IN-VITRO INACTIVATION OF YEAST GLUTATHION E-REDUCTASE BY TETRAMETHYLTHIURAM DISULFIDE

Previous in vivo investigations have shown that glutathione reductase is one of the sites of action of the dithiocarbamate fungicide tetrame thylthiuram disulphide (thiram) in the yeast Saccharomyces cerevisiae. The inactivation of glutathione reductase by thiram has now been demo nstrated in vitro. This inactivation was time-dependent and occurred o nly with the enzyme in the reduced state and in the absence of glutath ione. Since the turnover rate of the enzyme with thiram as a substrate was significantly higher than the rate of enzyme inactivation, it was suggested that more than one enzyme-inhibitor complex was involved in the reaction. Arguments supporting a covalent modification of glutath ione reductase were further obtained by experiments carried out with [ C-14]thiram and gel filtration. A kinetic scheme for the inactivation is proposed and the relevance of the in vitro data to previous in vivo studies is discussed taking into consideration current concepts of gl utathione reductase inactivation by affinity reagents.