Reduction of ATPase activity accompanied by photodecomposition of ergosterol by near-UV irradiation in plasma membranes prepared from Saccharomyces cerevisiae

When plasma membranes prepared from the yeast Saccharomyces cerevisiae were exposed to near-UV radiation, photodecomposition of ergosterol and reduction of ATPase activity occurred simultaneously. The V-max for ATPase activity decreased markedly with increasing near-UV dosage while the K-m 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 liposomes; 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% 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.