d-Lactate Dehydrogenase Links Methylglyoxal Degradation and Electron Transport through Cytochrome c

Glycolysis generates methylglyoxal (MGO) as an unavoidable, cytotoxic by-product in plant cells. MGO scavenging is performed by the glyoxalase system, which produces d-lactate as an end product. d-Lactate dehydrogenase (d-LDH) is encoded by a single gene in Arabidopsis (Arabidopsis thaliana; At5g06580). It catalyzes in vitro the oxidation of d-lactate to pyruvate using flavin adenine dinucleotide as a cofactor; knowledge of its function in the context of the plant cell remains sketchy. Blue native-polyacrylamide gel electrophoresis of mitochondrial extracts combined with in gel activity assays using different substrates and tandem mass spectrometry allowed us to definitely show that d-LDH acts specifically on d-lactate, is active as a dimer, and does not associate with respiratory supercomplexes of the inner mitochondrial membrane. The combined use of cytochrome c (CYTc) loss-of-function mutants and respiratory complex III inhibitors showed that CYTc acts as the in vivo electron acceptor of d-LDH. CYTc loss-of-function mutants, as well as the d-LDH mutants, were more sensitive to d-lactate and MGO, indicating that they function in the same pathway. In addition, overexpression of d-LDH and CYTc increased tolerance to d-lactate and MGO Together with fine-localization of d-LDH, the functional interaction with CYTc in vivo strongly suggests that d-lactate oxidation takes place in the mitochondrial intermembrane space, delivering electrons to the respiratory chain through CYTc These results provide a comprehensive picture of the organization and function of d-LDH in the plant cell and exemplify how the plant mitochondrial respiratory chain can act as a multifunctional electron sink for reductant from cytosolic pathways.