The effects of altered membrane fatty acid composition on the toxic interactions of heavy metals with Saccharomyces cerevisiae.

The effects of altered membrane fatty acid composition on the toxic interactions of heavy metals with Saccharomyces cerevisiae were examined. Saccharomyces cerevisiae was enriched with the polyunsaturated fatty acids (PUFAs) linoleate (18:2) and linolenate (18:3) by growth in 18:2or 18:3-supplemented medium. Incorporation of the exogenous PUF As resulted in them comprising greater than 65% and 40% of the total fatty acids in whole-cell and plasma membrane lipids, and nuclear membrane lipids, respectively. Incorporation of the exogenous PUF As had no discernible adverse effects on cell division. However, inhibition of cell division in the presence of Cd(N03)2 was accentuated by growth in the presence of the di-unsaturated fatty acid linoleate. Furthermore, susceptibility to both Cd2+ and Cu2+ -induced plasma membrane permeabilisation and whole cell toxicity was markedly accentuated in PUF A-enriched cells, and increased with the degree of fatty acid unsaturation. The increased sensitivity ofPUFA-enriched cells to membrane permeabilisation and whole-cell toxicity was correlated with increased levels of lipid peroxidation in these cells. Cu2+ and Cd2+_ induced lipid peroxidation was rapid and associated with a decline in plasma membrane lipid order, detected by fluorescence depolarization measurements. Levels of the lipid peroxidation products thiobarbituric acid-reactive substances (TBARS) and conjugated dienes were markedly higher in PUF A-enriched cells, compared with unsupplemented cells, following exposure to cadmium or copper. Thus, lipid peroxidation was demonstrated as a major means of heavy metal toxicity in a microorganism for the first time. In addition, the effects ofPUFA-enrichment on the interactions of heavy metals with cellular nucleic acids were examined. Exposure ofPUFA-enriched cells to the redox-active metals chromium and copper resulted in the uncoupling of DNA synthesis from cell division, leading to sequential S phases. For example, DNA levels of up to 8C were evident in 18:3-enriched cells after only 4.5 h exposure to 100 JJ.M Cu(N03h. Using flow cytometry, the heterogeneity in susceptibility to copper toxicity of exponential phase S. cerevisiae was also examined. Susceptibility towards copper toxicity was demonstrated to be cell cycle stage-dependent, whereby G2/M phase cells were found to be the most susceptible towards copper toxicity. Staining with the oxidantsensitive probe 2',7' -dichlorodihydrofluorescein diacetate (H2DCFDA) revealed that the greater copper sensitivity of G2/M phase cells correlated with elevated endogenous levels of reactive oxygen species in these cells. Abstracts, presentations and publicationss, presentations and publications Howlett, N. G. and Avery, S. V. (1995). Relationship between the surface properties of microorganisms and heavy metal uptake and toxicity. Society for General Microbiology 132 Meeting, University of Aberdeen, Scotland, Abstracts p. 102. Howlett, N. G. (1995). Influence of membrane lipid composition on cadmium toxicity towards Saccharomyces cerevisiae (oral presentation). Joint winner of Ph.D. student symposium, Oxford Brookes University, U.K . Howlett, N. G. and Avery, S. V. (1996). Membrane properties determining cadmiuminduced plasma-membrane permeabilisation in Saccharomyces cerevisiae. Society for General Microbiology 135th Meeting, University of Essex, UK. , Abstracts p. 53. Avery, S. V. , Howlett, N. G. , and Radice, S. (1996). Copper toxicity towards Saccharomyces cerevisiae: Dependence on plasma membrane fatty acid composition. Applied and Environmental Microbiology 62, 3960-3966. Howlett, N. G. and Avery, S. V. (1997). Induction of lipid peroxidation during heavy metal stress in Saccharomyces cerevisiae and influence of cellular fatty acid composition. American Society for Microbiology 97 General Meeting, Miami Beach, FLA, US.A. , Program p. 23. Recipient of ASM sustaining member student travel grant award.