Uptake of ATP analogs by isolated pea chloroplasts and their effect on CO2 fixation and electron transport

Abstract

1. The ATP analog, adenylyl-imidodiphosphate rapidly inhibited CO2-dependent oxygen evolution by isolated pea chloroplasts. Both alpha, beta- and beta, gamma-methylene adenosine triphosphate also inhibited oxygen evolution. The inhibition was relieved by ATP but only partially relieved by 3-phosphoglycerate. Oxygen evolution with 3-phosphoglycerate as substrate was inhibited by adenylyl-imidodiphosphate to a lesser extent than CO2-dependent oxygen evolution. The concentration of adenylylimidodiphosphate required for 50% inhibition of CO2-dependent oxygen evolution was 50 micronM. 2. Although non-cyclic photophosphorylation by broken chloroplasts was not significantly affected by adenylyl-imidodiphosphate, electron transport in the absence of ADP was inhibited by adenylyl-imidodiphosphate to the same extent as by ATP, suggesting binding of the ATP analog to the coupling factor of phosphorylation. 3. The endogenous adenine nucleotides of a chloroplast suspension were labelled by incubation with [14C]ATP and subsequent washing. Addition of adenylyl-imidodiphosphate to the labelled chloroplasts resulted in a rapid efflux of adenine nucleotides suggesting that the ATP analog was transported into the chloroplasts via the adenine nucleotide translocator. 4. It was concluded that uptake of ATP analogs in exchange for endogenous adenine nucleotides decreased the internal ATP concentration and thus inhibited CO2 fixation. Oxygen evolution was inhibited to a lesser extent in spinach chloroplasts which apparently have lower rates of adenine nucleotide transport than pea chloroplasts.