Two separate pathways for D-lactate oxidation by Saccharomyces cerevisiae mitochondria which differ in energy production and carrier involvement

In this work we looked at whether and how mitochondria isolated from Saccharomyces cerevisiae (SCM) oxidize D-lactate.We found that: (1) externally added D-lactate causes oxygen uptake by SCM with P/O ratio equal to 1.5; in the presence of antimycin A (AA), P/O ratio was 1.8, differently in the presence of the non-penetrant a-cyanocinnamate (a-CCN) no P/O ratio could be measured. Consistently, mitochondrial electrical membrane potential (Dw) generation was found, due to externally added D-lactate in the presence of antimycin A, but not of a-CCN. (2) SCM oxidize D-lactate in two different manners: (i) via inner membrane D-lactate dehydrogenase which leads to D-lactate oxidation without driving Dw generation and ATP synthesis and (ii) via the matrix D-lactate dehydrogenase, which drives Dw generation and ATP synthesis by using taken up D-lactate. (3) Pyruvate newly synthesised in the mitochondrial matrix is exported via the novel D-lactate/pyruvate antiporter. D-Lactate/pyruvate antiport proved to regulate the rate of pyruvate efflux in vitro. (4) The existence of the D-lactate/H+ symporter is also proposed as shown by mitochondrial swelling. The D-lactate carriers and D-lactate dehydrogenases could account for the removal of the toxic methylglyoxal from cytosol, as well as for the D-lactate-dependent gluconeogenesis.