Metabolism of melamine by Klebsiella terragena

Abstract

Experiments were conducted to determine the pathway of melamine metabolism by Klebsiella terragena (strain DRS-1) and the effect of added NH4+ on the rates and extent of melamine metabolism. In the absence of added NH4+, 1 mM melamine was metabolized concomitantly with growth. Ammeline, ammelide, cyanuric acid, and NH4+ accumulated transiently in the culture medium to maximal concentrations of 0.012 mM, 0.39 mM, trace levels, and 0.61 mM. respectively. In separate incubations, in which cells were grown on either ammeline or ammelide (in the absence of NH4+), ammeline was metabolized without a lag while ammelide metabolism was observed only after 3 h. In the presence of 6 mM added NH4+ (enriched with 5% 15N), ammeline, ammelide, and cyanuric acid accumulated transiently to maximal concentrations of 0.002 mM, 0.47 mM, and trace levels, respectively, indicating that the added NH4+ had little effect on the relative rates of triazine metabolism. These data suggest that the primary mode of melamine metabolism by K. terragena is hydrolytic, resulting in successive deaminations of the triazine ring. Use of 15N-enriched NH4+ allowed estimates of rates of triazine-N mineralization and assimilation of NH4+ -N versus triazine-N into biomass. A decrease in the percent 15N in the external NH'4+ pool, in conjunction with the accumulation of ammelide and/or triazine-derived NH4+ in the culture medium, suggests that the initial reactions in the melamine metabolic pathway may occur outside the cytoplasmic membrane.