic conditions and using pure human enzyme. The effects on the activity of the following were compared: (i) synthetic (N-benzyldihydronicotinamide, BNAH) or natural (nicotinamide riboside, NRH) co-substrates; (ii) synthetic (menadione) or natural (co-enzyme Q0, Q2) substrates; (iii) QR2 modulators and inhibitors (melatonin, resveratrol and S29434); (iv) a pro-drug activated via a redox cycle [CB1954, 5-(aziridin-1-yl)-2,4-dinitrobenzamide]. The results were also compared with those obtained with human QR1. The production of hydroxyl radicals is: (i) observed whatever the substrate/co-substrate used; ii) quenched by adding catalase; (iii) not observed with the specific QR2 inhibitor S29434; (iv) observed with the pro-drug CB1954. While QR2 produced free radicals with this pro-drug, QR1 gave no EPR signal showing the strong reducing capacity of QR2. In conclusion, EPR analysis of QR2 enzyme activity through free radical production enables modulators and effective inhibitors to be distinguished.

Cytoprotective Effect of Afobazole and Its Main Metabolite M-11

“…We modeled similar situation in mouse bone marrow cells by inhibiting NQO1 with effective concentration of dicumarol. At the same time, the choice of menadione allowed not only to provide the development of oxidative stress [13], but also made it possible to set the value of the pharmacological regulation of enzyme activity for the formation of protective action due to the high substrate specifi city of the model quinone to NQO2. Revealed effects of afobazole and M-11 with quantitatively different receptor spectra prove the participation of MT3 receptors in the mechanism of cytoprotection.…”

Chloroquine Binding Reveals Flavin Redox Switch Function of Quinone Reductase 2

“…In addition to an enzymatic role in quinone reduction, NQO2 stabilizes the p53 tumor suppressor against 20S proteasomal degradation in the presence of NRH (22,23). Furthermore, NQO2 is capable of generating reactive oxygen species (24). Yet the current understanding of NQO2 and its inhibition does not adequately explain the effects of PQ and CQ on malaria and non-malarial diseases.…”

Impact of melatonin on immunity: a review

“…In later research, the hypothesis that melatonin is a co-substrate quinone oxidoreductase 2 was rejected and strong inhibition of the enzyme by melatonin was shown even by melatonin in pharmacological concentrations [24,25]. It should be emphasized that quinone reductase 2 has hydrogen peroxide as a side product [26]. As will be discussed later in this article, melatonin is a potent antioxidant and its antioxidant potency is above that of other indoles [15].…”