Modeling of alternative pathways of electron transport to photosystem I in isolated thylakoids

Kinetics of dark decay of absorbance changes at 830 nm (DeltaA(830)) was examined in thylakoids isolated from leaves of pea seedlings at various concentrations of exogenous NADPH or NADH. Absorbance changes were induced by far-red light to avoid electron donation from photosystem II. In the presence of either biological reductant, the kinetics of DeltaA(830) decay reflecting dark reduction of P700(+), the primary electron donor of photosystem I, was fitted by a single exponential term. The rate of P700(+) reduction increased with the rise in the concentration of both NADPH and NADH. The values of K-M and V-max for P700(+) reduction estimated from concentration dependences were 105 +/- 21 muM and 0.32/s for NADPH or 21 +/- 8 muM and 0.12/s for NADH. The rate of P700(+) reduction by either NADPH or NADH significantly increased in the presence of rotenone, a specific inhibitor of chloroplast reductase. The value of V-max was changed only in the presence of rotenone, whereas K-m was practically unaffected. Unlike the chloroplasts of intact leaves, the only enzyme mediating the input of reducing equivalents from NADPH or NADH to the electron transport chain was concluded to be present in thylakoids.