Succinate Dehydrogenase, Succinate, and Superoxides: A Genetic, Epigenetic, Metabolic, Environmental Explosive Crossroad

Abstract

Research focused on succinate dehydrogenase (SDH) and its substrate, succinate, culminated in the 1950s accompanying the rapid development of research dedicated to bioenergetics and intermediary metabolism. This allowed researchers to uncover the implication of SDH in both the mitochondrial respiratory chain and the Krebs cycle. Nowadays, this theme is experiencing a real revival following the discovery of the role of SDH and succinate in a subset of tumors and cancers in humans. The aim of this review is to enlighten the many questions yet unanswered, ranging from fundamental to clinically oriented aspects, up to the danger of the current use of SDH as a target for a subclass of pesticides.

Keywords: Krebs cycle; SDH; SDHI; cancer; encephalopathy; mitochondria; mitotoxic; oxytoxic; pesticides; respiratory chain; succinate.

Figure 1

The succinate dehydrogenase. SDH’s four subunits (A–D) are conserved from microorganism to human. Animal SDH, as its plant counterpart, reduces a specific pool of ubiquinone that competes with other kinetically distinct quinone pools reduced by various mitochondrial dehydrogenases for the reduction of CIII-associated quinones. Depending on the reduction status of the redox centers of the enzyme, variable amounts of superoxides will be generated and used as physiological regulators of cell signaling pathways. However, in excess, superoxides can have multiple deleterious effects. Abbreviations/symbols: CII, CIII, respiratory chain complexes II, and III; FeS, iron-sulfur cluster; FAD, flavin adenine dinucleotide; Q, ubiquinone with variable length isoprenoid side chain according to species (CoQ₁₀ in human).

Figure 2

Four different methods, noticed 1 to 4, to estimate SDH-related activities. First, the measurement from succinate to DCPIP through PMS determines the electron transfer activity through SDHA and B subunits (noted 1 on the scheme). This activity is malonate-sensitive but TTFA-insensitive. Second, the reduction of DCPIP in the presence of the added short-chain homologue of ubiquinone, e.g., decylubiquinone, measures the activity of SDH through its four subunits A–D. This activity (2) is both malonate- and TTFA-sensitive. Third, the reduction rate of added cytochrome c by succinate (known as succinate cytochrome c reductase activity) measures the coupled activity of CII and CIII (3). This combined activity is often considered as a measure of SDH activity, which represents the rate limiting catalytic step (slowest step), making it rate limiting. This activity is sensitive to malonate, TTFA, and inhibitors of CIII, e.g., antimycin. Fourth, succinate-dependent oxygen consumption (4) measured either on isolated mitochondria, permeabilized muscle fibers, or various permeabilized cell types, can also be taken as reflecting SDH activity as long as the used conditions (e.g., mitochondria intactness, and non-limiting ADP availability) make SDH the limiting factor for oxygen consumption. Abbreviations/symbols: A–D, the four SDH subunits; c, cytochrome c; CIII, CIV, complexes III and IV of the respiratory chain; DCPIP, dichlorophenol indophenol; endo., endogenous; exog., exogenous (externally added); im, inner membrane of the mitochondria; PMS, phenazine methosulfate; Q endo., endogenous quinone, i.e., ubiquinone; Q exog., externally added quinones, e.g., decylubiquinone; TTFA, thenoyltrifluoroacetone.

Figure 3

An unconventional schematic diagram featuring the succinate dehydrogenase among some of its competitors/coworkers for access to supercomplexes existing in the heating respiratory chain. The number of dehydrogenases capable of reducing part of mitochondrial ubiquinone varies between organisms. Moreover, their activity, distribution, and proportion can differ markedly depending on the organ, particularly in humans. Due to this competition, a small variation (<20%) can result in significant pathological consequences in humans [84]. Abbreviations/symbols: AA catab, amino acid catabolism; Arg, arginine; dh, dehydrogenase; DHO, dihydroorotate; DHAP, dihydroxyacetone phosphate; D2HGdh, D-2-hydroxyglutarate dehydrogenase; ETF, electron transfer flavoprotein; FA ox, β fatty acid oxidation; Fum, fumarase; G3P, glycerol-3-phosphate; H₂S, hydrogen sulfide; metab, metabolism; Orn, ornithine; O, orotate; Pro, proline; Pyrr-Carb, pyrroline 5-carboxylate; Q, (ubi)quinone pool; Qo, quinone oxidase; Qr, quinone reductase; Succ, succinate; SuQr, sulfide quinone reductase; I-V, respiratory chain complexes I-V.

Figure 4

The involvement of the mitochondrial SDH in the fast segment of the Krebs bi-cycle. The Krebs cycle is represented as consisting of two segments, A and B, that are characterized by different measured flow velocities. * Respective fluxes through the segments A and B of the bi-cycle in nmol/min/mg prot. Abbreviations: ACO, aconitase; ADP, adenosine-diphosphate; FA, fatty acid; CoA, coenzyme A; CS, citrate-synthase; DLAT, dihydrolipoamide-S-acetyl-transferase (E2); DLD, dihydrolipoamide-dehydrogenase (E3); DLST, dihydrolipoamide-S-succinyl-transferase (E2); FH, fumarate-hydratase; GDP, guanosine-diphosphate; GPT, glutamate-pyruvate-transaminase; GOT, glutamate-oxaloacetate-transaminase; GLUD, glutamate-dehydrogenase; IDH, isocitrate-dehydrogenase; IRP, iron-responsive protein; LDH, lactate-dehydrogenase; LIP, lipoic acid; LIPT1, 2: lipoyltransferase; MDH, malate-dehydrogenase; ME, malic enzyme; NAD, nicotinamide-adenine-dinucleotide; NADP, nicotinamide-adenine-dinucleotide-phosphate; OGDH, oxoglutarate-dehydrogenase (α-ketoglutarate-dehydrogenase; OGDHE1: E1; OGDHL, E1-like); PC, pyruvate-carboxylase; PDH, pyruvate-dehydrogenase (PDHA1 and PDHB, α and β subunits of E1); PDHX, pyruvate-dehydrogenase compound X; PDKs, pyruvate-dehydrogenase-kinases; PDP, pyruvate-dehydrogenase-phosphatase; TPP, thiamine-pyrophosphate; SDH, succinate-dehydrogenase; SUCL, succinyl CoA-ligase.

Figure 5

Natural and chemical SDH inhibitors. Sites of action of a series of the best-known SDH inhibitors are indicated by red arrows. A subset of these inhibitors (bottom) acts either as competitive inhibitors (ci) or as a suicide inhibitor (si) by binding to the SDHA subunit. ATP, which is also capable of binding to this same subunit, acts as an activator of SDH activity (green arrow). Another group of inhibitors act by interacting with the endogenous quinone binding site (top). This type of inhibitors includes a large number of molecules used as fungicides in agriculture (a more complete list of SDHI inhibitors can be found at http://endsdhi.com/wp-content/uploads/2022/04/SDHI-structure-15-Avril-22.pdf, accessed on 22 July 2022). Abbreviations/symbols: ATP, Adenosine triphosphate; CII, CIII, respiratory chain complex II and III; FeS, iron-sulfur cluster; FAD, SDH A-bound flavin adenine dinucleotide; 3-NP, 3 nitropropionic acid; Q, ubiquinone with variable length isoprenoid side chain according to species (CoQ10 in human); Qp, Qd, proximal and distal Q binding sites, respectively.