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 H,O,. Hydrogen peroxide is essential for the ascorbateinduced 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, terf-butyl alcohol or poly(ethy1ene 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 q, ascorbate or other reductants and is subsequently reoxidized by H,O, 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.

Cryptococcus neoformans was unable to utilize catecholamines (epinephrine, norepinephrine, or dopamine) as sole carbon or nitrogen sources. Therefore, catecholamines are not essential growth factors for this fungus and the brain is not a preferred nutritional niche for its growth with regard to catecholamines. To establish whether the brain is a survival niche for C. neoformans and to explain the role of phenoloxidase as a virulence factor, a wild-type strain that had phenoloxidase activity and mutants which lacked it were exposed to an epinephrine oxidative system, and the survival of both strains was tested. The oxidative system contained epinephrine as an electron donor, Fe3+ as the catalytic transition metal ion, and hydrogen peroxide as an electron acceptor. The wild-type strain was found to be resistant to this oxidative system, whereas under the same conditions the mutant strain was susceptible and its survival decreased at a rate of 4 logs per h. Damage to high-molecular-weight DNA seems to be a causative factor of cell death after exposure of the mutants to the oxidative system. These results suggest that C. neoformnans may survive in the brain because of its ability to utilize catecholamines for melanogenesis and thus neutralize the harmful effects of catecholamines which are manifested in the presence of hydrogen peroxide and transition metal ions. The role of phenoloxidase in resistance to the epinephrine oxidative system is also discussed.

The protective effect of cyclic stable nitroxide free radicals, having SOD-like activity, against oxidative damage was studied by using Escherichia coli xthA DNA repair-deficient mutant hypersensitive to H2O2. Oxidative damage induced by H2O2 was assayed by monitoring cell survival. The metal chelator 1,10-phenanthroline (OP), which readily intercalates into DNA, potentiated the H2O2-induced damage. The extent of in vivo DNA scission and degradation was studied and compared with the loss of cell viability. The extent of DNA breakage correlated with cell killing, supporting previous suggestions that DNA is the crucial cellular target of H2O2 cytotoxicity. The xthA cells were protected by catalase but not by superoxide dismutase (SOD). Both five- and six-membered ring nitroxides, having SOD-like activity, protected growing and resting cells from H2O2 toxicity, without lowering H2O2 concentration. To check whether nitroxides protect against O2.(-)-independent injury also, experiments were repeated under hypoxia. These nitroxides also protected hypoxic cells against H2O2, suggesting alternative modes of protection. Since nitroxides were found to reoxidize DNA-bound iron(II), the present results suggest that nitroxides protect by oxidizing reduced transition metals, thus interfering with the Fenton reaction.

No abstract

SUMMARYEseherichia coli strain w adsorbs phage A very efficiently but the phage does not form plaques on this strain. In a very small fraction (lO -4) of the infected cells the phage grows and produces small bursts of progeny phage also unable to form plaques on strain w. E. coli strain w is lysogenic for a temperate phage, w~b, related to phage v 2. Non-restricting hosts for phage a became restricting hosts when made lysogenic for w~b. When ~2P-labelled a adsorbed to restricting wO lysogenic hosts, > 20% of the a~P become acidsoluble shortly after infection. No w~b specific modification was carried by the small number of ~ phages which escaped this restriction process. It is concluded that w~b controls a host-restriction mechanism but not a hostmodification process, and in parallel with other examples of host-controlled restriction and modification can be represented as r+m -or r+m °. ;tw mutants have been isolated which escape this restriction and which form plaques on strain w and w~ lysogenic strains with an efficiency of 1.0. With these mutants a w-specific host modification controlled by the genome of strain w was demonstrated. Mixed infection experiments with restricted ?t and unrestricted • ~w showed that that restricted phage did not block the growth of the unrestricted mutant nor did the mutant permit the restricted phage to grow. In addition it was shown that ~ obtained from bacteria mixedly infected with A and Aw was still unable to grow in restricting hosts and 2tw similarly obtained from mixedly infected bacteria still retained its ability to grow on restricting hosts. It is concluded that there is a nudeotide sequence in the DNA of phage A which, when ~ infects a restricting host, is specifically recognized by the restriction mechanism controlled by the w~b. The mutation to Aw involves an alteration to this sequence such that it is no longer recognized by the restriction mechanism of the w~b.Mutants of w~b were isolated not restrictive for phage A.