Effects of singlet oxygen on membrane sterols in the yeast Saccharomyces cerevisiae

• Published in: European journal of biochemistry Vol. 267; no. 6; pp. 1607 - 1618
• Main Authors: Bocking, T, Barrow, K.D, Netting, A.G, Chilcott, T.C, Coster, H.G.L, Hofer, M
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01.03.2000
• Subjects: Cell Membrane - chemistry; Cell Membrane - drug effects; Chromatography, High Pressure Liquid; ergosterol; Ergosterol - analogs & derivatives; Ergosterol - biosynthesis; Ergosterol - metabolism; Light; Membrane Lipids - metabolism; Oxidative Stress; oxygen; Oxygen - pharmacology; Photochemistry; plant biochemistry; plant physiology; plasma membrane; Saccharomyces cerevisiae; Saccharomyces cerevisiae - drug effects; Saccharomyces cerevisiae - metabolism; Saccharomyces cerevisiae - radiation effects; Singlet Oxygen; Steroid Isomerases - metabolism; sterol oxidation; sterols; Tolonium Chloride - pharmacology; Tolonium Chloride - radiation effects
• ISSN: 0014-2956; 1432-1033
• DOI: 10.1046/j.1432-1327.2000.01179.x

Photodynamic treatment of the yeast Saccharomyces cerevisiae with the singlet oxygen sensitizer toluidine blue and visible light leads to rapid oxidation of ergosterol and accumulation of oxidized ergosterol derivatives in the plasma membrane. The predominant oxidation product accumulated was identified as 5α,6α‐epoxy‐(22E)‐ergosta‐8,22‐dien‐3β,7α‐diol (8‐DED). 9(11)‐dehydroergosterol (DHE) was identified as a minor oxidation product. In heat inactivated cells ergosterol is photooxidized to ergosterol epidioxide (EEP) and DHE. Disrupted cell preparations of S. cerevisiae convert EEP to 8‐DED, and this activity is abolished in a boiled control indicating the presence of a membrane associated enzyme with an EEP isomerase activity. Yeast selectively mobilizes ergosterol from the intracellular sterol ester pool to replenish the level of free ergosterol in the plasma membrane during singlet oxygen oxidation. The following reaction pathway is proposed: singlet oxygen‐mediated oxidation of ergosterol leads to mainly the formation of EEP, which is enzymatically rearranged to 8‐DED. Ergosterol 7‐hydroperoxide, a known minor product of the reaction of singlet oxygen with ergosterol, is formed at a much lower rate and decomposes to give DHE. Changes of physical properties of the plasma membrane are induced by depletion of ergosterol and accumulation of polar derivatives. Subsequent permeation of photosensitizer through the plasma membrane into the cell leads to events including impairment of mitochondrial function and cell inactivation.