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Key Event Title
Succinate dehydrogenase, inhibited
|Level of Biological Organization|
Key Event Components
|succinate dehydrogenase activity||decreased|
|FAD metabolic process||succinate dehydrogenase complex||decreased|
|succinate metabolic process||succinate dehydrogenase complex||decreased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP||Point of Contact||Author Status||OECD Status|
|Succinate dehydrogenase inhibition leading to increased insulin resistance||MolecularInitiatingEvent||Simon Thomas (send email)||Under development: Not open for comment. Do not cite|
Key Event Description
Eukaryotic succinate dehydrogenase (SDH, EC220.127.116.11 (Brenda, IntEnz)) is an enzyme complex comprising four polypeptide chains (SDHA - SDHD) with associated FAD, Fe-S and haem prosthetic groups that catalyses the reversible oxidation (dehydrogenation) of succinate to fumarate with concomitant reduction of ubiquinone to ubiquinol, serving to channel reducing equivalents from succinate, a tricarboxylic acid (TCA) cycle intermediate, to ubiquinol, an intermediate of the mitochondrial electron transfer chain (Du et al, 2023).
The overall reaction:
succinate + ubiquinone = fumarate + ubiquinol
comprises two, reversible half-reactions:
(1) succinate + FAD = fumarate + FADH2
(2) FADH2 + ubiquinone = FAD + ubiquinol
each of which is catalysed at a different active site.
The active site of reaction 1 is in the hydrophilic protein SDHA that contains the covalently bound FAD group, and protudes from the inner mitochondrial membrane (IMM) into the mitochondrial matrix, making it available to exchange succinate and fumarate within the TCA cycle. The active site of reaction 2 is in a more hydrophobic region comprising transmembrane domains of proteins SDHC and SCHD that insert complex II into the IMM (Du et al, 2023), making it available to ubiquinol and ubiquinone shuttling within the IMM.
The presence of two distinct and different active sites enables SDH inibition to be effected in at least two ways: by inhibition of either active site, with potentially different biochemical and physiological consequences, and by inhibitors with differing characteristics.
Inhibition of SDH can result in reduction of mitochondrial electron transport, and subsequent inhibition of oxidative phosphorylation (e.g. Chen et al, 2021), and also generation of superoxide in the mitochondria, leading to with subsequently deleterious effects such as initiation of apoptosis or necrosis (Murphy et al, 2009).
How It Is Measured or Detected
Succinate dehydrogenase activity is generally measured by the spectrophotometric detection of colour change in the presence of an electron acceptor, with succinate (succinic acid) as substrate. Alteration in rate of colour change in the presence of a putative inhibitor determining the strength of that inhibition. The fact that the overall reaction is comprised of two consecutive sub-reactions enables the rate of each sub-reaction - and their inhibition - to be measured separately by appropriate choice of electron acceptor in the presence of succinate as a substrate (e.g. Miyadera et al, 2003). Activities are frequently measured in isolated mitochondria, in order to reduce interference by extra-cytosolic enaymes and intermediates; mitochondria can be sonicated to obviate rate limitation by mitochondrial upake of succinate (e.g. Guo et al, 2016).
Succinate dehydrogenase (SDH) activity corresponds to reaction (1), above. It can be measured by use of the water-soluble dye 2-(4,5-dimethyl-2-thiazolyl)-3,5-diphenyl-2H-tetrazolium bromide (MTT) in the presence of the intermediate electron carrier phenazine methosulfate (PMS), to intercept electrons before their transport to ubiquinone, and convey them to MTT, which changes colour following its reduction.
Succinate quinone reductase (SQR) activity corresponds to the overall reaction (i.e. 1 and 2), above. It can be measured by reduction of 2,6-dichlorophenolindophenol (DCPIP) in the presence of the 2,3-dimethoxy-6-methyl-1,4-benzoquinone (UQ2), which accepts electrons from the ubiquinone reduction site and transfers them to DCPIP, thus being a measure of the rate of the entire reaction catalysed by complex II.
Domain of Applicability
SDH inhibition by phthalate esters has been measured and quantified in mitochondria of hepatocytes of adult male CD rats (Melnick and Schiller, 1982; Melnick and Schiller, 1985). Inter-species differences in SDH structure may lead to different susceptibilities in different taxa.
SDH inhibition has been demonstrated by lonidamine, 3-nitroproprionic acid (3-NPA) and 2-thenoyltrifluoroacetone (TTFA) in DB-1, HepG2, HCT116 and HeLa cells, and by lonidamine in mitochondria isolated from adult mouse liver (Guo et al, 2016).
Brenda, "Information on EC 18.104.22.168 - succinate dehydrogenase", https://www.brenda-enzymes.org/enzyme.php?ecno=22.214.171.124, accessed 28/04/2023.
Chen, L. et al (2021) "Citrus-derived DHCP inhibits mitochondrial complex II to enhance TRAIL sensitivity via ROS-induced DR5 upregulation", Journal of Biological Chemistry, Vol 296, 100515
Du, Z. et al (2023) "Structure of the human respiratory complex II", Proceedings of the National Academy of Sciences", Vol 120, e2216713120.
Guo, L. et al (2016) "Inhibition of Mitochondrial Complex II by the Anticancer Agent Lonidamine", Journal of Biological Chemistry, Vol 291. pp42-57.
IntEnz, "IntEnz Enzyme Nomenclature, EC 126.96.36.199", https://www.ebi.ac.uk/intenz/query?cmd=SearchID&id=1525&view=INTENZ, accessed 28/04/2023.
Melnick, R.L. and Schiller, C.M. (1982), "Mitochondrial toxicity of phthalate esters", Environmental Healh Perspectives, Vol 45, pp51-56.
Melnick, R.L. and Schiller, C.M. (1985), "Effect of phthalate esters on energy coupling and succinate oxidation in rat liver mitochondria", Toxicology, Vol 34, pp13-27.
Miyadera, H. et al (2003) "Atpenins, potent and specific inhibitors of mitochondrial complex II (succinateubiquinone oxidoreductase)", Proceedings of the National Academy of Sciences, Vol 100, pp473-477.
Murphy, M.P. (2009), "How mitochondria produce reactive oxygen species", Biochemical Journal, Vol 417, pp1-13.