Summary: A freshwater Pseudomonas sp. was grown in continuous culture under steady-state conditions in l-lactate-, succinate-, glucose- or ammonium-limited media. Under carbon limitation, the NAD(H) (i.e. NAD + NADH) concentration of the organisms increased exponentially from approximately 2 to 7 μmol/g dry wt as the culture dilution rate (D) was decreased from 0·5 to 0·02 h−1. Organisms grown at a given D in any of the carbon-limited media possessed very similar levels of NAD(H). Therefore, under these conditions, cellular NAD(H) was only a function of the culture D and was independent of the nature of the culture carbon source. D had no influence on the NAD(H) content of cells grown under ammonium limitation. In contrast, cellular NADH concentration was not influenced by D in carbon- or ammonium-limited media. In l-lactate-limited medium, bacteria possessed 0·14 μmol NADH/g dry wt: very similar levels were found in organisms grown in the other media. The results are consistent with those of Wimpenny & Firth (1972) that bacteria rigidly maintain a constant NADH level rather than a constant NADH:NAD ratio. NADP(H) (i.e. NADP + NADPH) and NADPH levels were also not influenced by changes in the culture carbon source or in D: in l-lactate-limited medium these concentrations were 0·97 and 0·53 μmol/g cell dry wt, respectively. The NADPH:NADP(H) ratio was much higher than the NADH:NAD(H) ratio, averaging 55% in carbon-limited cells.
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