On the so-called “iodide oxidase”. Mechanism of iodide oxidation by <i>Aspergillus</i>

Abstract

IT is well known that different organisms have the property of oxidizing iodides to free iodine. This reaction was first described by Sch6nbein [1848]. It was used by Bach & Chodat [1904] and among other workers by Wolff [1914; 1917] who suggested that the oxidation was due to a secondary reaction between the iodide and the oxidation products of polyphenols. Raciborski [1905], however, found that A8pergillus niger excreted an iodide-oxidizing system into its medium, but that both phenol oxidases and peroxidase were absent. Hence he postulated the existence of a specific iodide ooxidase. Korczewski [1922] confirmed Ra"ciborski's observation and showed that the iodide-oxidizing system was destroyed by heat and acid and inhibited by KCN, NH20H and peroxide. Further, if the iodine is not removed an equilibrium is reached and the reaction soon comes to an end. He always found peroxide present in the medium, but, since its concentration was very low, he concluded that it could not be responsible for the oxidation of iodide. He found, however, that the addition of peroxidase caused a marked acoeleration of iodide oxidation. Korczewski concluded that the culture medium contains an oxygenase similar to that postulated by Bach & Chodat [1902; 1903;' 1904], and that this enzyme forms with molecular oxygen a superoxide which is responsible for the oxidation of iodide. Methods The methods used for preparing an active iodide-oxidizing medium were basedon Korczewski's work. A.' niger was grown on a medium composed of: 4 % sucrose, 0-2 % K2HPO4, 1 %(NH4)2S04, 0 05 % MgSO4, 7H20. It was found