Summary: The initial metabolism of some methylated and ethylated amines, which were used as a nitrogen source but not as the sole carbon source by the yeasts and involved a peroxisomal amine oxidase which produced ammonium ions, hydrogen peroxide, and formaldehyde or acetaldehyde. The aldehydes so formed were either oxidized via their corresponding carboxylic acids or, depending on the organism and the aldehyde, also partly assimilated into cell material. The synthesis of amine oxidase, which was paralleled by the development of peroxisomes in the cells, was repressed in the presence of ammonium ions and derepressed under nitrogen limitation. Amines were not required as inducers of enzyme synthesis. Utilization of ethylated amines, but not of methylated amines, as a nitrogen source resulted in a significant increase in cell yield. In both yeasts ammonium ions were assimilated mainly by way of NADPH-dependent glutamate dehydrogenase. The activity of this enzyme increased drastically in cells grown under ammonium or amine limitation or, under carbon limitation, in the presence of amines as the sole source of nitrogen. Under the latter conditions free ammonium was not detectable in the culture supernatant, while the amount of amines utilized was just sufficient to account for the amount of cell material produced. This indicated that during growth with amines as the nitrogen source the physiological condition that the cells experience is in fact one of ammonium limitation. Our results suggested that the rate of amine oxidation was determined by the intracellular concentration of ammonium and determined via repression of amine oxidase synthesis. Due to this control and the high nitrogen to carbon ratio of amines, sustained growth of the methylotrophic yeast on methykmine and of both species on ethylamine as a carbon source is not possible, even though these organisms are able to grow on the related compounds methanol and ethanol.


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