Binding of iron and inhibition of iron-dependent oxidative cell injury by the 'calcium chelator' 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA)

A role for increases in intracellular calcium (Ca²⁺) has been suggested in the pathophysiology of various forms of oxidant mediated cell injury. In recent studies, we found that iron bound to the Pseudomonas aeruginosa siderophore, pyochelin, augments oxidant-mediated endothelial cell injury by catalyzing the formation of hydroxyl radical (HO^.). To investigate the role of Ca²⁺ in this process, the effects of two Ca²⁺ chelating agents, Fura-2 and 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA), were assessed. BAPTA, but not Fura-2, was protective against H₂O₂/ferripyochelin-mediated injury. Subsequent data suggested that chelation of iron rather than Ca²⁺ by BAPTA was most likely responsible. Spectrophotometry demonstrated that both ferrous (Fe²⁺) and ferric (Fe³⁺) iron formed a complex with BAPTA. The affinity of BAPTA for the metals was Fe³⁺ > Ca²⁺ > Fe²⁺ BAPTA was found to decrease markedly iron catalyzed production of HO^. and/or ferryl species when analyzed by spin trapping. Although our results do not definitively prove that BAPTA protects endothelial cells from ferripyochelin-associated damage by chelating iron, these data indicate chat caution must be exercised in utilizing protective effects of intracellular 'Ca²⁺ chelating agents' as evidence for a role of alterations in cellular Ca²⁺ levels in experimental conditions in which iron-mediated oxidant production is also occurring.