TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 52, 2018

Fractionation of Stable Nitrogen Isotopes (15N/14N) during Enzymatic Deamination of Glutamic Acid: Implications for Mass and Energy Transfers in the Biosphere

Akiko S. Goto1,2,*, Kasumi Miura2, Takashi Korenaga2,3, Takashi Hasegawa1, Naohiko Ohkouchi4 and Yoshito Chikaraishi4,5

1Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
2Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397 Japan
3Faculty of Risk and Crisis Management, Chiba Institute of Science, 3 Shiomi-cho, Choshi, Chiba 288-0025, Japan
4Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
5Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo 060-0819, Japan

(Received January 5, 2017; Accepted December 7, 2017)

Abstract: Isotopic fractionation of nitrogen associated with the trophic transfer of amino acids in food webs has recently been used as a powerful tool to estimate the accurate trophic levels of heterotrophic organisms. During the grazing process (i.e., trophic transfer), the amino acid, glutamic acid, is significantly enriched in 15N (by ∼6‰–9‰) from diets to consumer heterotrophs; this is most likely caused by isotopic fractionation with the preferential deamination of 14N-amino group in glutamic acid during its metabolism. However, few studies determined the isotopic fractionation factor (α) of this process, which limits our understanding of the mechanism responsible for the isotopic fractionation and thus restricts its applicability in assessing the mass and energy transfers with respect to the amino acid assimilation/dissimilation cycle in the biosphere. In this study, we evaluate the α value associated with the enzymatic deamination of glutamic acid in vitro. Glutamic acid is gradually enriched in 15N by up to 4.0‰, when the deamination flux is increased up to 45.4%. The α value calculated is 0.9938 ± 0.0005 if the Rayleigh fractionation model is applied to the enrichment in 15N. Thus, we demonstrate the relationship between isotopic fractionation and deamination flux: for xample, 8.0‰ fractionation corresponds to that 72 ± 3% of the diet-derived glutamic acid is deaminated in the consumer species at each shift of trophic levels in a food web.
Key words: glutamic acid, deamination, isotopic fractionation factor (α), nitrogen isotopic fractionation, energy transfer

*Corresponding author E-mail: akigoto@staff.kanazawa-u.ac.jp