The Reaction between Iodoacetic Acid and Denatured Egg Albumin * by Lawrence Rosner

The reaction between iodoacetic acid and sulfhydryl compounds has received much attention in recent years. Dickens (1) has shown that this reaction proceeds with the formation of HI and the substitution of a carboxymethyl group for the hydrogen of the thiol group. Smythe (2) has measured the rate of reaction of iodoacetic acid with various sulfhydryl compounds by estimation of the carbon dioxide change in a COz-bicarbonate buffer as a result of the HI produced. Rapkine (3) showed that iodoacetic acid may react not only with the sulfhydryl groups of relatively simple molecules, such as cysteine and glutathione, but also with those of proteins. It was found by Rapkine that as denatured egg albumin stood with increasing amounts of iodoacetic acid the intensity of its nitroprusside test diminished until it finally disappeared. Mirsky and Anson (4) have used this reaction in their method for determining available thiol groups in proteins. These investigators found that iodoacetic acid might react with only a portion or with none of the sulfhydryl groups of native proteins, as evidenced by the continued presence after the reaction of part or all of the original protein cysteine in the protein hydrolysate. Denaturation of the protein, however, causes all of the sulfhydryl groups to be capable of interaction with iodoacetic acid, there being, in this case, no free cysteine in the protein hydrolysate after the reaction. In the experiments of Mirsky and Anson iodoacetic acid was allowed to react with coagulated denatured egg albumin; these