Enhancement of methylmercury toxicity by L-cystine in cultured mouse blastocysts

Expanded mouse blastocysts incubated with 1 to 2 microM methylmercury (MeHg) in modified Eagle's basal medium (BME + AA), which contains amino acids, collapsed and degenerated within 24 h. In contrast, blastocysts incubated with the same concentration of MeHg in egg culture medium (ECM), which does not contain amino acids, survived and remained expanded as control embryos did. By systematically omitting each BME amino acid from BME + AA and adding each BME amino acid to egg culture medium, we determined that L-cystine (0.5 mM in BME + AA) was the component of BME + AA that was responsible for the enhancement of the toxicity of MeHg. The shortest incubation time during which the cystine-enhanced MeHg toxicity became irreversible was 2 h, and the addition of any of the neutral BME amino acids (except threonine) or non-BME neutral amino acids (alanine, glycine, or serine) during the 2 h incubation eliminated or reduced the cystine-enhanced MeHg toxicity. Basic amino acids (except histidine) were less effective in protecting embryos: Glutamine and lysine reduced the toxic effect only slightly, and arginine had no effect. DL-buthionine sulfoximine (7.5 mM), a specific inhibitor of glutathione, also reduced cystine-enhanced MeHg toxicity. It therefore appears that cystine enhances MeHg toxicity indirectly, at least in part, by stimulating the synthesis of cellular glutathione, which may in turn enhance MeHg transport. In the absence of cystine, 10 microM MeHg (2 h incubation) was necessary to cause the collapse and degeneration of all blastocysts treated.(ABSTRACT TRUNCATED AT 250 WORDS)