Transient stimulation of light-triggered ATP hydrolysis by preillumination of chloroplasts in the presence of ATP

Abstract

Chloroplasts which were preilluminated in the presence of dithiothreitol and ATP + Mg 2+ show a 1.5–5-fold higher initial velocity of ATP hydrolysis in the subsequent dark than chloroplasts which were preilluminated in the absence of ATP. The half maximal time of ATP preincubation is about 1 min, the half maximal ATP concentration 15 μM. Nucleotide specificity with regard to the diastereomers of adenosine 5′-( O-1-thiotriphosphate) is the same as in nucleotide binding to CF 1 and as the substrate specificity in ATP hydrolysis. ADP abolishes the stimulating effect of ATP if simulaneously present in the light. ATP-stimulated ATP hydrolysis generates a lower transmembrane ΔpH than normal ATP hydrolysis and the relaxation of the flash-induced electrochromic absorption change is accelerated by ATP-preincubation in the light. These results suggest that the ATP pretreatment causes an increase of thylakoid membrane permeability. The target for ATP is most likely the ATPase itself. Dithiothreitol activation in the presence of ATP might induce a reversible dislocation of CF 1 relative to CF 0 resulting in transitory membrane leakage. Reversibility is demonstrated by the finding that the stimulated rate of ATP hydrolysis is normalized after about 1 min. Moreover ATP P i exchange which exhibits a lag under these conditions recovers up to the control rate after 1 min.