Unusual effects of reducing agents on 0-diphenoloxidase of Mycobacterium leprae

Reducing agents had no effect on the oxidation of 3,4-dihydroxyphenylalanine (DOPA) to quinone by Mycobacterium leprae; no quinone formation by o-diphenoloxidase of mammalian or plant origin was detected under similar experimental conditions. Ascorbic acid and reduced glutathione prevented further oxidation and polymerization of the quinone to melanin by M. leprae; cysteine was less effective. In the presence of reducing agents, the quinone (indole-5,6-quinone) formed from DOPA by M. leprae was not reduced back to diphenol. On the other hand, the quinone (dopachrome) produced from DOPA by mammalian or plant phenolase was rapidly decolorized by reducing agents. Oxidized glutathione and cystine had little effect on o-diphenoloxidase from all of the three sources. Cyanide, which completely inhibited mammalian and plant phenolases, had only a partial effect on the enzyme in the bacilli. Various lines of evidence suggest that the properties of o-diphenoloxidase in M. leprae are different from those of similar enzymes obtained from other sources.