THE ROLE OF MEMBRANE-BOUND PORPHYRIN-TYPE COMPOUND AS ENDOGENOUS SENSITIZER IN PHOTODYNAMIC DAMAGE TO YEAST PLASMA-MEMBRANES

The effect of visible light (400-600 nm) on Candida guilliermondii and Saccharomyces cerevisiae was studied, and irradiation fluences killin g the yeast cells without exogenous sensitizers were determined. The l ethal effects are strongly oxygen-dependent, suggesting the involvemen t of photodynamic reactions mediated by endogenous sensitizer(s). Repa ir-deficient strains of S. cerevisiae show the same photosensitivity a s the wild-type strain indicating that visible light does not photosen sitize repairable DNA lesions. As was demonstrated using the microfluo rometric method with the fluorochrome primulin, photodestruction of pl asma membrane permeability barriers is important for yeast cell lethal ity. Visible light at cell-killing fluences induces lipid peroxidation in plasma membrane ghosts isolated from C. guilliermondii. Data obtai ned suggest the important role of singlet oxygen photogenerated by end ogenous sensitizer(s) in initiating oxidative reactions. A spectrofluo rometric analysis of the plasma membrane ghosts revealed one compound fluorescent in the visible spectral region at 683 nm. Its fluorescence excitation and absorption spectra have structures typical for porphyr ins. The plasma membrane-bound porphyrin like compound is different in some fluorescence properties from mitochondrial porphyrins. Estimatio n of its amount gives a value of 0.1 nmol porphyrin per milligram of p rotein of the plasma membrane ghosts. This porphyrin-type compound is considered to be the most probable candidate for the role of the sensi tizer in photodynamic damage to yeast plasma membrane and cell inactiv ation by visible light.