Oxidative damage of bovine serum albumin and other enzyme proteins by iron-chelate complexes

Abstract

Direct oxidative protein damage by iron-nitrilotriacetate (NTA), as well as physiological iron complexes, iron-citrate and iron-ADP was studied in the presence or absence of H 2O 2, using bovine serum albumin (BSA), glucose-6-phosphate dehydrogenase (G-6-PD), glutathione reductase (GSSGRase) and catalase as the target proteins. Both Fe(III)NTA + H 2O 2 and Fe(II)NTA + H 2O 2 caused marked BSA fragmentation which accompanied the decrease in the intrinsic tryptophan fluorescence and appearance of bityrosine fluorescence. However, Fe(III)citrate + H 2O 2 showed only slight BSA fragmentation. In the absence of H 2O 2, Fe(II) NTA but not Fe(III)NTA caused similar but slight BSA fragmentation, which depended on the molecular oxygen. Fe(II)citrate also showed O 2-dependent BSA fragmentation to a comparable degree, however, Fe(II)ADP showed no detectable BSA damage. BSA fragmentation by Fe(II)NTA + O 2 and by Fe(III)NTA + H 2O 2 resulted in the appearance of the new α-amino groups. Electron spin resonance study using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping reagent showed DMPO-OH spin adduct, which suggests the presence of hydroxyl radical, in Fe(III)NTA + H 2O 2, but not in Fe(II)NTA + O 2 system. Fe(II)NTA inactivated G-6-PD and GSSGRase in a O 2-dependent manner, however, G-6-PD was more susceptible to the damage. This enzyme inactivation also accompanied the protein fragmentation and was not due to simple sulfhydryl oxidation. Catalase was not significantly inactivated nor fragmented by Fe(II)NTA + O 2. These findings suggest that the interaction between proteins and iron-chelate complexes is important in iron catalyzed oxidative damage, and that the structure of the chelating agent may determine the target molecules.