The role of an NAD-independent lactate dehydrogenase and acetate in the utilization of lactate by Clostridium acetobutylicum strain P262

Clostridium acetobutylicum strain P262 utilized lactate at a rapid rate [600 nmol min^-1 (mg protein)^-1], but lactate could not serve as the sole energy source. When acetate was provided as a co-substrate, the growth rate was 0.05h^-1. Butyrate, carbon dioxide and hydrogen were the end products of lactate and acetate utilization, and the stoichiometry was 1 lactate + 0.4 acetate →0.7 butyrate + 0.6H₂ + 1CO₂. Lactate-grown cells had twofold lower hydrogenase than glucose-grown cells, and the lactate-grown cells used acetate as an alternative electron acceptor. The cells had a poor affinity for lactate (K_s=1.1 mM), and there was no evidence for active transport. Lactate utilization was catabolyzed by an inducible NAD-independent lactate dehydrogenase (iLDH) that had a pH optimum of 7.5. The iLDH was fivefold more active with d-lactate than l-lactate, and the K _m for d-lactate was 3.2 mM. Lactate-grown cells had little butyraldehyde dehydrogenase activity, and this defect did not allow the conversion of lactate to butanol.