Studies in Enzyme Cytochemistry. IV. Kinetics of Aerial Oxidation of Indoxyl and Some of Its Halogen Derivatives

S. Cotson, S. J. Holt

Abstract

Aerial oxidation of indoxyls to indigoid dyes has been studied kinetically by a spectrophotometric method. With excess oxygen, the oxidation is first order with respect to the indoxyl under alkaline conditions. Under acidic conditions, the reaction is much slower and deviations from first-order kinetics occur. First-order velocity constants have been determined, where appropriate, and have been shown to lie between $1.8\times 10^{-3}$ and approximately $8\times 10^{-3}$ $\text{s}^{-1}$ at 20 degrees C and at pH 8.0. In all cases the oxidation rates are too low to permit accurate localization of enzymes in sites of radius $<10\mu$. The activation energy of aerial oxidation of unsubstituted indoxyl at pH 8.0 is 15.8 kcal. During oxidation, small amounts of the corresponding isatins and anthranilic acids are formed as by-products; also approximately one molecule of hydrogen peroxide is formed for every molecule of indoxyl oxidized. This may lead to certain staining artifacts if aerial oxidation of indoxyls is the chromogenic reaction in cytochemical staining processes. A free radical mechanism for the aerial oxidation, involving intermediate formation of leucoindigo, is postulated.