Laccase-Catalyzed Oxidation of Iodide and Formation of Organically Bound Iodine in Soils

Laccase oxidizes iodide to molecular iodine or hypoiodous acid, both of which are easily incorporated into natural soil organic matter. In this study, iodide sorption and laccase activity in 2 types of Japanese soil were determined under various experimental conditions to evaluate possible involvement of this enzyme in the sorption of iodide. Batch sorption experiment using radioactive iodide tracer (¹²⁵I^–) revealed that the sorption was significantly inhibited by autoclaving (121 °C, 40 min), heat treatment (80 and 100 °C, 10 min), γ-irradiation (30 kGy), N₂ gas flushing, and addition of reducing agents and general laccase inhibitors (KCN and NaN₃). Interestingly, very similar tendency of inhibition was observed in soil laccase activity, which was determined using 2,2′-azino-bis­(3-ethylbenzothiazoline-6-sulfonate) (ABTS) as a substrate. The partition coefficient (K_d: mL g^–1) for iodide and specific activity of laccase in soils (Unit g^–1) showed significant positive correlation in both soil samples. Addition of a bacterial laccase with an iodide-oxidizing activity to the soils strongly enhanced the sorption of iodide. Furthermore, the enzyme addition partially restored iodide sorption capacity of the autoclaved soil samples. These results suggest that microbial laccase is involved in iodide sorption on soils through the oxidation of iodide.