The oxidation of l-lactate by liver mitochondria

A single NAD +-dependent LDH was found in liver mitochondria, localized in the intermembrane space. Maximal aerobic oxidation of l-lactate requires externally added NAD +. Apparent K m for NAD + is the same for the aerobic oxidation of l-lactate by mitochondria, as for the oxidation of l-lactate by NAD + in the presence of mitochondrial LDH. Mitochondrial LDH contains bound NAD + which remains associated with the enzyme protein during acrylamide gel electrophoresis. Dismutation between l-lactate and acetoacetate by mitochondria is greatly stimulated by NAD + and this reaction is inhibited by DNP, while dismutation between pyruvate and acetoacetate is not. Results are interpreted in terms of a possible mitochondrial path of l-lactate oxidation. This process may involve an energy-dependent transfer of reducing equivalents between l-lactate + LDH-bound NAD +(H) and enzyme systems of the inner membrane (and matrix).