Sulfhydryl Groups in Proteins

Egg albumin in the native, unaltered state, does not give tests characteristic of sulfhydryl groups. When, however, this protein is treated in any one of several ways, such as by heat (8, 9, 17), ultraviolet irradiation (lo), shaking (lo), or by solution in urea or other amides (7), free sulfhydryl groups make their appearance. The amount of free -SH groups appearing in egg albumin through the action of urea, guanidine hydrochloride, and their derivatives has been estimated (6). Considerable differences in the number of such groups liberated, which depend on the nature and the concentration of the reagent used, were noted. The investigation of these groups has been extended to a number of proteins whose molecular weights have been reported to be smaller in urea solutions than in water. The three seed globulins, edestin, excelsin, and amandin, have practically the same molecular weight in water (4, 5, 15, 16). Their molecular weights in 4 M urea are also almost the same (4), but are reported to be only oneeighth as great as in water. Burk and Greenberg (5) showed that the molecular weight of horse hemoglobin in urea solution was 34,000, a value just half the molecular weight of this protein in water. The results of these investigators were confirmed by Wu and Yang (18) and by Steinhardt (14), the latter showing further that the molecular weight in acetamide solutions was comparable to that in urea. It has been suggested that the difference in the molecular weights of certain proteins in water and in urea is due in part to a scission of the S-S linkages of the protein in the latter solvent with the