Iron Mobilization from Crocidolite Results in Enhanced Iron-Catalyzed Oxygen Consumption and Hydroxyl Radical Generation in the Presence of Cysteine

Abstract

The reactivity of iron on crocidolite with O₂ in the presence of cysteine was determined and compared with that of iron mobilized from crocidolite by citrate, EDTA, or nitrilotriacetate (NTA). Suspension of crocidolite in 50 mM NaC1, pH 7.5, in the absence of a reducing agent, did not result in a measurable amount of O₂ consumption or OH˙ generation. Addition of cysteine in the absence of other iron chelators increased crocidolite-dependent O₂ consumption. Therefore, iron on crocidolite had limited redox activity in the presence of cysteine. However, when iron was mobilized from crocidolite it had increased redox activity. Citrate, EDTA, or NTA (1 mM) mobilized 75, 44, or 31 µM iron, respectively, in preincubations up to 76 h, and increased O₂ consumption upon addition of cysteine to 3.1, 2.1 or 1.1 nmol O₂ consumed/mg/min, respectively. Using ESR spectroscopy with the spin trap DMPO, the amount of OH˙ formed in the presence of crocidolite was determined and found to be directly related to the amount of iron mobilized from crocidolite by EDTA. In the presence of a chelator, both O₂ consumption and OH˙ generation depended only upon the presence of a component(s) mobilized from crocidolite by the chelator. Pretreatment of the crocidolite with desferrioxamine B (1 mM), an iron chelator, inhibited O₂ consumption in the presence or absence of citrate. The results of this study suggest that iron is responsible for the crocidolite-dependent reactions with O₂ and that mobilization of iron by citrate, NTA, or EDTA greatly enhances its reactivity with O₂. Furthermore, mobilization of iron from crocidolite by EDTA greatly enhanced the generation of OH˙ Therefore, intracellular mobilization of iron from asbestos may increase reactions with O₂ leading to OH˙ formation, DNA damage, and cancer.