Benzene 1,2,3-tricarboxylic acid was found to be a sufficiently potent inhibitor of citrate transport by rat liver mitochondria that it may be used in “inhibitor stop” experiments. It was thus possible to measure the initial rates of exchange reactions catalyzed by the citrate-transporting system and to study the dependence of the rate on temperature and the concentrations of citrate, malate, and Mg2+. Both the rate and extent of exchange of L-malate with intramitochondrial [14C]citrate were less than that of citrate with intramitochondrial [Wlcitrate. This difference in the rate and extent between the two exchanges was thought to be due to the fact that L-malate-[14C]citrate exchange resulted in a disequilibrium of either charge or pH across the mitochondrial membrane. It is proposed that malate2exchanges for citrate2rather than citrate3-, thereby setting up a pH differential which restricts further exchange.
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