Eukaryotic assimilatory nitrate reductase fractionates N and O isotopes with a ratio near unity
Karsh, KL and Granger, J and Kritee, K and Sigman, DM, Eukaryotic assimilatory nitrate reductase fractionates N and O isotopes with a ratio near unity, Environmental Science and Technology, 46, (11) pp. 5727-5735. ISSN 0013-936X (2012) [Refereed Article]
In order to (i) establish the biological systematics necessary to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (15N/14N and 18O/16O) in the environment and (ii) investigate the potential for isotopes to elucidate the mechanism of a key N cycle enzyme, we measured the nitrate N and O isotope effects (15ε and 18ε) for nitrate reduction by two assimilatory eukaryotic nitrate reductase (eukNR) enzymes. The 15ε for purified extracts of NADPH eukNR from the fungus Aspergillus niger and the 15ε for NADH eukNR from cell homogenates of the marine diatom Thalassiosira weissflogii were indistinguishable, yielding a mean 15ε for the enzyme of 26.6 ± 0.2‰. Both forms of eukNR imparted near equivalent fractionation on N and O isotopes. The increase in 18O/16O versus the increase in 15N/14N (relative to their natural abundances) was 0.96 ± 0.01 for NADPH eukNR and 1.09 ± 0.03 for NADH eukNR. These results are the first reliable measurements of the coupled N and O isotope effects for any form of eukNR. They support the prevailing view that intracellular reduction by eukNR is the dominant step in isotope fractionation during nitrate assimilation and that it drives the 18ε:15ε ≈ 1 observed in phytoplankton cultures, suggesting that this O-to-N isotope signature will apply broadly in the environment. Our measured 15ε and 18ε may represent the intrinsic isotope effects for eukNR-mediated N–O bond rupture, a potential constraint on the nature of the enzyme’s transition state.