EFFECTS OF GAMMA-IRRADIATION ON THE ACTIVITY OF TONOPLAST H-ATPASE FROM POTATO-TUBERS (SOLANUM-TUBEROSUM L)()

Tonoplast vesicles were prepared from potato tubers (Solanum tuberosum L.) on a step gradient (0% and 6%, w/w) of dextran T-70 to clarify th e mechanism by which the tonoplast H+-ATPase is inactivated by gamma-i rradiation. H+-ATPase activity and H+-pumping were examined after irra diation of tubers (in vivo irradiation) and of isolated tonoplast vesi cles (in vitro irradiation) at doses up to 1.0 kGy. Both in vivo irrad iation and in vitro irradiation resulted in significant decreases in A TPase and H+-pumping activities. The ATPase and H+-pumping activities 12 h after irradiation were much lower than those 2 h after irradiatio n. Solubilized H+-ATPase was inactivated, in a dose-dependent manner, by irradiation (enzyme irradiation) to a greater extent than was obser ved after in vitro irradiation or in vivo irradiation. The activity of ATPase 12 h after enzyme irradiation was almost the same as it was 2 h after enzyme irradiation. The free fatty acid content of vacuolar me mbranes was increased by in vivo irradiation and by in vitro irradiati on with an accompanying decrease in tonoplast H+-ATPase activity. Lipi ds from irradiated tonoplasts had a considerable inhibitory effect on the activity of solubilized H+-ATPase. This result suggests that the d irect inactivation of H+-ATPase in potato tonoplast by gamma-irradiati on is augmented by the effects of deterioration of membrane lipids tha t is induced by the irradiation.