Membrane-bound, pyridine nucleotide-independent L-lactate dehydrogenase of Rhodopseudomonas sphaeroides

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Abstract
R. sphaeroides has a pyridine nucleotide-independent L-lactate dehydrogenase associated with the membrane fraction of cells grown aerobically or phototrophically. The dehydrogenase is present in cells grown on a variety of C sources, but at levels less than 20% of that found in cells grown with DL-lactate. The dehydrogenase was purified 45-fold from membranes of strain L-57, a non-photosynthetic mutant, by steps involving solubilization with lauryl dimethylamine oxide and 3 anion-exchange chromatography steps. The purified enzyme was specific for the L-isomer of lactate. The Km of the purified enzyme for L-lactate is 1.4 mM, but that of the membrane-associated enzyme is 0.5 mM. The enzyme activity was inhibited competitively by D-lactate and non-competitively by oxalate and oxamate. Quinacrine, a flavin analog, also inhibited the activity. The inducible enzyme may serve as a marker of membrane protein in studies of membrane development.