Addition of ethanol and some other primary alcohols, except methanol, to cells and protoplasts (but not membrane particles) considerably stimulated the rate of oxygen consumption. This additional respiration was strongly inhibited by 0.1 mM KCN. The cyanide inhibition curve of endogenous substrate oxidation was slightly biphasic while in the presence of ethanol it became clearly biphasic having Ki values of approx. 0.1 and 0.5 mM. Based on the steady-state cytochrome spectra in the presence of 0.1 mM KCN, we attributed the lower Ki to cytochrome a602. Proteolysis of protoplasts external membrane proteins did not change the rate of endogeneous substrate oxidation but prevented the inhibition of this respiration by low concentrations of KCN and stimulation of oxygen consumption by ethanol. The activity of NAD(+)-dependent ethanol dehydrogenase in the cytoplasm was found to be 520 nmol NADH- x min-1 x mg-1 protein. Proteolysis of external membrane proteins apparently inhibits the operation of the cytochrome a602-containing electron transport branch inducing the suppression of electron flow from NADH to oxygen.
The strictly aerobic bacterium Micrococcus luteus was grown in the presence of lactate as sole carbon source under conditions of excess substrates (in batch culture) or under strict lactate (C) or ammonium (N) limitation (in a chemostat, D=O.O2 h-l). KCN (2mM) stimulated the respiration of batch-grown and N-limited cells, and inhibited the oxidase activity of C-limited cells by 4040%. The content and composition of dytochromes and the KCN-dependences of the oxidase activities were found to be the same for the membranes of C-limited and batchgrown cells. The KCN sensitivity of the oxidase activities of isolated membranes decreased in the order NADH > malate > lactate. NAD-dependent lactate and malate dehydrogenase activities were observed in the cytoplasm of C-limited cells but not in that of batch-grown cells. The stimulation of cell respiration by lactate, malate, pyruvate or ethanol caused a marked increase of the steady-state level of NAD+ reduction only in Climited cells. It is assumed that the slow growth of C-limited cells is accompanied by an increase in the formation of NADH, which is oxidized by the KCN-sensitive, tightly-coupled, main branch of the M. luteus respiratory chain. Under non-limited conditions of growth the respiration is due mainly to the direct oxidation of lactate via the weakly coupled alternative branch. The role of this switching of the electron flow from one pathway to the other during the adaptation of the bateria to the slow C-and energy-limited growth is discussed.
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