A Two-Translocator Model for the Transport of 2-Oxoglutarate and Glutamate in Chloroplasts during Ammonia Assimilation in the Light

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Abstract
This study examines the transport of 2-oxoglutarate (2-OG) and other dicarboxylates during ammonia assimilation in illuminated spinach [Spinacia oleracea] chloroplasts. The transport of all dicarboxylates examined was strongly inhibited by NH4Cl preincubation in the light. Treatment with NH4Cl caused a rapid depletion of the endogenous glutamate pool and a corresponding increase in endogenous glutamine content. The inhibition of transport activity by NH4Cl was apparently linked to its metabolism in the light because inhibition of glutamine synthetase activity by the addition of L-methionine sulfoximine or carbonylcyanide-m-chlorophenylhydrazone abolished this affect. Measurements of endogenous metabolite pools showed that malate was most rapidly exchanged during the uptake of all exogenous dicarboxylates examined. Depending on the exogenous substrates used, the apparent half-times of efflux measured for endogenous malate, aspartate and glutamate were 10, 10 to 30, and 15 to 240 seconds, respectively. The transport of 2-OG was also inhibited by malate. But chloroplasts preincubated with malate in the presence or absence of NH4Cl were found to have high transport activity similar to untreated chloroplasts. A two-translocator model is proposed to explain the stimulation of 2-OG transport as well as the stimulation of (NH3, 2-OG)-dependent O2 evolution by malate (KC Woo, CB Osmond 1982 Plant Physiol. 69: 591-596) in isolated chloroplasts. In this model the transport of 2-OG on the 2-OG translocator and glutamate on the dicarboxylate translocator is coupled to malate counter-exchange in a cascade-like manner. This results in a net 2-OG/glutamate exchange with no net malate transport. Thus, during NH3 assimilation the transport of 2-OG into and the export of glutamate out of the chloroplast occurs via the 2-OG and the dicarboxylate translocators, respectively.