Light-induced Mg2+ ATPase activity of coupling factor in intact chloroplasts.

Intense illumination isolated, intact, spinach chloroplasts triggers the well known proton-pumping Mg2+ ATPase activity of coupling factor, which can be assayed in subsequently lysed chloroplasts by monitoring ATP-driven quenching of 9-aminoacridine fluorescence. The light-triggered ATPase activity decays slowing in the dark and is inhibited by N,N'-dicyclohexylcarbodiimide. After osmotic lysis and washing of the chloroplasts, preillumination no longer triggers maximal proton-pumping ATPase until methylviologen and dithiothreitol are added to the medium. It is suggested that intact organelles contain soluble or loosely bound cofactors necessary for light-triggering of coupling factor ATPase. On osmotic lysis, these endogenous cofactors are diluted or inactivated and must be replaced by addition of a dithiol reagent and an electron acceptor.