Abstract
For the three Gram-negative bacteria, Pseudomonas fluorescens, Escherichia coli, and Erwinia amylovora, p-benzoquinone was the principal bactericidal agent formed in vitro during the oxidation of hydroquinone by horseradish peroxidase, whereas no toxicity could be associated with either phenolic or oxygen-free radicals. Even the continuous generation of p-benzosemiquinone during the simultaneous reduction of p-benzoquinone by xanthine oxidase and reoxidation of hydroquinone by peroxidase was no more toxic than p-benzoquinone alone. Anaerobiosis had no effect on the toxicity of either p-benzoquinone or the peroxidase reaction and the generation of superoxide and hydroxyl radicals catalyzed by xanthine oxidase was not bactericidal. Substitutions on the p-benzoquinone ring decreased quinone toxicity in rough proportion to the decrease in quinone redox potential, suggesting that strong oxidizing potentials are important for such quinone toxicity.
Highlights
For the threeGram-negative bacteriaP, s e ~ ~ oanio~n, wohic~h rapidly dismutes to yield p-benzoquinone and fluoreseeras,~ s e ~ e r icoeli~, aind~Erwinia u ~ ~ louor uh,ydroq~none
If xanthine and xanthine oxidase are added to a mixture of pbenzoquinone, peroxidase?and hydrogen peroxide, semiquinone can be generated by a cycle involving reduction of pbenzoquinone by xanthine oxidase and oxidation of hydroquinoneby peroxidase.Continuousgenerationof semiquinone did not increase the toxicity of p-benzoquinone, methyl-pbenzoq~noneo, r 2~6-dirnethyl-p-~nzo~ino(Fnige
Earlier reports that found roughly equivalent toxicity of hydroquinone when compared with p-benzoquinone in bactericidal assays [12, 23] did notcontrol for the autooxidation of hydroquinone, which readily occurs in thepresence of trace metals [24].The use of buffers having low metal contamination (Hepes) or of metal chelators in phosphatebuffer drastically reduces the toxicity of hydroquinone in thepresence of hydrogen peroxide
Summary
0 1985by The American Society of BioIogid Chemists, Inc, Voi. 260, No 27,Issue of November, pp. 14604-14609,1985 printed hU..A. Anaerobiosis had nefofect on the toxicity of either p-benzoquinone or the peroxidase cem, a Gram-negative soil and water saprophyte that has been widely used as a comparison organismin planpt athology due to its taxonomic relationship with a broad range of plant pathogenicpseudomonads [11].Experiments were carried out using Escherichia coli and Erwinia amylovora. The latter bacterium causes fire blight in pears, and hydroquinone and reaction and the generation of superoxide and hy- its @-glucoside,arbutin, are implicated in the resistance of droxyl radicals catalyzed bxyanthine oxidase was not pear trees to infection by Er. amylovora [12].
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