The Determination of Protein Sulfhydryl Groups with Iodine and O-Iodosobenzoate by an Amperometric Titration

Iodimetric titrations have been used extensively to determine reducing matter in many biological systems. Iodine itself in acid media reacts not only with such low molecular weight reductants as ascorbic acid and glutathione but also with some proteins. Hess and Sullivan (6) found that the amount of iodine reduced by native proteins in acid solution corresponded to their cysteine contents as determined by colorimetric analysis of acid hydrolysates. It is fairly well established that there are several degrees of reactivity or availability of the sulfhydryl groups of proteins. Anson (1) found that iodine will react with all of the sulfhydryl groups of native egg albumin and iodoacetamide with about half of them but that the reagents nitroprusside and acid ferricyanide show a negative test.