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Effect of Dissolved Oxygen Tension on the Metabolism of Methylated Amines in Hyphomicrobium X in the Absence and Presence of Nitrate: Evidence for ‘Aerobic’ Denitrification
Abstract
Hyphomicrobium X possesses two different enzymes for the oxidation of dimethylamine, namely dimethylamine dehydrogenase and dimethylamine mono-oxygenase. During growth of the organism in batch culture at dissolved oxygen tensions (DOT) in excess of 30 mmHg in media containing trimethylamine as the carbon and energy source, dimethylamine mono-oxygenase was the only enzyme involved in dimethylamine oxidation. The apparent K m of the mono-oxygenase for oxygen was relatively high (23.2 μ m). The enzyme was less sensitive to inhibition by trimethylamine (K i 4·2 mm) than was dimethylamine dehydrogenase (K i 7·1 μ m) and therefore dimethylamine did not accumulate in the culture medium under these conditions. This was in contrast to observations made during anaerobic growth on trimethylamine. During growth of the organism in dimethylamine-limited chemostat cultures, the specific activities of the mono-oxygenase and the dehydrogenase were dependent on the DOT in the culture. When the DOT in the culture growing at a dilution rate of 0.10 h−1 was decreased below 30 mmHg, the activity of the mono-oxygenase also decreased. In contrast, the activity of dimethylamine dehydrogenase increased, indicating that this enzyme gradually took over at the lower DOT. Below values of 5 mmHg the culture became oxygen-limited and below 3 mmHg it was washed out. When the organism was grown at low DOT in medium supplemented with nitrate, essentially the same results were obtained, except that wash-out of the culture under anaerobic conditions did not occur. The organism was able to carry out denitrification under ‘partly aerobic’ conditions (DOT 0 to 20 mmHg at a dilution rate of 0·10 h−1). Over this range of DOT, the mono-oxygenase hardly played a role in dimethylamine oxidation in vivo because the enzyme was inhibited by the nitrite which accumulated in the culture to a concentration of 3 mm. The potential of Hyphomicrobium X to denitrify in the presence of oxygen was dependent on the growth rate of the organism. At low growth rates (0·01 h−1) synthesis of nitrate reductase, which largely determined the rate of nitrate reduction in the culture, was already significant at relatively high DOT (up to 50 mmHg). At high growth rates (0·15 h−1) nitrate reductase activity became apparent only at DOT values below 6 mmHg. The kinetics and the possible ecological significance of this ‘aerobic’ denitrification process are discussed.
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© Society for General Microbiology, 1980