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

Photodynamic treatment of the yeast Saccharomyces cerevisiae with the singl et oxygen sensitizer toluidine blue and visible light leads to rapid oxidat ion of ergosterol and accumulation of oxidized ergosterol derivatives in th e plasma membrane. The predominant oxidation product accumulated was identi fied as 5 alpha,6 alpha-epoxy-(22E)-ergosta-8,22-dien-3 beta,7 alpha-diol ( 8-DED). 9(11)-dehydroergosterol (DHE) was identified as a minor oxidation p roduct. In heat inactivated cells ergosterol is photooxidized to ergosterol epidioxide (EEP) and DHE. Disrupted cell preparations of S. cerevisiae con vert EEP to 8-DED, and this activity is abolished in a boiled control indic ating the presence of a membrane associated enzyme with an EEP isomerase ac tivity. Yeast selectively mobilizes ergosterol from the intracellular stero l ester pool to replenish the level of free ergosterol in the plasma membra ne during singlet oxygen oxidation. The following reaction pathway is propo sed: singlet oxygen-mediated oxidation of ergosterol leads to mainly the fo rmation of EEP, which is enzymatically rearranged to 8-DED. Ergosterol 7-hy droperoxide, a known minor product of the reaction of singlet oxygen with e rgosterol, is formed at a much lower rate and decomposes to give DHE. Chang es of physical properties of the plasma membrane are induced by depletion o f ergosterol and accumulation of polar derivatives. Subsequent permeation o f photosensitizer through the plasma membrane into the cell leads to events including impairment of mitochondrial function and cell inactivation.