Methylmercury efflux from brain capillary endothelial cells is modulated by intracellular glutathione but not ATP

Abstract

To reach its target tissue, methylmercury must traverse brain capillary endothelial cells, the site of the blood-brain barrier. Methylmercury uptake from blood plasma into these cells is mediated in part by an amino acid carrier that transports the methylmercury-L-cysteine complex; however, the mechanism by which it is released from the endothelial cells into brain interstitial space is unknown. Using bovine brain capillary endothelial cells in culture, the present study examined the hypothesis that methylmercury is transported out of these cells as a glutathione (GSH) complex, GSH concentration in cultured bovine brain capillary endothelial cells was 13.1 ± 3.3 nmol/mg protein. Depletion of intracellular GSH in [203Hg]methylmercury-preloaded cells by exposure to 1-chloro-2,4-dinitrobenzene or diethyl maleate decreased the rate of [203Hg]methylmercury efflux. Incubation of [203Hg]methylmercury-preloaded cells with high concentrations of S-methylglutathione, S-ethylglutathione, S-butylglutathione, and sulfobromophthalein-glutathione inhibited [203Hg]methylmercury efflux. The GSH analogs γ-glutamylglycylglycine and ophthalmic acid also inhibited [203Hg]methylmercury efflux, but to a lesser degree than the glutathione S-conjugates, whereas L-leucine, L-methionine, and L-alanine bad no effect. Efflux was not affected by depletion of intracellular ATP with 2-deoxyglucose or antimycin A. These results indicate that complexation with GSH and subsequent transport of the complex by an ATP-independent mechanism may be involved in the transport of methylmercury out of brain capillary endothelial cells.