MITOCHONDRIAL LDH CONTENT, LACTATE OXIDATION, AND RESPONSE TO CINNAMATE.

89 Isolated mitochondria readily oxidize pyruvate and other NADH-linked substrates such as glutamate which donate electrons to the Electron Transport Chain via Complex I. Isolated mitochondria also oxidize other substrates, including succinate which donates electrons via Complex II.α-cyano-4-hydroxycinnamate (CHC) is a known inhibitor of some forms of monocarboxylate transport proteins, specifically isoforms which transport pyruvate and lactate. In contrast to the volume of literature describing mitochondrial pyruvate oxidation, few reports describe lactate oxidation. We incubated isolated rat liver and skeletal muscle mitochondria with either 10 mM pyruvate (+ 1.0 mM malate) or 10 mM lactate (+ 1.0 mM malate), with and without 2.5 mM CHC. Mitochondria readily oxidized pyruvate and lactate, with similar State 3 and 4 respiratory rates, respiratory control (State 3/State 4) and ADP/O ratios. Cinnamate blocked the respiratory response to added ADP, but the block was bypassed by addition of succinate or glutamate. Isolated mitochondria were found to contain the heart (H) type isoform of lactic dehydrogenase (LDH). We conclude that the monocarboxylates pyruvate and lactate gain entry into mitochondria by facilitated diffusion through a CHC-sensitive transporter. Mitochondria can oxidize exogenous lactate because of an internal LDH pool which facilitates lactate to pyruvate conversion and oxidation. Results support presence of an “Intracellular Lactate Shuttle.”