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Correlation Between the Loss of the Chaperone‐like Activity and the Oxidation, Isomerization and Racemization of Gamma‐irradiated Alpha‐crystallin ¶

Alpha‐crystallin possesses a molecular chaperone‐like activity that prevents proteins from aggregating; however, the mechanism of this activity is not well known. Here we have taken gamma‐irradiated alpha‐crystallin and studied the relationship between the decrease in chaperone‐like activity and the modifications such as oxidation, isomerization and racemization of amino acids in this molecule. We found that the chaperone‐like activity of alpha‐crystallin decreased with increasing gamma irradiation. After 4000 Gy gamma irradiation the activity of alpha‐crystallin was reduced to 40% of the level of nonirradiated, native alpha‐crystallin. The circular dichroism spectrum showed that the secondary structure of the irradiated alpha‐crystallin had not changed. However, its tertiary structure appeared to change following more than 1000 Gy irradiation. Sodium dodecyl sulfate‐polyacrylamide gel electrophoresis also indicated that cross‐linking of alpha‐crystallin increased with increasing radiation doses. Irradiated and nonirradiated alpha‐crystallin was subjected to trypsin digestion and peptide analysis by reverse‐phase high‐performance liquid chromatography and mass and sequence analysis. Depending on the radiation dose, Met‐1 of alpha A‐crystallin was oxidized to methionine sulfoxide. In addition, Asp‐151 of alpha A‐crystallin was isomerized to the beta‐Asp form after irradiation, and racemization of Asp‐151 decreased. Thus, the loss of the chaperone‐like activity of alpha‐crystallin is related to changes in its isomerization, oxidation and racemization.