The mitochondrial lactate oxidation complex: endpoint for carbohydrate carbon disposal.

The lactate shuttle concept has revolutionized our understanding and study of metabolism in physiology, biochemistry, intermediary metabolism, nutrition, and medicine. Seminal findings of the mitochondrial lactate oxidation complex (mLOC) elucidated the architectural structure of its components. Here, we report that the mitochondrial pyruvate carrier (mPC) is an additional member of the mLOC in mouse muscle and C2C12 myoblasts and myotubes. Immunoblots, mass spectrometry, and co-immunoprecipitation experiments of mitochondrial preparations revealed abundant amounts of mitochondrial lactate dehydrogenase (mLDH), monocarboxylate transporter (mMCT), basigin (CD147), cytochrome oxidase (COx), and pyruvate carriers 1 and 2 (mPC1 and 2). In addition, using confocal laser scanning microscopy (CLSM) and in situ proximity ligation, we also demonstrated planar and three-dimensional (3-D) colocalization of pyruvate and lactate transporters with COx in fixed mouse skeletal muscle sections and C2C12 myoblasts and myotubes skeletal muscle sections, mouse muscle and C2C12 myoblasts and myotubes myotubes, and C2C12 myoblasts. This work serves as a landmark for configuring the final pathway of carbohydrate oxidation. We expand on knowledge of the architectural design of the mitochondrial lactate oxidation complex (mLOC); key members are: mitochondrial lactate dehydrogenase (mLDH), monocarboxylate transporter 1 (mMCT1), cytochrome oxidase (COx), basigin scaffolding protein (CD147), and the mitochondrial pyruvate carrier (mPC). The mLOC is key in creating the lower end of the concentration gradient for disposal of lactate and pyruvate.