INHIBITION BY AMMONIA OF METHANE UTILIZATION IN METHYLOCOCCUS-CAPSULATUS (BATH)

The kinetics of methane uptake by Methylococcus capsulatus (Bath) and its inhibition by ammonia were studied by stopped-flow membrane-inlet mass spectrometry. Measurements were done on suspensions of cells grown in high- and low-copper media. With both types of cells the kinetics of methane uptake are hyperbolic when oxygen is in excess. The apparent K(m) and V(max) for methane uptake are both higher in low-copper cells than in high-copper cells. Ammonia is a simple competitive inhibitor of methane uptake in high-copper cells when the oxygen concentration is above a few mu-M. The findings agree with the assumption that ammonia is a weak alternative substrate for particulate methane monooxygenase. In low-copper cells the effect of ammonia is complicated and cannot be explained in terms of current assumptions on the mechanism of soluble methane monooxygenase. Our data indicate that ammonia inhibition is likely to be a more serious problem in connection with cultivation in low-copper medium than in high-copper medium.