Biodegradation Of Nitrate Esters By Fungi

Substituted nitrate esters have a fairly low aqueous solubility. Glycerol tri-nitrate saturates at 1.5 g/l, whilst fully substituted nitro-cellulose is completely insoluble in water (Williams and Bruce 2000). Although these compounds are relatively stable, there is some evidence that in the natural environment they can be reductively transformed, forming alcohols and nitrates (Williams and Bruce 2000). There is a dearth of published literature on fungal biodegradation of nitrate esters and the possible pathways by which such degradation could occur. The few fungi reported as having some degradative effects on nitrate esters are distributed across the mitosporic fungi and wood rotting Basidiomycete genera. Geotrichium candidum was shown to have denitration capability, generating glycerol dinitrate and glycerol mono-nitrate with glycerol-2 mononitrate the predominant product (Ducroq et al. 1990). This work demonstrated a regio-specificity in the de-nitration, and they suggested that different enzymes were involved in the de-nitration reactions. P. chrysogenum appears also to have the ability to denitrate nitrate esters and in this Basidio-mycete, the enzymes responsible for these reactions appear to be glutathione-S-transferases (GST) (Servent et al. 1991; 1992). Glutathione-S-transferase enzyme activity has been located in the cytosol. Furthermore, these workers also identified a cytosolic and microsomal P450-like enzyme with similar activity. There are therefore at least two different classes of enzymes involved in the de-nitration of GTN, an oxygen insensitive glutathione-dependent enzyme that liberates nitrate, and an oxygen-sensitive NADPred dependent P450-like activity that liberates nitrous oxide. In all cases of fungal degradation of nitrate esters, additional carbon sources have had to be supplied, and degradation is only partial. Even when a cellulolytic species is combined in co-culture with a de-nitrating species (Sclerotium rolfsii plus Fusarium solani) decomposition is incomplete (Sharma et al. 1995; Sundaram et al. 1995).

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