lldD, also known as L-lactate dehydrogenase, is present in a wide variety of organisms, including plants and animals. It is an oxidoreductase which catalyses the interconversion of pyruvate and lactate with concomitant interconversion of NADH and NAD+. As it can also catalyze the oxidation of hydroxybutyrate, it is occasionally called Hydroxybutyrate Dehydrogenase (HBD). Recombinant E.coli lldD protein, fused to His-tag at Nterminus, was expressed in E.coli and purified by using conventional chromatography.