Electrochromic effects in relation to energy transduction and energy coupling in chloroplast membranes

A study was made on the kinetics of the flash-induced P515 electrochromic bandshift signal in spinach leaves and isolated chloroplasts. It was found that part of the signal (i.e. the slow component, also called reaction 2), normally present in dark-adapted membranes is absent from the signal under conditions where the membrane is energized preceding the actinic light flash. This energization can be brought about either by light-driven electron transport or by reverse electron flow caused by ATP hydrolysis. The activation of the chloroplast ATPase was found to be dependent on endogeneous factors determining ΔGATP such as of the adenylate kinase. The acceleration of the overall decay of the flash-induced P515 response found after short periods of illumination was found to be independent of the H<font size="-1">+</font>- permeability of the membrane as has been suggested by others. It is shown that this acceleration can be fully explained by the suppression of the reaction 2 component as a consequence of membrane energization by an activated ATPase. The occurrence of reaction 2 appeared to be dependent on the functional integrity of the membrane and a possible correlation between factors determining the structural organization of the thylakoid and the kinetics of the P515 response was found. It is concluded that the reaction 2 component of the P515 response is the reflection of an intramembranal electrical event, presumably associated with the liberation and subsequent stabilization of protons in inner-membrane domains. It appears that this stabilizing ability is lost upon the addition of a lipophilic protonophore.