The stable nitrogen isotope ratios of amino acids in organisms have been employed to understand the diet-resource utilization and trophic energy transfer among organisms in food webs. This method is proposed by empirical observations that a universal isotopic enrichment in ¹⁵N is found in glutamic acid for diverse species. In general, glutamic acid is deaminated with enzymatic reaction, incorporated directly into the tricarboxylic acid cycle, and extracted energy under aerobic metabolism. However, little knowledge is available for change in the ratio associated with ‘anaerobic metabolism’ such as glycolysis and fermentation in organisms. Therefore, we investigated laboratory-cultured samples to see the change in the ratio under anaerobic metabolisms. The eukaryote Saccharomyces cerevisiae and the bacteria Lactococcus lactis were incubated with organic substrates and were compared the ratios between before (i.e., t=0, non-fermented) and after incubation (i.e., t=f, fermented). We found negligible isotopic fractionation for phenylalanine, as well as for isoleucine, proline, and glutamic acid, but variable for glycine, alanine, valine, and leucine in the samples. Our results suggest that activity of anaerobic metabolism can be recorded in the latter amino acids, and that comparison of the isotope ratios between two metabolically-different amino acids (e.g., alanine and glutamic acid) will be useful to evaluate the contribution of anaerobic metabolism in biological and environmental samples.

Primary Presenter: Yuko Takizawa, Hokkaido University (takizaway@lowtem.hokudai.ac.jp)

Authors:

Yoshito Chikaraishi, Hokkaido University  (ychikaraishi@lowtem.hokudai.ac.jp)