Catecholamines (CAs: epinephrine, norepinephrine, dopamine, L-DOPA, 6-hydroxydopamine) ando-diphenols (DOPAC and catechol) enhanced dihydrolipoamide dehydrogenase (LADH) inactivation by Cu(II) /H2O2(Cu-Fenton system). The inhibition of LADH activity correlated with Cu(II), H2O2and CA concentrations. Similar inhibitions were obtained wit! the assayed CAs and o-diphenols. CAs enhanced HO radical production by Cu(II) /H2O2, as demonstrated by benzoate hydroxylation and deoxyribose oxidation; LADH counteracted the pro-oxidant effect of CAs by scavenging hydroxyl radicals. Captopril, dihydrolipo amide, dihydrolipoic acid, DL-dithiothreitol, GSSG, try-panothione and histidine effectively preserved LADH from oxidative damage, whereas N-acetylcysteine, N-(2-mercaptopropionylglycine) and lipoamide were less effective protectors. Catalase (though neither bovine serum albumin nor superoxide dismutase) protected LADH against the Cu(II)/H2O2/CAs systems. Dena tured catalase protected less than the native enzyme, its action possibly depending on Cu-binding. LADH in creased and Captopril inhibited epinephrine oxidation by Cu(II)/H2O2and Cu(II). The summarized evidence supports the following steps for LADH inactivation: (1) reduction of LADH linked-Cu(II) to Cu(I) by CAs; (2) production of HO*from H2O2by LADH-linked Cu(I) (Haber-Weiss reaction) and (3) oxidation of aminoacid residues at the: enzyme active site by site-specifically generated HO*radicals. Hydrogen peroxide formation from CAs autoxidation may contribute to LADH inactivation.