Thiol-Modulated Mechanisms of the Cytotoxicity of Thimerosal and Inhibition of DNA Topoisomerase IIα

Abstract

Thimerosal is an organic mercury compound that is widely used as a preservative in vaccines and other solution formulations. The use of thimerosal has caused concern about its ability to cause neurological abnormalities due to mercury accumulation during a normal schedule of childhood vaccinations. While the chemistry and the biological effects of methylmercury have been well-studied, those of thimerosal have not. Thimerosal reacted rapidly with cysteine, GSH, human serum albumin, and single-stranded DNA to form ethylmercury adducts that were detectable by mass spectrometry. These results indicated that thimerosal would be quickly metabolized in vivo because of its reactions with protein and nonprotein thiols. Thimerosal also potently inhibited the decatenation activity of DNA topoisomerase II alpha, likely through reaction with critical free cysteine thiol groups. Thimerosal, however, did not act as a topoisomerase II poison and the lack of cross-resistance with a K562 cell line with a decreased level of topoisomerase II alpha (K/VP.5 cells) suggested that inhibition of topoisomerase II alpha was not a significant mechanism for the inhibition of cell growth. Depletion of intracellular GSH with buthionine sulfoximine treatment greatly increased the K562 cell growth inhibitory effects of thimerosal, which showed that intracellular glutathione had a major role in protecting cells from thimerosal. Pretreatment of thimerosal with glutathione did not, however, change its K562 cell growth inhibitory effects, a result consistent with the rapid exchange of the ethylmercury adduct among various thiol-containing cellular reactants. Thimerosal-induced single and double strand breaks in K562 cells were consistent with a rapid induction of apoptosis. In conclusion, these studies have elucidated some of the chemistry and biological activities of the interaction of thimerosal with topoisomerase II alpha and protein and nonprotein thiols and with DNA.

Thimerosal in childhood vaccines contributes to accumulating mercury toxicity in the kidney

“…Thimerosal rapidly reacted with cysteine and GSH to form etHg-cysteine and etHg-GSH adducts in human leukemia K562 cells (Wu et al 2008). Thimerosal depleted intracellular GSH levels in these cells by 71% which markedly inhibited cell growth, corroborating a previous study using glioblastoma cells (James et al 2005).…”

Thimerosal Exposure and the Role of Sulfation Chemistry and Thiol Availability in Autism

“…In addition, TM quickly reacts with other thiols forming Et-Hg adducts, reducing the availability of thiols in the cell [59]. Studies have shown that TM and other forms of Hg decrease the levels of total cellular thiols in general [60,61].…”

Probing Conformational Changes in Human DNA Topoisomerase IIα by Pulsed Alkylation Mass Spectrometry

“…However, previous studies demonstrated that hsTop2␣ is sensitive to thiol-reacting reagents and that cysteine modifications participate in a number of unique reactions (38 -41). For instance, thimerosal, a mercury-containing compound that rapidly reacts with thiol group, inhibited the topoisomerase II-decatenating activity (39). Thiopurines also inhibited topoisomerase II ATPase activity by alkylating thiol groups of cysteines (40).…”