NADH/NADratios and internal pyruvate concentrations were determined during switches between aerobic and anaerobic steady-state conditions of glucose-limited chemostat cultures of . During the switch experiments, changes in catabolic fluxes were observed: transition from anaerobic to aerobic conditions resulted in a complete and instantaneous conversion of glucose into acetate and CO via the pyruvate dehydrogenase complex, while during a switch from aerobic to anaerobic conditions the culture became homolactic. A similar switch to a homolactic fermentation was observed upon release of the limitation by addition of a glucose pulse to the culture. In sharp contrast to this, a pyruvate pulse resulted in an increase of both pyruvate formate-lyase and pyruvate dehydrogenase complex activity. Furthermore, acetoin was formed during a pyruvate pulse, probably due to a dramatic increase in internal pyruvate concentration. Regulation of the catabolic fluxes over the various pyruvate-catabolizing enzymes is discussed in view of the observed changes in internal pyruvate concentrations and NADH/NADratios.


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