J. Basic Microbiol. 42 (2002) 6, 408– 419 © 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0233-111X/02/0612-0408 $ 17.50+.50/0 (Department of General and Environmental Microbiology 1 , Central Research Laboratory 3 , University of Pécs, Hungary; Department of Microbiology and Biotechnology 2 , University of Debrecen, Debrecen, Hungary) Chromate sensitivity in fission yeast is caused by increased glutathione reductase activity and peroxide overproduction MIKLÓS PESTI 1 *, ZOLTÁN GAZDAG 1 , TAMÁS EMRI 2 , NELLI FARKAS 3 , ZSUZSA KOÓSZ 1 , JÓZSEF BELÁGYI 3 and ISTVÁN PÓCSI 2 (Received 17 May 2002/Accepted 17 June 2002) The Cr(VI)-sensitive mutant chr-51S of the Schizosaccharomyces pombe accumulated chromate (CrO 4 2– ) and reduced Cr(V) to much greater extent, than did its parental strain 6 chr + . Sublethal doses of K 2 Cr 2 O 7 did not induce any adaptive stress response, while H 2 O 2 or menadione pretreatment proved protective against the cell injuries caused by Cr(VI). The intracellular GSH concentration in chr-51S cells was approximately half of that for the 6 chr + . Moreover, the glutathione disulfide reducing capacity of chr-51S was characterized by significantly increased glutathione reductase (GR) and glucose-6-phosphate dehydrogenase activities. These data strongly suggested that, instead of GSH, the NADPH/GR system was the major one-electron Cr(VI) reductant in vivo. The increased Cr(V) reduction in chr-51S mutant was accompanied by high intracellular superoxide and peroxide concentrations, required for formation of the hydroxyl radical ( • OH). The decreased intracellular GSH levels and the Cr(VI)-sensitive phenotype of the chr-51S cells indicates that GSH might act effectively against chromate by scavenging • OH. Chromate [Cr(VI)] compounds are known to be mutagenic, cytotoxic and carcinogenic (COHEN et al. 1993, KLEIN et al. 1998, SHI et al. 1999, SUGDEN et al. 2000, CHENG et al.). When Cr(VI) enters a cell, it undergoes several processes: (i) reduction of Cr(VI), (ii) reac- tion with reactive oxygen species (ROS) that results in formation of the harmful hydroxyl radical ( • OH), and (iii) neutralization by cellular constituents, including detoxifying enzymes and antioxidants (HALLIWELL and GUTTERIDGE 1999). The yeast cell membrane is regarded as the first target site of the chromate anion (CrO 4 2– ). This interaction results in an increased membrane fluidity, with concomitant formation of Cr(V) (BELÁGYI et al. 1999). It is noteworthy that Cr(III), which cannot cross the cell mem- brane at all, also increases the fluidity of the membrane substantially and consequently is deleterious for yeast cells (PESTI et al. 2000). Cr(VI) enters the cell through non-specific sulfate transporters by facilitated diffusion (WIEGAND et al. 1985). The Cr(VI) tolerance of yeasts seems to be proportional to their ability to take up sulfate from the environment (PEPI and BALDI 1992, SMITH et al. 1995). The Cr(VI) gradient between the two sides of the cell membrane is maintained by the metabolically active cells themselves, which continuously reduce the accumulated Cr(VI) to lower oxidation states in both enzymatic processes (involving flavoenzymes) and non- enzymatic processes (glutathione (GSH), NADPH and ascorbate) (SHI et al. 1999, ARSLAN et al. 1987). Among the low molecular mass reductants, GSH is widespread in yeasts (PENNINCKX and ELSKENS 1993) and seems to be the most important agent participating in * Corresponding author: Prof. Dr. PESTI; e-mail: micro@ttk.pte.hu