Dissipation of the mitochondrial proton motive force drives sepsis pathogenesis and explains hyperlactatemia’s predictive value in sepsis mortality

Figure 2 Antioxidant enzyme inhibition: The mitochondrial proton motive force maintains cellular redox homeostasis. The electron transport chain (ETC) is a major source of hydrogen peroxide (H₂O₂). Up to 4% of electrons “leak” out prematurely from the ETC into the mitochondrial matrix forming superoxide anion radical (O₂^•ˉ), which is dismutated to H₂O₂ by superoxide dismutase. H₂O₂ is highly toxic to cells and must be disposed of. This detoxification process relies on two key regenerative redox enzyme systems—glutathione peroxidase (GPx) and peroxiredoxin (PRx)—which facilitate the reduction of H₂O₂ to water using glutathione (GSH) and reduced thioredoxin (TRX-r), respectively. During this reaction, GSH is oxidized to glutathione disulfide (GSSG), while TRX-r is converted to oxidized thioredoxin (TRx-o). To regenerate these reducing agents, glutathione disulfide reductase (EC 1.8.1.7) and thioredoxin disulfide reductase (EC 1.8.1.9) catalyze the reduction of GSSG and TRx-o, respectively, using NADPH as an electron donor. NADPH is replenished by nicotinamide nucleotide transhydrogenase (NNT; EC 7.1.1.1), which facilitates electron transfer from NADH to NADP⁺. This reaction is driven by the proton motive force (detailed below), linking mitochondrial energy metabolism to redox homeostasis. Thus, mitochondrial enzymatic redox buffering systems critically depend on the proton motive force (PMF) to sustain H₂O₂ detoxification and maintain cellular redox homeostasis. However, the functionality of this tightly interdependent antioxidant system can be compromised by excessive H₂O₂ production during the early hypermetabolic stage of sepsis. An acute H₂O₂ load can deplete GSH and Trx-r, leading to the accumulation of H₂O₂ followed by hyperoxidation of GPx and PRx, resulting in their conversion to dehydroalanine (DHA) and sulfinic acid derivatives, respectively (orange dashed arrows). These oxidative modifications deactivate both enzymes, which are responsible for eliminating approximately 99% of mitochondrial H₂O₂. Deactivation of these enzymes results in significantly increased free H₂O₂ accumulation within the mitochondrion, which disrupts mitochondrial energy metabolism initiating dissipation of the PMF (Figure 1B). Dissipation of the PMF compromises NNT functionality, further reducing mitochondrial capacity for H₂O₂ detoxification leading to toxic systemic levels of H₂O₂. NNT: Nucleotide transhydrogenase; GDR: Glutathione disulfide reductase; TDR: Thioredoxin disulfide reductase; GPx: Glutathione peroxidase; PRx: Peroxiredoxin; TRX-r: Reduced thioredoxin; TRx-o: Oxidized thioredoxin; GSH: Glutathione; GSSG: Glutathione disulfide; ETC: Electron transport chain; SOD: Superoxide dismutase; H₂O₂: Hydrogen peroxide; DHA: Dehydroalanine.