Epoxidation of (+/-)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene during (bi)sulfite autoxidation: activation of a procarcinogen by a cocarcinogen.

Abstract

The (bi)sulfite ion undergoes extensive autoxidation in neutral aqueous media with the formation of sulfur trioxide radical anion that is detected by ESR. The radical anion subsequently reacts with molecular oxygen to form a peroxyl radical. We find that when (±)-trans-7, 8-dihydroxy-7,8-dihydrobenzo[a]pyrene is included in this autoxidation system, BP-7,8-diol is converted to diolepoxides, ultimate carcinogenic derivatives of benzo [a]pyrene. This epoxidation occurs with a stereoselectivity consistent with either a peroxyl radical or a peracid as the epoxidizing agent. The epoxidation is dependent on the concentration of both (bi)sulfite and oxygen. In the presence of 10 IAM butylated hydroxyanisole, which abolishes (bi)sulflte autoxidation, no (bi)sulfite-dependent epoxidation occurs. These results are discussed in regard to the mechanism of (bi)sulfite autoxidation, and in relationship to the cocarcinogenicity of sulfur dioxide [anhydrous (bi)sulfite] for benzo [a]pyrene-induced pulmonary neoplasia.Sulfur dioxide, an environmental pollutant, exists primarily as the (bi)sulfite anion in aqueous solution at near neutral pH. The metal-catalyzed autoxidation of the (bi)sulfite anion, studied for decades, is an oxygen-consuming chain reaction (1-3). The initiating one-electron oxidation yields the sulfur trioxide radical anion ('SO-), a predominantly sulfur-centered radical (4) that reacts with molecular oxygen. Superoxide initiates (bi)sulfite oxidation by forming 'SO (1, 5) but is not a chain propagator (2, 5), implying superoxide is not formed by reactions of the '0j radical. Presumably the reaction product of SO with oxygen is a peroxyl free radical.S53 + 02 -03SOO. [1] Huie and Neta (6) presented a kinetic argument favoring this oxygen addition reaction. Since this pathway produces a peroxyl free radical, we sought to obtain chemical support for the formation of -03SOO during (bi)sulfite autoxidation based on products of a specific trapping reaction.The trapping agent employed was (±)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene . This compound is stereoselectively epoxidized to form (+)-7r,8t-dihydroxy-9t, 10t-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE) by either peroxyl radicals (7-10) or peracids (11)(12)(13). The inclusion of BP-7,8-diol in an aqueous system for (bi)sulfite autoxidation did indeed lead to the production of anti-BPDE. In this paper we present a characterization of this epoxidation and consider the implications ofthis reaction in regard to the mechanism of (bi)sulfite autoxidation and the Butylated hydroxyanisole (BHA), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), superoxide dismutase, and diethylenetriaminepentaacetic acid (DETAPAC) were from Sigma. Potassium peroxomonosulfate (OXONE, DuPont) was obtained from Aldrich. Polyoxyethylene sorbitan monolaurate (Tween 20) and all chromatography solvents were from Fisher.Reaction Conditions. Tritiated and unlabeled BP-7,8-diol, in ethanol, were added to the reaction vessel, and the solvent was evaporated under a stream of N2. The r...

Xanthine oxidase/hydrogen peroxide generates sulfur trioxide anion radical (SO.−3) from sulfite (SO2−3)

“…Second, this result points to a new enzymatic pathway for salfite metabolism and toxicity, since the SO;-radical has been implicated in the mechanism of toxic reactions resulting from sulfite exposure [2][3][4][5][6][7][8][9]. Third, we noted that while the mechanism of SO;-generation from SOl-via autoxidation and trace-metal catalyzed oxidation has been studied in detail [10][11][12][13][14], relatively little is known about the mechanism of SO;-generation through enzymatic pathway, with the exception of some recent studies [15][16][17][18][19]. In particular, Mason and coworkers [17][18][19] utilized ESR and ESR spin trap methodology to identify SO;-formation during the prostaglandin/hydroperoxidase-catalyzed oxidation of SOl- [17,18], and SO;-/SO]-radical formation during SOl-oxidation by peroxidase/H,.O~ [19].…”

“…As mentioned in the Introduction, sulfit¢ solutions generate SO;-radical due to trace-metal ion catalyzed oxidation [10][11][12][13][14]. To prevent this, 2.0 mM DETAPAC was added to all of the reaction mixtures, following Mottley and Mason [19].…”

“…Indeed, the amount of SO;-generated was found to ing ~ As (e) but inactivated X.O. SO;-adduct formed in other systems [11,13,17,19,[22][23][24][25], demonstrating the formation of the SO;-radical in the reaction mixture. Omission of any one component caused a dramatic decrease in SO~" generation (Fig.…”

Use of Electron Paramagnetic Resonance Spectroscopy to Evaluate the Redox StateIn Vivo

“…Spin trapping of sulfur-centered radicals-The formation of free radical intermediates (SO3· 3− , SO4· 3− ) from autoxidation or enzyme-initiated oxidation of sulfite contributes to the mechanism of sulfite-induced toxicity (78). Direct in vivo spin trapping has successfully detected sulfite radicals using low frequency EPR (1,200 MHz, L-band) (59).…”

Stimulatory effects of sulfur and nitrogen oxides on carcinogen activation in human polymorphonuclear leukocytes.

“…The mechanism underlying the effect probably involves oxidation of sulfite to the sulfur trioxide radical anion (SO ; ), which in turn may react with molecular oxygen to yield a sulfur peroxyl radical ( OOSO3-). The peroxyl radical may direcdy or via the corresponding acid epoxidize BP-7,8-dihydrodiol to BPDE (18 GST Activity (pmole /min/ mg protein) and subsequent nonenzymatic or enzymatic 2 e--oxidation) is a probable candidate as is e 3. Covalent binding of BP-7,8-dihydrodiol inter-the triol epoxide (formed by hydroxylation tes to exogenous DNA in the presence of poly-at the 1 or 3 position of BPDE) (Figure 2).…”