THE FUNCTION OF ASCORBIC-ACID IN PHOTOSYNTHETIC PHOSPHORYLATION

Ascorbate is oxidized to the free radical monodehydroascorbate by O-2( .-) and by H2O2 (through the action of ascorbate peroxidase) formed in the Mehler reaction by isolated spinach (Spinacia oleracea) thylakoid s. Light-dependent electron transport from water to monodehydroascorba te is shown to be coupled to ATP formation with a ratio ATP:O-2 of 2. In the presence of ascorbate the net O-2 exchange balance of the Mehle r reaction is close to zero, and the synthesis of ATP is increased 2 t o 3 times due to the extra electron transport to the monodehydroascorb ate free radical. A scheme of the electron transport in the presence o f ascorbate is discussed.