Stimulation of citrate oxidation and transport in human placental mitochondria by L-malate.

1. Citrate was oxidized by human placental mitochondria at a rate half that for isocitrate, and cis-aconitate at a rate by 20% lower as with isocitrate. The apparent Km values for these substrates were similar. 2. Oxidation of citrate was stimulated by L-malate but the stimulation was abolished by benzene 1,2,3-tricarboxylate and butylmalonate. 3. Citrate uptake by placental mitochondria was inhibited by benzene 1,2,3--tricarboxylate, and raised specifically by L-malate. Stimulation by fumarate and succinate was found to be due to conversion of these substrates to L-malate. 4. It is concluded that human placental mitochondria contain malate-stimulated tricarboxylate carrier system.