Eukaryotic Assimilatory Nitrate Reductase Fractionates N and O Isotopes with a Ratio near Unity

• DOI: 10.1021/es204593q
• Publication Date: 2012-04-25T15:47:07Z
• Authors: Kristen L. Karsh; Julie Granger; K. Kritee; Daniel M. Sigman
• Keywords: Kinetic isotope effect; Isotopes of nitrogen; Nitrogen Assimilation

In order to (i) establish the biological systematics necessary interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate ((15)N/(14)N (18)O/(16)O) in environment (ii) investigate potential for isotopes elucidate mechanism a key N cycle enzyme, we measured O effects ((15)ε (18)ε) reduction by two assimilatory eukaryotic reductase (eukNR) enzymes. The (15)ε purified extracts NADPH eukNR from fungus Aspergillus niger NADH cell homogenates marine diatom Thalassiosira weissflogii were indistinguishable, yielding mean enzyme 26.6 ± 0.2‰. Both forms imparted near equivalent fractionation on isotopes. increase (18)O/(16)O versus (15)N/(14)N (relative their natural abundances) was 0.96 0.01 1.09 0.03 eukNR. These results are first reliable measurements coupled any form They support prevailing view that intracellular is dominant step during assimilation it drives (18)ε:(15)ε ≈ 1 observed phytoplankton cultures, suggesting this O-to-N signature will apply broadly environment. Our (18)ε may represent intrinsic eukNR-mediated N-O bond rupture, constraint nature enzyme's transition state.