Carrier-mediated transport of metabolites in purified bean mitochondria

The mechanism of transport of Krebs cycle intermediates, phosphate and sulfurcontaining compounds across the membrane of purified bean mitochondria was investigated by directly measuring die exchange between intramitochondrial labelled substrates and external anions and by testing die inhibitor sensitivity of diese transport processes. The exchange between intramitochondrial phosphate and external phosphate or sulfite is insensitive to N-ediylmaleimide or butylmalonate when either is added alone, but is completely inhibited by N-ethylmaleimide plus butylmalonate or by mersalyl. Internal phosphate is exchanged with malate, succinate, oxaloacetate, sulfate and thiosulfate; these reactions are inhibited by butylmalonate but not affected by N-ethylmaleimide. Internal sulfate is exchanged with malate, malonate, succinate, phosphate and sulfite in a butylmalonate- and mersalyl-sensitive reaction. Also the exchanges of malonate with phosphate, sulfate and sulfite are inhibited by butylmalonate and mersalyl. On the other hand, the exchange between intra- and extramitochondrial malonate is completely inhibited only by the combination of butylmalonate and 1,2,3-benzenetricarboxylate. Citrate is exchanged with some di- and tricarboxylates and phosphoenolpyruvate (but not with phosphate, sulfate, oxoglutarate, trans-aconitate and benzenetricarboxylates). These exchanges are inhibited by 1,2,3-benzenetricarboxylate, but not by 1,2,4-benzenetricarboxylate or 1,3,5-pentanetricarboxylate. Oxoglutarate is exchanged with succinate, malate, malonate and oxaloacetate (but not with phosphate, citrate or phosphoenolpyruvate) in a mersalyl-insensitive, butylmalonate- and phenylsuccinate-sensitive reaction. We concluded that bean mitochondria contain the following transport systems: a phosphate carrier inhibited by N-ethylmaleimide or mersalyl, a dicarboxylate carrier inhibited by butylmalonate or mersalyl, a citrate carrier inhibited by 1,2,3-benzenetricarboxylate and an oxoglutarate carrier inhibited by phenylsuccinate or butylmalonate but insensitive to mersalyl.