Abstract
The stable nitrogen isotope ratio of glutamic acid is increased by 〜8.0‰ during trophic level changes in ecological food webs. It has been hypothesized that significant isotopic fractionation occurs associated with glutamic acid metabolism during transamination or deamination in heterotrophic animals. However, few studies have investigated the isotopic fractionation factor (α) in this process, even though knowledge of this factor is critical in quantitatively evaluating the flux of amino acid metabolism in animals. In this study we determined the value of a associated with an enzymatic transamination of glutamic acid to aspartic acid in vitro. We found 0.1‰ and 2.2‰ enrichment of 15N following the enzymatic transamination of glutamic acid for 10 and 60min, respectively. Using a Rayleigh model, the α value in this reaction was estimated to be 0.9958. Although this value is associated with much uncertainty (0.9949-0.9968) when applying it to test for an 8.0‰ enrichment, 8‰ of 15N-enrichment for a α value of 0.9958 corresponds to 86% of the glutamic acid being reacted during transamination.
