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.