D-Lactate/malate antiporter, in response to the cellular carbonyl stress mediated by methylglyoxal, in yeast mitochondria

Methylglyoxal is a highly reactive dicarbonyl degradation product formed from triose phosphates during glycolysis. Methylglyoxal-derived advanced glycation end-products are involved in neurodegenerative disorders and in the clinical complications of diabetes. Yeast are an outstanding cell model for investigating intracellular protein glycation and its implications in cell physiology and aging. In Saccharomyces cerevisiae a established pathway for MG detoxification by the action of the glyoxalase system converts, in the cytosol, MG into D-Lactate in the presence of glutathione. Previous work has demonstrated that isolated Saccharomyces cerevisiae mitochondria can take up and oxidise D-lactate externally added to them and shown the existence of two carriers for Dlactate mitochondrial metabolism: D-lactate/H+ symporter and D-lactate/pyruvate antiporter. Here, we found that as a result of D-lactate uptake and metabolism by Saccharomyces cerevisiae mitochondria, reducing equivalents were exported from the mitochondrial matrix to the cytosol in the form of malate. The rate of malate efflux, as measured photometrically using NADP+ and malic enzyme, depended on the rate of transport across the mitochondrial membrane. It showed saturation characteristics (Km = 20 μM; Vmax = 6 nmol/min mg of mitochondrial protein) and was inhibited by non-penetrant compounds. These findings demonstrate that reducing equivalent export from mitochondria is due to the occurrence of a putative D-lactate/malate antiporter which differs from D-lactate/pyruvate antiporter as shown by the different inhibitor sensitivity and pH profile.