Organic free radicals and proteins in biochemical injury: electron- or hydrogen-transfer reactions?

The reactions of organic free radicals, acting as either reductants or oxidants, have been studied by pulse radiolysis in neutral aqueous solution at room temperature. Manyhydroxyl-substituted aliphatic carbon-centred radicals and one-electron adducts have been shown to be good one-electron reductants, while several oxygen-, sulphur- and nitrogen- (but not carbon-) centred free radicals have been shown to be good one-electron oxidants. Several carbon-centred radicals can be reduced rapidly by hydrogen transfer, from undissociated thiol compounds which can thus act as catalysts facilitating the overall reduction of a carbon-centred radical by an electron-donating molecule. Kinetic considerations influenced by the one-electron redox potentials of the radical-molecule couples involved, determine whether a particular reaction predominates. In this paper examples of such reactions, involving a water-soluble derivative of vitamin E (Trolox C) and the coenzyme NADH, are described, together with studies showing (a) that even in complex multi-solute systems some organic peroxy radicals can inactiviate alcohol dehydrogenase under conditions where the superoxide radical does not, and (b) the superoxide radical can be damaging if urate is also present, and this damage can be reduced by the presence of superoxide dismutase.

Peroxy free radicals, enzymes and radiation damage: sensitisation by oxygen and protection by superoxide dismutase and antioxidants.

In vitro studies have shown that in the presence of the DNA base thymine, the enzyme alcohol dehydrogenase can be extensively damaged by exposure to only 1 gray of cobalt-60 gamma radiation. When irradiated solutions are purged with oxygen-free nitrogen or contain the enzyme superoxide dismutase or various antioxidants, the extent of inactivation of the dehydrogenase is considerably reduced. Peroxy free radicals are considered to be responsible for the inactivation and cysteine and methionine residues are considered the most likely sites of initial damage. The results presented, with those obtained with ADP and urate instead of thymine and with various amino acids and foreign compounds, are discussed in the light of previous statements concerning the relevance of enzyme damage in radiobiology and the role of oxygen free radicals in tissue injury generally.

Uric acid substantially enhances the free radical-induced inactivation of alcohol dehydrogenase.

Lactate dehydrogenase (LDH) and yeast alcohol dehydrogenase ( YADH ) are inactivated when attacked by hydroxy free radicals (OH). Organic molecules with a high rate constant of reaction with OH such as ascorbate or urate can compete with the enzymes for these strongly oxidising radicals. However, although 10(-3)M ascorbate can substantially protect both LDH and YADH from OH attack, in the presence of 10(-3)M urate only LDH is protected. In the case of YADH an even greater degree of inactivation than with OH occurs. The extent of inactivation is considerably reduced when oxygen is absent, in agreement with a urate peroxy radical perhaps being partly responsible for the increased inactivation of the enzyme.