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.