Oxaloacetate transport into plant mitochondria

The properties of oxaloacetate (OA) transport into mitochondria from potato (Solanum tuberosum) tuber and pea (Pisum sativum) leaves were studied by m easuring the uptake of C-14-labeled OA into liposomes with incorporated mit ochondrial membrane proteins preloaded with various dicarboxylates or citra te. OA was found to be transported in an obligatory counterexchange with ma late, 2-oxoglutarate, succinate, citrate, or aspartate. Phtalonate inhibite d all of these countertransports. OA-malate countertransport was inhibited by 4,4'-dithiocyanostilbene-2,2'-disulfonate and pyridoxal phosphate, and a lso by p-chloromercuribenzene sulfonate and mersalyl, indicating that a lys ine and a cysteine residue of the translocator protein are involved in the transport. From these and other inhibition studies, we concluded that plant mitochondria contain an OA translocator that differs from all other known mitochondrial translocators. Major functions of this translocator are the e xport of reducing equivalents from the mitochondria via the malate-OA shutt le and the export of citrate via the citrate-OA shuttle. In the cytosol, ci trate can then be converted either into 2-oxoglutarate for use as a carbon skeleton for nitrate assimilation or into acetyl-coenzyme A for use as a pr ecursor for fatty acid elongation or isoprenoid biosynthesis.