Free Radical Inactivation of Lactate Dehydrogenase

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Abstract
The enzyme lactate dehydrogenase (LDH) was irradiated under various conditions to assess the relative contributions of .cntdot.H, .cntdot.OH, H2O2 and .cntdot.O2- to LDH inactivation. .cntdot.OH is the only important inactivating species. The effect of the selective free radicals, .cntdot.(SCN)2-, .cntdot.Br2- and .cntdot.I2- on the activity was studied. In neutral solution, the order of inactivating effectiveness is .cntdot.I2- > .cntdot.OH > .cntdot.Br2- > .cntdot.(SCN)2-. At pH 8.6, .cntdot.OH and .cntdot.Br2- are approximately equal in effectiveness, whereas .cntdot.(SCN)2- is the least efficient. The radiation inactivation of LDH is accompanied by a loss of sulfhydryl groups, and 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 NAD binding protects LDH.

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