Physicochemical studies on ovalbumin. 3. The sulphydryl and disulphide contents of ovalbumin and an iodine-modified derivative

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Abstract
The reaction between ovalbumin and iodine at 0[degree] in the presence of M-potassium iodide has been studied the end point, obtained potentiometrically, indicated a stoicheiometry of 5 iodine atoms/molecule of ovalbumin. On standing with excess of iodine, a total of 6-7 iodine atoms react. An amperometric-titration procedure, with either mercuric chloride or methylmercuric iodide, has been used to estimate the sulphydryl and disulphide content of both the native material and the product obtained after reaction with 5 equiv. of iodine. The results indicate that ovalbumin possesses 4 sulphydryl and 2 disulphide groups/molecule, and the modified product has 2 sulphydryl and 2 disulphide groups/molecule. Iodine analysis of the iodine-treated material with a radiochemical procedure shows the presence of 1 iodine atom/molecule. No change in optical rotation accompanies the iodine treatment, but the solubility of the product is greatly reduced compared with the native material. Solutions of the product are more stable at low temperatures than solutions of the native material. The effect of environmental conditions on the stoicheiometry indicates that several products can potentially be formed. Under the conditions used here, the 5:1 product is formed first. No conditions are yet known under which iodine is a specific reagent for sulphydryl groups alone.