RESUMO
Background: In spite of the fact that the standard test for nitrate reductase activity is negative for Mycobacterium avium, it can grow in a defined minimal medium with either nitrate (NO3) or nitrite (NO2) as sole nitrogen sources. Methods: NO3-and NO2-reductase activities were measured in soluble and membrane fractions of aerobically grown cells of M. avium and those grown aerobically and shifted to anaerobiosis. Results: NO3- and NO2-reductase activities were only detected in the membrane fractions and the two enzyme activities were significantly reduced if cells were grown aerobically in the presence of ammonia (NH4). The NO2-reductase activity of membrane fractions was 2-fold higher than that of NO3-reductase consistent with the fact that NO3-reductase activity of M. avium cannot be detected if measured by nitrite formation. Membrane fractions of M. avium cells grown 1 week aerobically and then 2 weeks under anaerobic conditions had NO3-and NO2-reductase activities. Conclusion: The results are consistent with the presence of assimilatory NO3-and NO2-reductase activities in cells of M. avium grown under aerobic conditions. Further, the data suggest that a shift to anaerobic conditions results in the appearance of ammonium-insensitive NO3-and NO2-reductase activities; quite possibly that function in a dissimilatory role (redox balancing).
