title: Spectroscopic identification of ortho-quinones as the products of polycyclic aromatic trans-dihydrodiol oxidation catalyzed by dihydrodiol dehydrogenase. A potential route of proximate carcinogen metabolism.
abstract: The homogeneous dihydrodiol dehydrogenase of rat liver cytosol catalyzes the NADP-dependent oxidation of polycyclic aromatic trans-dihydrodiols, a reaction that may suppress their carcinogenicity provided the products of the reaction are noncarcinogenic. This report demonstrates that the products of naphthalene and benzo[a]pyrene trans-dihydrodiol oxidation are electrophilic o-quinones, which arise via autoxidation of catechols produced from the dihydrodiols by the action of dihydrodiol dehydrogenase. Oxidation of the trans-1,2-dihydrodiol of naphthalene or the 7,8-dihydrodiol of benzo[a]pyrene by the homogeneous rat liver dehydrogenase in 50 mM glycine at pH 9.0 led to the formation of multiple products by TLC, none of which co-migrated with the corresponding o-quinone standards. An identical result was obtained when these standards were incubated with buffer alone, suggesting that o-quinones were formed enzymatically from the dihydrodiols, and then underwent addition reactions with the glycine buffer. In subsequent reactions, the o-quinones formed from the enzymatic oxidation of the trans-dihydrodiols of naphthalene and benzo[a]pyrene were trapped by conducting the reactions in phosphate buffer containing 2-mercaptoethanol. The products of these reactions were identified by 500 MHz nmr and electron impact mass spectrometry as adducts of the 1,2-quinone of naphthalene (m/e M+ = 234) and the 7,8-quinone of benzo[a]pyrene (m/e M+ = 358), which contained mercaptoethanol as a thioether at C-4 and C-10, respectively. Kinetic analysis of the reactivity of the 1,2-quinone of naphthalene showed that the cellular nucleophiles, cysteine and glutathione, react very rapidly with the quinone. The 7,8-quinone of benzo[a]pyrene also reacted with glutathione and cysteine to form water-soluble metabolites, but did not react with adenosine or guanosine. These results suggest that o-quinones formed by enzymatic dihydrodiol oxidation may be effectively scavenged by cellular nucleophiles, resulting in their detoxification.
authors.affiliation: Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6084.
authors.forename: T E
authors.initials: TE
authors.lastname: Smithgall
authors.forename: R G
authors.initials: RG
authors.lastname: Harvey
authors.forename: T M
authors.initials: TM
authors.lastname: Penning
mesh_terms: D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D001564:Benzo(a)pyrene / Q000378:metabolism; D002273:Carcinogens / Q000378:metabolism*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003545:Cysteine / Q000378:metabolism; D005978:Glutathione / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D009281:Naphthalenes / Q000378:metabolism; D010088:Oxidoreductases; D044925:Oxidoreductases Acting on CH-CH Group Donors; D011809:Quinones
chemical_list: D002273:Carcinogens; D009281:Naphthalenes; D011809:Quinones; C031721:naphthalene; D001564:Benzo(a)pyrene; D010088:Oxidoreductases; D000429:Alcohol Oxidoreductases; C023125:dihydrodiol dehydrogenases; D044925:Oxidoreductases Acting on CH-CH Group Donors; C116788:cis-1,2-dihydro-1,2-dihydroxynaphthalene dehydrogenase; D005978:Glutathione; D003545:Cysteine