J Physiol. 2020 May 02.
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 modeling 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 s upply 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. Abstract figure legend Credit for the discovery of what would become known as mitochondria is given to Rudolf Albrecht von Kölliker in 1857; these structures were subsequently described in greater detail by Richard Altmann. In 1898, Benda used a derivation of the Greek words for 'thread' and 'granule' to name these structures 'mitochondria'. In 1907, Fletcher and Hopkins reported the disappearance of lactate in the presence of O2 in previously stimulated muscles. Approximately two decades later, Meyerhof's work on O2 consumption and lactate (La- ) resynthesis into glycogen during the recovery of isolated skeletal muscles from prior contractions was an early hint at the intersection of glycolysis and aerobic phosphorylation. Warburg related these phenomena to the metabolic physiology of cancer. Research by both Meyerhof and Emden led to discovery of the glycolytic pathway. In the 1930s, the work of Lundsgaard, Krebs, Kalckar, the Coris, Belitzer and Szent-Gyorgi, and subsequently Lipmann, Ochoa, Bensley & Hoerr and Claude in the 1940s led to establishing the bioenergetics of glycolysis and the TCA cycle and compounds of high phosphoryl transfer potential. The 1950s heralded the age of research using isolated, functioning mitochondria to explore bioenergetics, and featured prominently the work of Lehninger, Estabrook & Saktor, and Chance & Williams. In the 1960s, Peter Mitchell first proposed the chemiosmotic theory of oxidative phosphorylation, for which he was awarded the Nobel Prize. During this same decade, work by Borst clarified the malate-aspartate shuttle, wherein the exchange of anionic aspartate for undissociated glutamate (one negative charge exported from the matrix per exchange) is driven by the membrane potential (ΔΨ). Work by Skulachev in this decade and beyond further clarified mitochondrial bioenergetics and mitochondrial morphology. Boyer elucidated the nature of the ATP synthase, ultimately winning the Nobel Prize for his work. In the 1980s, David Nicholls further clarified mitochondrial bioenergetics, and the work of George Brooks initiated the era of the Cell-to-Cell Lactate Shuttle. Starting in the 1990s, research emerged suggesting that mitochondria are capable of transporting La- across the inner membrane and oxidizing it without the support of the cytosolic-mitochondrial electron shuttles (i.e. the malate-aspartate and glycerol-3-phosphate shuttles). The ultimate combustion of La- obviously takes place in the mitochondria; there is no question about that simple conclusion. However, our view is that La- is not directly oxidized by LDH in the mitochondrial matrix, but rather La- must first be converted to pyruvate (Pyr- ) in the cytosol or intermembrane space. Rationale for this view includes the high activity of the near-equilibrium enzyme LDH, which exceeds glycolytic capacity, the highly oxidized NAD+ /NADH ratio relative to the mitochondrial matrix, and the thermodynamic necessity for an energy-driven accumulation of shuttle species (e.g. ΔΨ-dependent aspartate-glutamate exchanger). Modeling in silico demonstrates that an active LDH in the matrix would render mitochondria nearly incapable of oxidizing Pyr- , a result which is inconsistent with decades of studies from hundreds of laboratories using both isolated mitochondria and permeabilized cells in which the mitochondrial reticulum remains intact. Healthy mitochondria function well, even at low O2 levels such that dysoxia is rare and low O2 is likely a minor factor in the increasing concentrations of La- typical with exercise or even many acute critical care situations.2 This article is protected by copyright. All rights reserved.