FIGURE 8.

ESI-MS characterization of l-Trp (A), the reaction product of l-Trp and H₂¹⁶O₂ catalyzed by ferric TDO (B), and the TDO reaction using isotope-labeled H₂¹⁸O₂ as the oxygen donor (C). A ¹⁶O-NFK sample, generated from the TDO reactivation process, was dissolved in an ¹⁸O-enriched water (¹⁶O/¹⁸O ratio of 3:5) after concentrating the reaction solution and removing the enzyme by filtration (D). A copy of sample D was further diluted by ¹⁶O water to reach a final ¹⁶O/¹⁸O ratio of 60:1 (1.6% ¹⁸O) (E). TDO reaction was carried out in H₂¹⁸O-based Tris-HCl buffer (F).