Unusual C3-Acetylation of Quinoxalin-2(1H)-one via Oxidative C–C and C–O Bond Cleavages of PEG-400

Aerobic oxidative tandem conversion of PEG-400 to acetyl radical via C-C and C-O bond cleavages followed by silver catalyzed menisci type addition to the C3 position of quinoxalin-2(1H)-one described. This...

Mechanism of 17α,20-Lyase and New Hydroxylation Reactions of Human Cytochrome P450 17A1

Cytochrome P450 (P450) reactions can involve C–C bond cleavage, and several of these are critical in steroid and sterol biosynthesis. The mechanisms of P450s 11A1, 17A1, 19A1, and 51A1 have been controversial, in the context of the role of ferric peroxide (FeO2−) versus perferryl (FeO3+, compound I) chemistry. We reinvestigated the 17α-hydroxyprogesterone and 17α-hydroxypregnenolone 17α,20-lyase reactions of human P450 17A1 and found incorporation of one 18O atom (from 18O2) into acetic acid, consonant with proposals for a ferric peroxide mechanism (Akhtar, M., Lee-Robichaud, P., Akhtar, M. E., and Wright, J. N. (1997) J. Steroid Biochem. Mol. Biol. 61, 127–132; Akhtar, M., Wright, J. N., and Lee-Robichaud, P. (2011) J. Steroid Biochem. Mol. Biol. 125, 2–12). However, the reactions were supported by iodosylbenzene (a precursor of the FeO3+ species) but not by H2O2. We propose three mechanisms that can involve the FeO3+ entity and that explain the 18O label in the acetic acid, two involving the intermediacy of an acetyl radical and one a steroid 17,20-dioxetane. P450 17A1 was found to perform 16-hydroxylation reactions on its 17α-hydroxylated products to yield 16,17α-dihydroxypregnenolone and progesterone, suggesting the presence of an active perferryloxo active species of P450 17A1 when its lyase substrate is bound. The 6β-hydroxylation of 16α,17α-dihydroxyprogesterone and the oxidation of both 16α,17α-dihydroxyprogesterone and 16α,17α-dihydroxypregnenolone to 16-hydroxy lyase products were also observed. We provide evidence for the contribution of a compound I mechanism, although contribution of a ferric peroxide pathway in the 17α,20-lyase reaction cannot be excluded.

Rat ventral prostate xanthine oxidase–mediated metabolism of acetaldehyde to acetyl radical

Alcohol drinking is known to lead to deleterious effects on prostate epithelial cells from humans and experimental animals. The understanding of the mechanisms underlying these effects is relevant to intraprostatic ethanol treatment of benign prostatic hyperplasia and to shed some light into the conflictive results linking alcohol consumption to prostate cancer. In previous studies, we provided evidence about the presence in the rat ventral prostate of cytosolic and microsomal metabolic pathways of ethanol to acetaldehyde and 1-hydroxyethyl radical and about the low levels of alcohol dehydrogenase and aldehyde dehydrogenase. Acetaldehyde accumulation in prostate tissue and oxidative stress promotion were also observed. In this study, we report that in the ventral prostate cytosolic fraction, xanthine oxidoreductase is able to metabolize acetaldehyde to acetyl radical. The identification of the acetyl was performed by GC-MS of the silylated acetyl-PBN adduct. Reference adduct was generated chemically. Formation of acetyl was also observed using pure xanthine oxidase. The generation of acetyl by the prostate cytosol was inhibited by allopurinol, oxypurinol, diphenyleneiodonium chloride, folate, and ellagic acid. Results suggest that metabolism of ethanol to acetaldehyde and to 1-hydroxyethyl and acetyl radicals could be involved in the deleterious effects of alcohol drinking on prostate epithelial cells.