[M-IS14] Biogeochemistry

[MIS14-08] Carbon isotopic compositions of 16 proteinogenic amino acids and their enantiomers for biogeochemical studies

*Naohiko Ohkouchi¹, Yunchen Sun^1,2, Nanako O Ogawa¹, Yoshinori Takano¹, Naoto F Ishikawa¹, Yuta Isaji¹, Thomas M Blattmann^1,3, Junichiro Kuroda^1,2, Hodaka Kawahata^2,4 (1.Japan Agency for Marine-Earth Science and Technology, 2.Atmosphere and Ocean Research Institute, University of Tokyo, 3.ETH Zurich, 4.Waseda University)

Keywords:Biogeochemistry, Carbon isotopes, Amino acids

Here we discuss what controls the carbon isotopic compositions of 16 proteinogenic amino acids and how this information will be used for biogeochemical study. As has been intensively studied, the isotope ratio of nitrogen (¹⁵N/¹⁴N) in the amino groups of amino acids is known to be primarily regulated by aminotransfer reactions and oxidative deamination in metabolic processes. In contrast, the isotope ratio of carbon (¹³C/¹²C), forming the backbone of amino acids, requires careful consideration, as C-C covalent bonds are relatively strong and decarboxylation may not be as crucial in metabolism. According to previous studies, metabolic processes lead to ¹³C-enrichment of certain amino acids, while others exhibit little change. To delve deeper into this issue, we have established a new analytical method utilizing HPLC and nano-EA/IRMS, enabling the accurate determination of carbon isotope ratios for a greater number of amino acids. The method was applied to marine consumers and cultured fish. A wide distribution of ~30‰ was observed in 16 amino acids from marine organisms, with a very consistent ¹³C distribution. In particular, the glycolytic amino acids glycine and serine were found to be quite enriched in ¹³C, sometimes showing values above 0‰. The changes in ¹³C due to predation were particularly large for the non-essential amino acids, +4.4‰ for serine and –2.6‰ for alanine. We also examined the d13C of the D- and L-amino acids (Ala and Glu) of the microbial peptidoglycan, which showed similar values (although there was a surprisingly large (up to 20‰) difference in d¹⁵N between the enantiomers). This difference should originate from the enzymatic transformation of L-Glu to D-Glu, which is catalyzed by non-pyridoxal-5’-phosphate (non-PLP) dependent enzyme. Such isotopic characteristics of amino acids in the environment and in organisms are strongly controlled by their dynamics (synthesis, degradation, predation, transport, etc.) and can be used as a tool to trace amino acids circulating in nature.