Production of superoxide/hydrogen peroxide by the mitochondrial 2-oxoadipate dehydrogenase complex

• The protein encoded by DHTKD1 is a 2-oxoadipate dehydrogenase.
• 2-Oxoadipate is produced by catabolism of tryptophan, lysine and hydroxylysine.
• Oxidation of 2-oxoadipate by rat skeletal muscle mitochondria generates superoxide/H2O2 through multiple sites of the mitochondrial electron transport chain and substrate oxidation.
• The 2-oxoadipate dehydrogenase complex (OADHC) contributes directly to superoxide/H2O2 production.
• Site AF in OADHC is the eleventh identified mitochondrial site that can produce superoxide/H2O2 at a measurable rate.

In humans, mutations in dehydrogenase E1 and transketolase domain containing 1 (DHTKD1) are associated with neurological abnormalities and accumulation of 2-oxoadipate, 2-aminoadipate, and reactive oxygen species. The protein encoded by DHTKD1 has sequence and structural similarities to 2-oxoglutarate dehydrogenase, and the 2-oxoglutarate dehydrogenase complex can produce superoxide/H2O2 at high rates. The DHTKD1 enzyme is hypothesized to catalyze the oxidative decarboxylation of 2-oxoadipate, a shared intermediate of the degradative pathways for tryptophan, lysine and hydroxylysine. Here, we show that rat skeletal muscle mitochondria can produce superoxide/H2O2 at high rates when given 2-oxoadipate. We identify the putative mitochondrial 2-oxoadipate dehydrogenase complex as one of the sources and characterize the conditions that favor its superoxide/H2O2 production. Rates increased at higher NAD(P)H/NAD(P)+ ratios and were higher at each NAD(P)H/NAD(P)+ ratio when 2-oxoadipate was present, showing that superoxide/H2O2 was produced during the forward reaction from 2-oxoadipate, but not in the reverse reaction from NADH in the absence of 2-oxoadipate. The maximum capacity of the 2-oxoadipate dehydrogenase complex for production of superoxide/H2O2 is comparable to that of site IF of complex I, and seven, four and almost two-fold lower than the capacities of the 2-oxoglutarate, pyruvate and branched-chain 2-oxoacid dehydrogenase complexes, respectively. Regulation by ADP and ATP of H2O2 production driven by 2-oxoadipate was very different from that driven by 2-oxoglutarate, suggesting that site AF of the 2-oxoadipate dehydrogenase complex is a new source of superoxide/H2O2 associated with the NADH isopotential pool in mitochondria.

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