Chronic Exposure to Sulfide Causes Accelerated Degradation of Cytochrome c Oxidase in Ethylmalonic Encephalopathy

Abstract

Ethylmalonic encephalopathy (EE) is an autosomal recessive, invariably fatal disorder associated with mutations in ETHE1, a gene encoding a mitochondrial sulfur dioxygenase (SDO). The main consequence of the absence of Ethe1-SDO is the accumulation of sulfide (H₂S) in critical tissues, including colonic mucosa, liver, muscle, and brain. To make progress in the elucidation of the biochemical mechanisms leading to cytochrome c oxidase (COX) deficiency, we (i) generated tissue-specific conditional Ethe1 knockout mice to clarify the different contributions of endogenous and exogenous H₂S production, and (ii) studied the development of H₂S-driven COX deficiency in Ethe1 ^−/− mouse tissues and human cells. Ethe1 ^−/− conditional animals displayed COX deficiency limited to the specific targeted tissue. The accumulation of H₂S over time causes progressive COX deficiency in animal tissues and human cells, which is associated with reduced amount of COX holoenzyme, and of several COX subunits, including mitochondrially encoded cytochrome c oxidase 1 (MTCO1), MTCO2, COX4, and COX5A. This reduction is not paralleled by consistent downregulation in expression of the corresponding mRNAs. Tissue-specific ablation of Ethe1 causes COX deficiency in targeted organs, suggesting that failure in neutralizing endogenous, tissue-specific production of H₂S is sufficient to cause the biochemical defect but neither to determine a clinical impact nor to induce the biomarker profile typical of EE. The mechanism by which H₂S causes COX deficiency consists of rapid heme a inhibition and accelerated long-term degradation of COX subunits. However, the pleiotropic devastating effects of H₂S accumulation in EE cannot be fully explained by the sole defect of COX in critical tissues, but are likely consequent to several toxic actions on a number of enzymatic activities in different tissues, including endothelial lining of the small vessels, leading to multiorgan failure. Antioxid. Redox Signal. 15, 353–362.