Inactivation of myocardial dihydrolipoamide dehydrogenase by myeloperoxidase systems: Effect of halides, nitrite and thiol compounds

Dihydrolipoamide dehydrogenase (LADH) Lipoamide reductase activity decrease d whereas enzyme diaphorase activity increased after LADH treatment with my eloperoxidase (MPO) dependent systems (MPO/ H2O2/halide, MPO/NADH/halide an d MPO/H2O2/ nitrite systems. LADH inactivation was a function of the compos ition of the inactivating system and the incubation time. Chloride, iodide, bromide, and the thiocyanate anions were effective complements of the MPO/ H2O2 system. NaOCl inactivated LADH, thus supporting hypochlorous acid (HOC l) as putative agent of the MPO/H2O2/NaCl system. NaOCl and the MPO/H2O2/Na Cl system oxidized LADH thiols and NaOCl also oxidized LADH methionine and tyrosine residues. LADH inactivation by the MPO/NADH/ halide systems was pr evented by catalase and enhanced by superoxide dismutase, in close agreemen t with H2O2 production by the LADH/NADH system. Similar effects were obtain ed with lactoperoxidase and horseradish peroxidase suplemented systems. L-c ysteine, N-acetylcysteine, penicillamine, N-(2-mercaptopropionylglycine), C aptopril and taurine protected LADH against MPO systems and NaOCl. The effe ct of the MPO/_H2O2/NaNO2 system was prevented by MPO inhibitors (sodium az ide, isoniazid, salicylhydroxamic acid) and also by L-cysteine, L-methionin e, L-tryptophan, L-tyrosine, L-histidine and reduced glutathione. The summa rized observations support the hypothesis that peroxidase-generated "reacti ve species" oxidize essential thiol groups at LADH catalytic site.