When cells of the yeast Sacchammyces cerevisiae were exposed to near-UV (300-400 nm), their absorption spectra changed slightly within the range 220-300 nm with increasing dosage. Difference spectra, calculated by subtracting the curve recorded in cells exposed to near-UV from the curve of unexposed cells, decreased with increasing dosage over a broad band with peaks at 272,282 and 295 nm and a shoulder at 265 nm. These peaks were in agreement with the absorption maxima of ergosterol, which is one of the major components of the plasma membrane of yeast. Near-UV radiation induced a simultaneous decrease in absorption spectra and reduction of ergosterol content in the plasma membrane. Photochemical decomposition of ergosterol by near-UV radiation was revealed in who, although ergosterol is generally known to be photoconverted to previtamin D, industrially by UV radiation in witm. In order to remove photosensitizers, liposomes were prepared from phospholipids and glycolipids, with or without ergosterol from purified yeast plasma membranes. Liposomal ergosterol in the orientated state was photochemically decomposed by near-UV radiation but ergosterol in the disorientated state in a homogeneous solution was not. Near-UV radiation also induced a decrease in activity of membrane-bound ATPase. Dose-response curves for the reduction of ATPase activity were similar to that for decomposition of ergosterol, suggesting that near-UV caused membrane function damage.

When plasma membranes prepared from the yeast Sacchammyces cerevisiae were exposed to near-UV radiation, photodecomposition of ergosterol and reduction of ATPase activity occurred simultaneously. The V-for ATPase activity decreased markedly with increasing near-UV dosage while the K, value remained constant. When ATPase solubilized from the plasma membrane was exposed to near-UV, the activity remained constant irrespective of dosage, indicating that the ATPase molecule itself was not damaged by near-UV irradiation. The relationship between content of ergosterol and ATPase activity was examined using liposomes constructed with lipids extracted from the membrane. Maximum activity of ATPase was seen at 5% ergosterol in Iiposomes; this activity was 2.5 times greater than that in liposomes without ergosterol. Activity of ATPase bound to liposomes with 5% ergosterol was reduced after near-UV irradiation, while the activity remained unchanged in the case of the liposomes without ergosterol. Fluidity of the liposomes with 5 YO ergosterol also decreased with increasing near-UV dosage. Dosage-response curves for reduction of ATPase activity and for decrease in fluidity were similar to that for photodecomposition of ergosterol. These results suggested that the reduction of ATPase activity in the membrane by near-UV irradiation was not caused by photochemical degradation of the primary structure of the ATPase molecule, but was attributable to conformational change resulting from an alteration in the higher-order structure of the membrane due to photochemical decomposition of ergosterol.

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