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Methylmercury (MeHg) effects on cell cycle kinetics were investigated to help identify its mechanisms of action. Flow cytometric analysis of normal human fibroblasts grown in v i m in the presence of BrdU allowed quantitation of the proportion of cells in G,, S, G, and the next G, phase. This technique provides a rapid and easily performed method of characterizing phase lengths and transition rates for the complete cell cycle. After first exposure t o MeHg the cell cycle time was lengthened due to a prolonged G,. At 3 ,DM MeHg the G, phase length was 25% longer than the control. The Gl/S transition rate was also decreased in a dose-related manner. Confluent cells exposed to MeHg and replated with MeHg respond in the same way as cells which have not been exposed to MeHg before replating. Cells exposed for long times to MeHg lost a detectable G , effect, and instead showed an increase in the G, percentage, which was directly related to MeHg concentration and length of exposure. After 8 days at 5 ,DM MeHg, 45% of the population was in G,. The

We have studied the localization and affinity of methyl mercury hydroxide (MeHg) binding sites on microtubules. There is one class of binding sites for MeHg on tubulin, a high affinity class with fifteen sites. MeHg binds to tubulin stoichiometrically within microtubules, and does not induce microtubule disassembly at this low binding ratio. MeHg binds in microtubules either in the presence or absence of free tubulin subunits but free subunits act as uncompetitive inhibitors for MeHg binding to the polymer. These stoichiometric polymer surface binding sites for MeHg apparently do not interfere with subsequent polymerization, in contrast to the multiple sites in the free dimer whose occupation blocks subsequent assembly. In assembly cycles that follow MeHg binding to polymers, we continue to find MeHg bound to microtubules at substoichiometric ratios. Dimers with higher levels of MeHg binding are rendered assembly incompetent. These results show MeHg to have one class of binding site on tubulin, and the MeHg binding site, both to the polymer surface and to the free dimer, to be the same.

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