AN INVESTIGATION OF N METABOLISM AND PH REGULATION IN SPHAGNUM USING IN-VIVO NUCLEAR-MAGNETIC-RESONANCE AND STABLE-ISOTOPE MASS-SPECTROMETRY

Glutamine synthetase (GS) and glutamate dehydrogenase (GDH) are both p resent in Sphagnum and in principle both enzymes could be involved in the assimilation of ammonium. (NH4+)-N-15 application led to the forma tion gamma-N-15-glutamine, followed by a-amino glutamate/glutamine as observed by in vivo N-15 NMR. No assimilation was detected in the pres ence of the GS-specific inhibitor methionine sulfoximine. This labelli ng pattern suggests, that ammonium assimilation in Sphagnum is mediate d solely by the GS/glutamate synthase pathway. The application of N-15 -labelled glutamate led to the liberation of (NH4+)-N-15. indicating t hat GDH mainly catalyses the deamination of glutamate. it was possible to record good-quality in vivo P-31 NMR spectra of Sphagnum flexuosum tissue, and the chemical shift of the orthophosphate resonances indic ated that cytoplasmic pH was not affected during the ammonium applicat ion, whereas the vacuolar pH was slightly reduced.