Mitochondrial lactate metabolism: history and implications for exercise and disease

Abstract

Mitochondrial structures were probably observed microscopically in the 1840s, but the idea of oxidative phosphorylation (OXPHOS) within mitochondria did not appear until the 1930s. The foundation for research into energetics arose from Meyerhof's experiments on oxidation of lactate in isolated muscles recovering from electrical contractions in an O₂ atmosphere. Today, we know that mitochondria are actually reticula and that the energy released from electron pairs being passed along the electron transport chain from NADH to O₂ generates a membrane potential and pH gradient of protons that can enter the molecular machine of ATP synthase to resynthesize ATP. Lactate stands at the crossroads of glycolytic and oxidative energy metabolism. Based on reported research and our own modelling in silico, we contend that lactate is not directly oxidized in the mitochondrial matrix. Instead, the interim glycolytic products (pyruvate and NADH) are held in cytosolic equilibrium with the products of the lactate dehydrogenase (LDH) reaction and the intermediates of the malate-aspartate and glycerol 3-phosphate shuttles. This equilibrium supplies the glycolytic products to the mitochondrial matrix for OXPHOS. LDH in the mitochondrial matrix is not compatible with the cytoplasmic/matrix redox gradient; its presence would drain matrix reducing power and substantially dissipate the proton motive force. OXPHOS requires O₂ as the final electron acceptor, but O₂ supply is sufficient in most situations, including exercise and often acute illness. Recent studies suggest that atmospheric normoxia may constitute a cellular hyperoxia in mitochondrial disease. As research proceeds appropriate oxygenation levels should be carefully considered.

Keywords: NADH shuttles; dysoxia; glycolysis; hypoxia; lactic acid; mitochondria; modeling in silico; oxidative phosphorylation; oxygen.

Figure 1.

Schematic representation of the interaction among mitochondrial electron shuttles and mitochondrial lactate (La⁻) oxidation (Kane, 2014; Ferguson et al., 2018). Mono- and dicarboxylate anions can move between the cytosol and the mitochondrial intermembrane space by crossing the outer mitochondrial membrane via the voltage-dependent anion channels (VDAC). Due to the action of the glycerol phosphate and malate-aspartate shuttles, the cytosolic NAD⁺/NADH ratio can be orders of magnitude greater than the mitochondrial matrix, but decreases during exercise along with decreasing mitochondrial membrane potential (ΔΨ) spanning the inner membrane (inset; (Sahlin et al., 1987)). The electrogenic transport of glutamate (Glu²⁻) across the inner mitochondrial membrane via the aspartate-glutamate exchanger (AGE) is a key regulator of mitochondrial lactate oxidation vis-à-vis aerobic glycolysis and the malate-aspartate shuttle. The putative mitochondrial lactate oxidation complex comprised of mLDH, CD147, cytochrome c oxidase, and monocarboxylate transporter is depicted (Hashimoto et al., 2006), as is a matrix mLDH (Brooks et al., 1999). In the text, we argue against the likelihood of LDH in the mitochondrial matrix and suggest that the necessity and/or role of the lactate oxidation complex requires further study. Abbreviations: 2-OG²⁻ 2-oxoglutarate, I Complex I/NADH oxidoreductase of the mitochondrial electron system, II Complex II/succinate dehydrogenase of the mitochondrial electron system, III Complex III of the mitochondrial electron transport system, IV/COX complex IV/cytochrome c oxidase, AAT aspartate aminotransferase, AGE Aspartate-glutamate exchanger, Asp²⁻ aspartate, C cytochrome c, cG3P DH cytosolic glycerol 3-phosphate dehydrogenase, CoA coenzyme A, DHAP²⁻ dihydroxyacetone phosphate, G3P²⁻ glycerol 3-phosphate, Glu²⁻ glutamate, LDH L-lactate dehydrogenase, Mal²⁻ malate, MCT monocarboxylate transporter, MDH malate dehydrogenase, mG3P DH mitochondrial glycerol 3-phosphate dehydrogenase, mLDH mitochondrial lactate dehydrogenase, MOE malate-2-oxoglutarate exchanger, MPC mitochondrial pyruvate carrier, OAA²⁻ oxaloacetate, PDH pyruvate dehydrogenase complex, Pyr− pyruvate, Q quinone, TCA cycle tricarboxylic acid cycle

Figure 2.

Results of modeling isolated mitochondrial energetics in silico. A) Typical experiment with Pyruvate + Malate, 10 mM + 2.5 mM and zero LDH in the mitochondrial matrix; B) Experiment with Lactate + Malate, 10 mM + 2.5 mM and matrix LDH activity set to equal PDH V_max activity; C) Experiment with Pyruvate + Malate, 10 mM + 2.5 mM and matrix LDH activity set to equal PDH V_max activity; D) Simulated state 3 rates with either Pyruvate + Malate or Lactate + Malate as substrates as matrix LDH activity is titrated from 0% to 100% of PDH V_max. Complete details of the results are provided in the text. Abbreviations: LDH lactate dehydrogenase, PDH pyruvate dehydrogenase, P:O ratio of ATP synthesized to atomic oxygen consumed, RCR respiratory control ratio, JO₂ mitochondrial respiratory O₂ flux, V_max maximal reaction velocity

Figure 3.

A) Schematic diagram showing the effect of hypothetical matrix LDH on the oxidation of Lactate. Green lines show the path of energy conservation and ATP production. Red line shows small loss of matrix pyruvate to buffer via the mitochondrial pyruvate carrier. B) Schematic diagram showing the effect of hypothetical matrix LDH on the oxidation of pyruvate. Green lines show the path of energy conservation and ATP production. Red lines show loss of matrix redox pressure (LDH catalyzes the oxidation of NADH produced by the TCA cycle) and export of pyruvate carbon as lactate due to LDH activity and a monocarboxylate transporter. Abbreviations:, C V Complex V, ETC electron transport chain, LDH lactate dehydrogenase, MCT monocarboxylate transporter, MPC mitochondrial pyruvate carrier, PDH pyruvate dehydrogenase, TCA tricarboxylic acid cycle