Cell‐free extracts of Rhizobium meliloti contain a soluble lactate dehydrogenase (LDH‐EC 1.1.1.27.). This was purified 250‐fold by ammonium sulfate precipitation and filtration on different Sephadex gels. This enzyme catalyses the reduction of pyruvate to lactate in the presence of NADH and for the first time we report its ability to reduce indole‐3‐pyruvic acid (IPyA) to indole‐3‐lactic acid (ILA). Optimal conditions for activity and K m values for both substrates were determined. In the presence of NAD the reverse reaction could be demonstrated with the aliphatic substrate (lactate), but under our conditions it was not possible to achieve the oxidation of ILA to IPyA. The role of this LDH in the indole metabolism is discussed and a general reaction scheme is suggested.
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Trinchant et al. (1974) studied this question.