Role of mitochondrial lactate dehydrogenase and lactate oxidation in the intracellular lactate shuttle

To evaluate the potential role of mitochondrial lactate dehydrogenase (LDH) in tissue lactate clearance and oxidation in vivo, isolated rat liver, car diac, and skeletal muscle mitochondria were incubated with lactate, pyruvat e, glutamate, and succinate. As well, alpha-cyano-4-hydroxycinnamate (CINN) , a known monocarboxylate transport inhibitor, and oxamate, a known LDH inh ibitor were used. Mitochondria readily oxidized pyruvate and lactate, with similar state 3 and 4 respiratory rates, respiratory control (state 3/state 4), and ADP/O ratios. With lactate or pyruvate as substrates, alpha-cyano- 4-hydroxycinnamate blocked the respiratory response to added ADP, but the b lock was bypassed by addition of glutamate (complex I-linked) and succinate (complex II-linked) substrates. Oxamate increased pyruvate (approximate to 10-40%), but blocked lactate oxidation. Gel electrophoresis and electron m icroscopy indicated LDH isoenzyme distribution patterns to display tissue s pecificity, but the LDH isoenzyme patterns in isolated mitochondria were di stinct from those in surrounding cell compartments. In heart, LDH-1 (H4) wa s concentrated in mitochondria whereas LDH-5 (M4) was present in both mitoc hondria and surrounding cytosol and organelles. LDH-5 predominated in liver but was more abundant in mitochondria than elsewhere. Because lactate exce eds cytosolic pyruvate concentration by an order of magnitude, we conclude that lactate is the predominant monocarboxylate oxidized by mitochondria il l vivo. Mammalian liver and striated muscle mitochondria can oxidize exogen ous lactate because of an internal LDH pool that facilitates lactate oxidat ion.