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Summary
The enzyme lactate dehydrogenase (LDH) has been irradiated under various conditions to assess the relative contributions of ·H, ·OH, H2O2 and ·O2− to LDH inactivation, and it is concluded that ·OH is the only important inactivating species. Further the effect of the selective free radicals, ·(SCN)2−, ·Br2− and ·I2− on the activity has been studied. In neutral solution, the order of inactivating effectiveness is ·I2− > ·OH > ·Br2− > ·(SCN)2−. At pH 8·6, ·OH and ·Br2− are approximately equal in effectiveness, whereas ·(SCN)2− is the least efficient. The radiation inactivation of LDH is accompanied by a loss of sulphydryl groups, and it is suggested that the primary target for radiation damage in LDH is the active site cysteine-165. Subsequent conformational changes are suggested to account for the apparent loss of coenzyme-binding ability and changes in the enzyme's kinetic parameters. The effect of bound coenzyme (NAD) on radiation-induced inactivation of N2O and air-saturated solutions was also investigated, and it is shown that NAD binding protects LDH.