SDH, inhibited leads to Superoxide generation, increased

Succinate dehydrogenase (SDH) is a 4 subunit mitochondrial enzyme that oxidises succinate to fumarate, with the corresponding reduction of FAD to FADH₂. FADH₂ is subsequently reoxidised by transfer of electrons to the mitochondrial electron transfer chain (ETC). SDH thus links the tricarboxylic acid (TCA) cycle to mitochondrial electron transfer. It is the only TCA cycle enzyme to be embedded in the mitochondrial inner membrane, as opposed to located free in the mitochondrial matrix, comprising complex II of the ETC. The mitochondrial ETC is the largest source of superoxide radicals in most mammalian cells, complex II being one source of those radicals.

Inhibition of mammalian complex II activity has been recorded for many chemicals, including fungicides, other environmental chemicals and pharmaceuticals, being associated with an increase in superoxide generation in some instances. Because of the size and complexity of complex II, with multipled potential binding sites for inhibitors, generation of superoxide radical is not, per se, a guaranteed outcome of inhibition of succinate oxidation. The site and nature of the enzyme-inhibitor interaction is potential determining factor.

This KER is concerned with the direct reversible or irreversible binding of inhibitory chemicals to complex II, leading to a reduction in the dehydrogenation of succinate, and the potential for consequent generation of mitochondrial superoxide radicals. Other effects, such as reduction in expression of complex II components, are not covered.

The following search was executed in Pubmed on 27/04/2023:

("complex II"[Title] and inhibit* and mitochond*) OR ("succinate dehydrogenase" AND inhibition)

generating 2341 hits. In order to reduce the hits to a more tractable set a further specification of "superoxide" was added to the second bracketed term

("complex II"[Title] and inhibit* and mitochond*) OR ("succinate dehydrogenase" AND inhibition AND superoxide)

generating 286 hits whose abstracts were reviewed for further reference to direct chemical inhibition of SDH or complex II activity. As the search term contains "superoxide", some hits from this search also included information about potential impact on superoxide generation. Several articles provided evidence for multiple interactions by a chemical that could also potentially give rise to increase in superoxide concentrations (e.g. interactions at both ETC complexes II and III). Such articles have not been cited as supporting evidence for this KER. In those articles that have been cited in support of this KER, the lack of evidence presented for additional interactions, other than SDH inhibition, that could potentially give rise to increased superoxide production does not rule out the possibility of such interactions being observed in the future.

Evidence is provided in terms of (i) the biological plausibility of the KER, and (ii) empirical evidence that supports, quantitatively or qualitatively, the manifestation of the relationship in mammals in vivo, or in mammalian ex vivo or in vitro systems.

Succinate dehydrogenase is a large, multi-protein, multi-centre enzyme, with separate binding sites for succinate (which is oxidised to fumarate) and ubiquinone (which is reduced to ubiquinol) in the two part-reactions that together comprise the full reaction (succinate + ubiquinone -> fumarate + ubiquinol). Electrons can be released at different sites for the reduction of oxygen to superoxide, so the relationship between inhibition of two-electron succinate oxidation, inhibition of two-electron ubiquinone reduction, and stiumlation of one-electron reduction of oxygen to superoxide depends on the particular inhibition mechanism and characteristics of a specific inhibitor. Thus, 3-nitroproprionate (3-NPA), a structural analogue of succinate, inhibits the succinate oxidation step, but does not generate ROS (Guo et al, 2016), so there is a complete disconnect between inhibition of the enzyme and generation of superoxide for this compound.

For compounds that both inhibit the reaction, and generate superoxide, a quantitative linkage has to be defined in terms of:

1. On the one hand the rate of superoxide generation, and
2. On the either hand:
   1. The extent of inhibition of succinate oxidation, and/or
   2. The extent of inhibition of ubiquinone reduction.

These relationships may not be quantitatively equal.

The simultaneous inibition of the reduction of succinate to fumarate, and the generation of ROS has been demonstrated in isolated mouse liver mitochondria (Guo et al, 2016)

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

Guo, L. et al (2016) "Inhibition of Mitochondrial Complex II by the Anticancer Agent Lonidamine", Journal of Biological Chemistry, Vol 291, pp42-57.