title: Prostaglandin H synthase-catalyzed oxidation of all-trans- and 13-cis-retinoic acid to carbon-centered and peroxyl radical intermediates.
abstract: Due to the importance of all-trans-retinoic acid (RA) in the treatment of various dermatological conditions and the wide distribution of prostaglandin H synthase (PGHS) in tissues, we have further examined the mechanisms involved in the hydroperoxide-dependent cooxidation of RA and its isomer, 13-cis-retinoic acid ((13Z)-RA), by PGHS. Hydroperoxide-dependent, PGHS-catalyzed oxidation of RA and (13Z)-RA was shown to form free radical adducts, using electron spin resonance (ESR) spin trapping techniques and 5-phenyl-4-penten-1-yl hydroperoxide (PPHP) or 13-hydroperoxy-9-cis-11-trans-octadecadienoic acid (13-OOH-18:2) as hydroperoxide substrates. Utilization of the spin trap alpha-phenyl-N-tert-butylnitrone (PBN) resulted in the detection of (13Z)-RA-PBN and RA-PBN adducts whose spectra were characterized by hyperfine coupling constants of aH = 4.16/aN = 15.69 and aH = 3.01/aN =15.92, respectively. Identical experiments under anaerobic conditions were carried out using the spin trap 2-methyl-2-nitrosopropane (NtB) which yielded nitroxide adducts whose spectra were characterized by a triplet of doublets with values of aH = 3.49/aN = 15.84 for the (13Z)-RA adduct and aH = 3.49/aN = 15.88 for the RA adduct. These results are indicative of secondary carbon-centered radical formation. We also used (+)-benzo[a]pyrene 7(S),8(S)-dihydrodiol ((+)-BP-7,8-diol) as a peroxyl radical probe. The results demonstrated the formation of (+)-BP-7,8-diol-derived tetrols, with the trans-anti tetrol representing the major oxidation product in systems undergoing PPHP-dependent, PGHS-catalyzed oxidation of (13Z)-RA or RA. These results are consistent with the formation of peroxyl radicals in these systems. In all experiments, the (13Z)-RA isomer appeared to be a better substrate for the enzyme compared to the all-trans isomer. Collectively these results provide further evidence to support the previously proposed mechanism for retinoid oxidation by PGHS involving the intermediacy of C4 carbon-centered radicals which subsequently react with dioxygen, yielding retinoid-derived peroxyl radicals.
authors.affiliation: Division of Toxicology, University of Arkansas for Medical Sciences, Little Rock 72205, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Freyaldenhoven
authors.forename: R V
authors.initials: RV
authors.lastname: Lloyd
authors.forename: V M
authors.initials: VM
authors.lastname: Samokyszyn
mesh_terms: D000693:Anaerobiosis; D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003497:Cyclic N-Oxides; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D015474:Isotretinoin / Q000737:chemistry*; D007641:Keratolytic Agents / Q000737:chemistry*; D008297:Male; D009589:Nitrogen Oxides / Q000737:chemistry; D009603:Nitroso Compounds / Q000737:chemistry; D010084:Oxidation-Reduction; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism*; D012756:Sheep; D013113:Spin Labels; D014212:Tretinoin / Q000737:chemistry*
chemical_list: D003497:Cyclic N-Oxides; D005609:Free Radicals; D007641:Keratolytic Agents; D009589:Nitrogen Oxides; D009603:Nitroso Compounds; D013113:Spin Labels; C029217:phenyl-N-tert-butylnitrone; D014212:Tretinoin; D011451:Prostaglandin-Endoperoxide Synthases; D015474:Isotretinoin; C034526:tert-nitrosobutane