Free radical inactivation of lactate dehydrogenase.

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- greater than -OH greater than -Br2- greater than -(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.