Citrate Transport in Corn Mitochondria

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Abstract
Citrate uptake by corn mitochondria (Zea mays L. B73 × Mol9) was investigated by osmotic swelling and [14C]citrate accumulation. Uptake driven by passive influx, ammonium gradients, and respiration was followed. There was no requirement for phosphate and/or malate to secure citrate uptake, although under some conditions these additives were promotive. Inhibition of the phosphate and dicarboxylate carriers did not eliminate citrate uptake. Citratein/malateout exchange occurs, but at a rate too slow to account for observed citrate uptake, and depletion of endogenous malate only reduced citrate uptake by 38%. It was concluded that citrate can be rapidly accumulated by a mechanism other than by exchange for dicarboxylates. The effect of uncoupler on respiration-driven [14C]citrate accumulation, and studies of passive swelling using ionophores and uncouplers indicated that the major avenue of citrate uptake is by H+/citrate co-transport with a pH optimum near 4.5. The in vivo role of this mechanism is not yet understood.