Inhibition of assimilatory nitrate reductase activity in soil by glutamine and ammonium analogs

Recent work in our laboratory indicated that the inhibitory effect of ammonium (NH4+) on assimilatory nitrate reductase (ANR) activity in soil is not due to NH4+ per se but to glutamine formed by microbial assimilation of NH4+. To test this conclusion, we studied the effects of eight analogs of L-glutamine (L-glutamic acid gamma-methyl ester, L-glutamic acid gamma-hydrazide, L-glutamic acid gamma-hydroxamate, L-glutamic acid gamma-ethyl ester, L-glutamic acid dimethyl ester, L-asparagine, L-aspartic acid beta-methyl ester, and L-aspartic acid beta-hydroxamate) and two analogs of ammonium (hydroxylamine and methylamine) on ANR activity in soil slurries. The studies with the L-glutamine analogs showed that all except L-glutamic acid dimethyl ester inhibited ANR activity in soil. The sharp contrast observed between the strong inhibitory effect of L-glutamic acid gamma-methyl ester on ANR activity and the complete lack of an inhibitory effect with the corresponding dimethyl ester suggests that only the free-acid form of glutamine effectively inhibits ANR activity. The studies with hydroxylamine and methylamine showed that both of these ammonium analogs inhibited ANR activity in soil and that this inhibition was dependent upon glutamine synthetase activity. This dependence indicates that inhibition of ANR activity by hydroxylamine and methylamine was due to formation of the glutamine analogs L-glutamic acid gamma-hydroxamate and L-glutamic acid gamma-methylamide, respectively. These observations support the conclusion that the inhibitory effect of NH4+ on ANR activity in soil is due to glutamine formed by microbial assimilation of NH4+.