On the cytotoxicity of vitamin C and metal ions: A site‐specific Fenton mechanism

The toxicity of ascorbate towards phage lambda and the phages T2–T7 has been investigated. At room temperature the T‐odd and lambda bacteriophages are highly susceptible to ascorbate‐induced damage, whereas the T‐even phages are practically resistant. The toxicity of ascorbate is dependent on the presence of copper (or iron) and oxygen, although oxygen is not required in the presence of H₂O₂. Hydrogen peroxide is essential for the ascorbate‐induced phage inactivation and the damage is prevented by catalase. At the concentrations used, most of the copper ions are bound to the phage particles. Chelating agents such as EDTA or histidine fully protect the phages, whereas salicylate only reduces the rate of phage inactivation. OH scavengers such as sucrose, formate, mannitol, tert‐butyl alcohol or poly(ethylene glycol) have no protective effect. Experiments with DNA labeled phages indicate that both phage adsorption and DNA injection are impaired as a result of the exposure to ascorbate and copper. The failure to express the viral genetic information as a result of single and double‐strand breaks in the DNA, probably also contribute to the loss of the plaque‐forming ability of the phages. The results are interpreted in terms of a ‘site‐specific’ Fenton mechanism according to which the binding of the transition metal ions to the biological target is a prerequisite for the production of damage. The bound metal ion is reduced either by O⁻₂, ascorbate or other reductants and is subsequently reoxidized by H₂O₂ yielding OH^˙ radicals. This cyclic redox reaction of the metal generates OH^˙ radicals which react with vital macromolecules with a high probability of causing ‘multi‐hit’ damage. This ‘site‐specific’ formation of OH^˙ radicals, which takes place near the target molecules, accounts both for the high damaging efficiency and for the failure of OH^˙ scavengers to protect against it.