Extra-cellular chromate-reducing activity of the yeast cultures

This paper reports on the experimental data supporting an essential role of extra-cellular reduction in chromate detoxification by baker’s and non-conventional yeasts. A decrease of chromate content in the yeast culture coincides with an increase of Cr(III) content in extra-cellular liquid. At these conditions, cell-bound chromium level was insignificant and a dominant part of extra-cellular Cr(III) species was detected in the reaction with chromazurol S only after mineralization of the cell-free samples. This phenomenon of chromium “disappearance” can be explained by the formation of Cr(III) stable complexes with extra-cellular yeast-secreted components which are “inaccessible” in the reaction with chromazurol S without mineralization. It was shown that increasing sucrose concentration in a growth medium resulted in an increase of chromate reduction. A strong inhibition of chromate reduction by 0.25 mM sodium azide, a respiration inhibitor and a protonophore, testifies that extra-cellular chromate detoxification depends on energetic status of the yeast cells. It was shown that Cr(III)-biochelates produced in extra-cellular medium are of a different chemical nature and can be separated into at least two components by ion-exchange chromatography on anionit Dowex 1x10. A total yield of the isolated Cr(III)-biocomplexes is approximately 65 % (from initial level of chromate) with a relative molar ratio 8:5.