Differential Effects of Sodium Selenite and Methylmercury(II) on Membrane Permeability and DNA Replication in HeLa S3 Carcinoma Cells: A Preliminary Report Regarding the Modification of Organomercurial Toxicity by Selenium Compounds

Abstract

When viewed in terms of their concentration in the growth medium, sodium selenite and methylmercuric hydroxide — administered individually to HeLa S3 cells — are of equal efficacy in inhibiting DNA synthesis: the dose-response curves overlap and 50% residual DNA synthesis occurs at 6.13 µM of either chemical. A different picture, however, emerges if replication is expressed as a function of the actual amounts of toxicant bound per cell. Now, the dose-response curves do not overlap and Na₂SeO₃ is much more toxic than CH₃HgOH: 50% inhibition of DNA replication exists at 5.37 × 10 ⁻¹⁷ moles of Se bound per cell and at 3.63 × 10⁻¹⁵ moles of Hg bound per cell. Further, selenite is taken up by the cells more slowly than methylmercury and its (limiting) cellular concentration is below that of the organomercurial. Lastly, much higher levels of selenite in the growth medium are required to bring about the same degree of membrane damage as the one caused by methylmercury. These differential effects may have a bearing on the observation, well-known but thus far unexplained, that selenite and methylmercury are strikingly less toxic to animals when administered simultaneously than they are when administered individually: selenium may counteract the membrane-destabilizing characteristics of methylmercury and it may retard its binding to the cells. Data on the inhibition of DNA synthesis have been obtained when selenite and methylmercury are administered simultaneously to HeLa S3 cells in varied molar ratios. Best mutual protection appears to exist when the two chemicals are present in equimolar amounts or when there is a slight excess of selenite.