Abstract
When Escherichia coli, K-10, was anaerobically grown on lactate in the presence of nitrate, the products formed were acetate (recovery in C atoms, 25%), formate (17%) and small amounts of pyruvate and ethanol. Nitrate was quantitatively reduced to nitrite. 14CO2 was formed from [2- 14C] lactate, whereas 14CO2 formation from [2-14C] glucose was almost nil. Citrate synthase, isocitrate dehydrogenase, 2-oxoglutarate dehydrogenase, succinyl-CoA synthetase, and malate dehydrogenase were twice as active in extracts from cells grown on lactate and nitrate compared with extracts from cells grown aerobically on glucose and 4-12 times as acitve with extracts from cells grown on glucose and nitrate. Activities of aconitate hydratase, succinate dehydrogenase, and fumarate hydratase were present in cells grown on lactate and nitrate. These enzymes showed maxima during the exponential phase of growth and activities of aconitate hydratase as well as succinate dehydrogenase fell to almost zero during the stationary phase. Essentially no activities of these three enzymes were found in cells grown on glucose and nitrate. Oxidation of lactate to carbon dioxide during nitrate respiration seems to be due to the operation of the tricarboxylic acid cycle in lactate-grown cells. Glucose- grown cells seem to lack this cycle.
