Reactions of radicals. 41. Reactivity patterns of the methyl radical

Methylmercury Degradation by Trivalent Manganese

Methylmercury (MeHg) is a potent neurotoxin and has great adverse health impacts on humans. Organisms and sunlight-mediated demethylation are well-known detoxification pathways of MeHg, yet whether abiotic environmental components contribute to MeHg degradation remains poorly known. Here, we report that MeHg can be degraded by trivalent manganese (Mn(III)), a naturally occurring and widespread oxidant. We found that 28 ± 4% MeHg could be degraded by Mn(III) located on synthesized Mn dioxide (MnO_2-x) surfaces during the reaction of 0.91 μg·L^-1 MeHg and 5 g·L^-1 mineral at an initial pH of 6.0 for 12 h in 10 mM NaNO₃ at 25 °C. The presence of low-molecular-weight organic acids (e.g., oxalate and citrate) substantially enhances MeHg degradation by MnO_2-x via the formation of soluble Mn(III)-ligand complexes, leading to the cleavage of the carbon-Hg bond. MeHg can also be degraded by reactions with Mn(III)-pyrophosphate complexes, with apparent degradation rate constants comparable to those by biotic and photolytic degradation. Thiol ligands (cysteine and glutathione) show negligible effects on MeHg demethylation by Mn(III). This research demonstrates potential roles of Mn(III) in degrading MeHg in natural environments, which may be further explored for remediating heavily polluted soils and engineered systems containing MeHg.

Free radicals, antioxidant enzymes, and carcinogenesis

Free radicals are found to be involved in both initiation and promotion of multistage carcinogenesis. These highly reactive compounds can act as initiators and/or promoters, cause DNA damage, activate procarcinogens, and alter the cellular antioxidant defense system. Antioxidants, the free radical scavengers, however, are shown to be anticarcinogens. They function as the inhibitors at both initiation and promotion/transformation stage of carcinogenesis and protect cells against oxidative damage. Altered antioxidant enzymes were observed during carcinogenesis or in tumors. When compared to their appropriate normal cell counterparts, tumor cells are always low in manganese superoxide dismutase activity, usually low in copper and zinc superoxide dismutase activity and almost always low in catalase activity. Glutathione peroxidase and glutathione reductase activities are highly variable. In contrast, glutathione S-transferase 7-7 is increased in many tumor cells and in chemically induced preneoplastic rat hepatocyte nodules. Increased glucose-6-phosphate dehydrogenase activity is also found in many tumors. Comprehensive data on free radicals, antioxidant enzymes, and carcinogenesis are reviewed. The role of antioxidant enzymes in carcinogenesis is discussed.

Free radical modification of prosthetic heme groups

Hemoproteins catalyze reductive and oxidative one-electron transformations. Not infrequently, the radicals produced by these one-electron reactions add to the prosthetic heme group of the enzyme and modify or terminate its catalytic function. Reactions of the radicals with the heme group include additions to the iron atom, pyrrole nitrogens, pyrrole carbons, vinyl groups, and meso carbons. The radicals involved in these reactions derive from the oxidizing agent, the substrate, or the amino acid residues of the catalytic site. The mechanism by which the radicals are generated, their steric and electronic properties, and the extent to which they have access to the heme group determine the nature and regiospecificity of the reaction. The reaction of heme prosthetic groups with radicals is relevant to the inhibition of hemoprotein enzymes, the normal and pathological degradation of heme, and our understanding of hemoprotein function.

Why is the hydroxyl radical the only radical that commonly adds to DNA? Hypothesis: it has a rare combination of high electrophilicity, high thermochemical reactivity, and a mode of production that can occur near DNA

Free radicals do not commonly add to nucleotides in DNA, despite the fact that radicals are produced in all aerobically metabolizing cells. Why is this? For oxy-radicals, the ratio of the rate constant for addition to double bonds divided by that for H-abstraction from good H-donors parallels the electrophilicity of the radical, and among oxy-radicals the hydroxyl radical is the most electrophilic, with an unusually high ratio of Kad/kH. The hydroxyl radical also is very reactive in H-atom abstraction reactions, with a large absolute value of kH. However, the hydroxyl radical's high reactivity makes it unselective and relatively nondiscriminating between H-abstraction from a sugar moiety in DNA and penetration to, and reaction with, a base. Oxy-radicals such as alkoxyl and peroxyl radicals do not have as high electrophilicity or as high reactivity. Interestingly, carbon-centered radicals (such as the methyl radical) also can both add to double bonds and abstract H-atoms, but carbon-centered radicals are not commonly observed to add to DNA bases. However, they cannot be generated near DNA in vivo. In contrast, hydroxyl radical generating systems appear to complex with DNA and produce the hydroxyl radical in the immediate vicinity of the DNA, producing a type of DNA damage that is called site specific. Thus, addition of a radical to a DNA base may require all three features possessed by the hydroxyl radical: high electrophilicity, high thermokinetic reactivity, and a mechanism for production near DNA.

Autoxidation of polyunsaturated fatty acids. Part I. Effect of ozone on the autoxidation of neat methyl linoleate and methyl linolenate

Neat samples of polyunsaturated fatty acids were exposed to ozone in air in a flow system, and the formation of peroxides, conjugated dienes and thiobarbituric acid (TBA)-reactive material was followed as a function of time. The effect of ozone is to shorten the induction period normally observed in autoxidation studies, but the ozone, at the concentrations used here (0-1.5 ppm), appears to have no effect on the rates of product formation after the induction period. During the induction period, increasing ozone concentrations give rise to substantially increased rates of peroxide (or materials which titrate like peroxide) formation, a slightly increased rate of conjugated diene formation, and no significant increase in the rate of production of TBA-reactive material. Vitamin E lengthens the induction period but appears to have no other effect. Some of these data are in conflict with earlier reports of Menzel et al.