Studies on the mechanism of action of ionizing radiations. X. Effect of x-rays on some physicochemical properties of proteins

Abstract

On x-irradiation of aqueous solutions of serum albumin, serum globulin, and egg albumin, there was an increase in the absorption of ultraviolet light, the increase being more marked around 2400 A. The increase in optical density was proportional to the x-ray dose and inversely proportional to protein concentration. Addition of salts protected greatly the protein solutions against these changes. The increase in optical density was more marked in alkaline solutions. Irradiation of oxygenated solutions showed a greater increase in optical density around 2400 A. The absorption spectrum changes seem to be due to oxidation tyrosine residues and of other oxidizable groups. Irradiation of 0.07% aqueous solutions of serum albumin with 75,000 r. at 25 °C. produced precipitation, which did not occur on irradiation at the temperature of ice water. Precipitation was avoided by increasing the protein concentration or by the addition of salts. The viscosity of proteins increased slightly on irradiation with 50,000 r. There were no marked changes in the electrophoretic mobilities or the sedimentation constants. On irradiation with 100,000 and 200,000 r. there was appearance of a second component, which may be taken as a dimer. Addition of cysteine prevented the appearance of the dimer. However, once the dimer was formed, cysteine did not revert the protein to its original structure.