Antioxidant systems in the pathogenic fungi of man and their role in virulence

In the last two decades, a variety of fungal antioxidants have attracted considerable interest, largely arising from their hypothetical role as virulence determinants. Melanin is a potent free radical scavenger and in Cryptococcus neoformans, there is now good evidence that the production of melanin is a significant virulence determinant. There is also recent evidence linking melanin biosynthesis to the virulence of Aspergillus fumigatus conidia. Superoxide dismutases are important housekeeping antioxidants and have an additional hypothetical role in virulence; however, although these enzymes have been biochemically characterized from Aspergillus and Cryptococcus, there is as yet no firm evidence that these enzymes are involved in pathogenicity. Catalase production may play some role in the virulence of Candida albicans but this enzyme has not been shown, as yet, to influence the virulence of A. fumigatus. There are some data supporting an antioxidant function for the acyclic hexitol mannitol in C. neoformans, but further investigations are required in this area. Research into the putative antioxidant activities of a range of other fungal enzymes, such as acid phosphatases, remains limited at this time.