Nitrogen assimilation during growth of on methylated amines as sole nitrogen source involves NADP-dependent glutamate dehydrogenase. Changes in enzyme activities during the adaptation of the yeast from growth on ammonium to growth on trimethylamine were examined. No ammonia, dimethylamine or monomethylamine could be detected in the medium during growth on trimethylamine. When two methylated amines were supplied together, they were used simultaneously, although monomethylamine was metabolized more quickly than the others. When cells were grown on a low concentration of ammonium plus higher concentrations of di- or trimethylamine, the ammonium was used first. NADP-dependent glutamate dehydrogenase was the first enzyme to be derepressed, followed by methylamine oxidase and formaldehyde dehydrogenase. Di- and trimethylamine mono-oxygenase activities only appeared when the ammonium concentration fell below 0.5 mM. At this point amine utilization could be detected and no diauxic lag was observed in the growth curve. During growth on limiting ammonium, there was an increase in the activity of methylamine oxidase (150-fold) and catalase (5-fold) in the absence of any amine, but no amine mono-oxygenase activity was detected. Addition of ammonium ions to cultures growing on dimethylamine produced an immediate repression of synthesis of methylamine oxidase, NADP-dependent glutamate dehydrogenase and the two amine mono-oxygenases. An inverse correlation was found between intracellular ammonium concentration and methylamine oxidase activity. Ammonium ions also inhibited the uptake of dimethylamine or trimethylamine by washed suspensions of dimethylamine-grown cells. It is concluded that the control of methylamine oxidase and catalase and (independently) of NADP-dependent glutamate dehydrogenase is by repression of enzyme synthesis by ammonium, while expression of amine mono-oxygenases seems to require the amine to be present in the medium. Formaldehyde and formate dehydrogenases seem also to be induced by their respective substrates.


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