On the active species in bicarbonate stimulation of Hill reaction in thylakoid membranes

At 5 degree C addition of CO2 or HCO3- to CO (HCO3-)-depleted thylakoids (containing 100 mM formate) initiates, within 10 s, the activation of the Hill reaction in light. In contrast to HCO3- addition, where there is a lag of 6-8 s, the activation by CO2 addition is almost instantaneous. With CO2, prior addition of carbonic anhydrase produces a lag of about 6 s that approaches the lag observed by the HCO3- addition. These data suggest that CO2 is an active species involved in stimulating the Hill reaction. Binding of CO2 (HCO3-) to a component on the external side of the thylakoid membranes is supported by the present study.