MECHANISMS OF CHROMATE DETOXIFICATION IN YEASTS

Abstract

The main pathway of Cr(VI) (hexavalent chromium) detoxification in microorganisms involves its reduction to Cr(III). Characteristics of chromate reduction in yeasts (isolation, purification and characterization of Cr(III)-biochelated complexes from the culture medium) and the non-enzymatic processes responsible for efficient detoxification of chromate are described. The yeast Saccharomyces cerevisiae was shown to be able to reduce chromate extra-cellularly by the formation of at least two types of stable Cr(III)-biochelated complexes. Selenite-resistant mutants of Phaffia rhodozyma were studied with respect to their ability to reduce Cr(VI). The cells of selenite-resistant mutant sit11 which show strong sensitivity toward Cr(VI), generated an increasing pool of a very toxic radical Cr(V). The role of some extracellular agents such as sulfate and riboflavin in Cr(VI) detoxification in the flavin-overproducing yeast Pichia guilliermondii was also investigated. Our data on modulation of chromate toxicity by riboflavin provide some evidence for the involvement of riboflavin (or its derivatives) in chromate detoxification. The identification of the extracellular chromate-reducing compounds and elucidation of their role in detoxification of chromate will promote the application of the studied yeast cells for Cr(VI) bioremediation and its transformation to Cr(III) bio-complexes with potential pharmaceutical and nutritional importance.