CATECHOLAMINES ENHANCE DIHYDROLIPOAMIDE DEHYDROGENASE INACTIVATION BYTHE COPPER FENTON SYSTEM - ENZYME PROTECTION BY COPPER CHELATORS

Catecholamines (CAs: epinephrine, norepinephrine, dopamine, L-DOPA, 6- hydroxydopamine) and o-diphenols (DOPAC and catechol) enhanced dihydro lipoamide dehydrogenase (LADH) inactivation by Cu(II)/H2O2 (Cu-Fenton system). The inhibition of LADH activity correlated with Cu(II), H2O2 and CA concentrations. Similar inhibitions were obtained with the assa yed CAs and o-diphenols. CAs enhanced HO. radical production by Cu(II) /H2O2, as demonstrated by benzoate hydroxylation and deoxyribose oxida tion; LADH counteracted the pro-oxidant effect of CAs by scavenging hy droxyl radicals. Captopril, dihydrolipoamide, dihydrolipoic acid, DL-d ithiothreitol, GSSG, trypanothione and histidine effectively preserved LADH from oxidative damage, whereas N-acetylcysteine, N-(2-mercaptopr opionylglycine) and lipoamide were less effective protectors. Catalase (though neither bovine serum albumin nor superoxide dismutase) protec ted LADH against the Cu(II)/H2O2/CAs systems. Denatured catalase prote cted less than the native enzyme, its action possibly depending on Cu- binding. LADH increased and Captopril inhibited epinephrine oxidation by Cu(II)/H2O2 and Cu(II). The summarized evidence supports the follow ing steps for LADH inactivation: (1) reduction of LADH linked-Cu(II) t o Cu(I) by CAs; (2) production of HO. from H2O2 by LADH-linked Cu(I) ( Haber-Weiss reaction) and (3) oxidation of aminoacid residues at the e nzyme active site by site-specifically generated HO. radicals. Hydroge n peroxide formation from CAs autoxidation may contribute to LADH inac tivation.