Cr(VI) reduction by gluconolactone and hydrogen peroxide, the reaction products of fungal glucose oxidase: Cooperative interaction with organic acids in the biotransformation of Cr(VI)

- Growing cells of strain Ed8 of A. tubingensis reduce Cr(VI) with high efficiency.
- The ability to reduce Cr(VI) and extracellular GOX activity level were correlated.
- Purified GOX from A. niger reduce Cr(VI) in a glucose-dependent reaction.
- Cr(VI) reduction by GOX is stimulated by organic acids, generating hydroxyl radicals.
- Gluconolactone and hydrogen peroxide reduce Cr(VI) in the absence of the GOX enzyme.

The Cr(VI) reducing capability of growing cells of the environmental A. tubingensis Ed8 strain is remarkably efficient compared to reference strains A. niger FGSC322 and A. tubingensis NRRL593. Extracellular glucose oxidase (GOX) activity levels were clearly higher in colonies developed in solid medium and in concentrated extracts of the spent medium of liquid cultures of the Ed8 strain in comparison with the reference strains. In addition, concentrated extracts of the spent medium of A. tubingensis Ed8, but not those of the reference strains, exhibited the ability to reduce Cr(VI). In line with this observation, it was found that A. niger purified GOX is capable of mediating the conversion of Cr(VI) to Cr(III) in a reaction dependent on the presence of glucose that is stimulated by organic acids. Furthermore, it was found that a decrease in Cr(VI) may occur in the absence of the GOX enzyme, as long as the reaction products gluconolactone and hydrogen peroxide are present; this conversion of Cr(VI) is stimulated by organic acids in a reaction that generates hydroxyl radicals, which may involve the formation of an intermediate peroxichromate(V) complex. These findings indicated that fungal glucose oxidase acts an indirect chromate reductase through the formation of Cr(VI) reducing molecules, which interact cooperatively with other fungal metabolites in the biotransformation of Cr(VI).

Fungal glucose oxidase (GOX) acts as an in direct chromate reductase through the formation of Cr(VI) reducing molecules. Higher levels of extracellular GOX activity (A upper) and remarkable Cr reducing capability in growing cultures (A lower) of A. tubingensis Ed8, as compared to reference strains A. niger FGSC322 and A. tubingensis NRRL593. Purified commercial GOX of A. niger shows capability to diminish Cr(VI) levels (B), converting it to Cr(III) (C), and GOX reaction products gluconolactone and hydrogen peroxide act together to reduce Cr(VI) levels in the absence of GOX enzyme (D).