BLUE LIGHT INDUCED, REVERSIBLE IN ACTIVATION OF THE TONOPLAST‐TYPE H+‐ATPase FROM CORN COLEOPTILES IN THE PRESENCE OF FLAVINS

Irradiation (λ max 447 nm; 58.5 W m ‐2 ) of a microsomal membrane fraction of corn coleoptiles for 5 min in the presence of the in vivo concentration of riboflavin inactivates the tonoplast‐type H + ‐ATPase. This inhibition is O 2 ‐dependent, is enhanced in D 2 O and suppressed by NaN 3 , indicating participation of singlet molecular oxygen in the inactivating mechanism. Besides singlet oxygen, the superoxide anion (O 2 ‐ ) is generated during irradiation, which obviously has no effect on the H + ‐pumping activity. However, in the presence of superoxide dismutase (SOD), O 2 ‐ is transformed into H 2 O 2 which causes an additional strong inhibition of H + . ATPase activity. This inhibition can be increased by ethylenediaminetetraacetic acid (EDTA), which is known to be an electron donor of the excited flavin molecule. In contrast, catalase prevents the H 2 O 2 ‐mediated photoinactivation of the H + ‐ATPase. The light dependent inactivation of H + ‐transport does not occur if reduced glutathion (GSH) is added prior to or after irradiation. These results indicate that the blue light mediated inhibition of the H + ‐ATPase is mediated by singlet oxygen and H 2 O 2 which oxidize essential SH‐groups of the enzyme into disulfides. Reduction of the formed disulfides by GSH restores the activity of the enzyme.