Citrate Transport in Corn Mitochondria

Citrate uptake by corn mitochondria (Zea mays L. B73 x Mol9) was investigated by osmotic swelling and [(14)C]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. Citrate(in)/malate(out) 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 [(14)C]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.