The decrease in level of serum thyroxine by 2,2',4,5,5'-pentachlorobiphenyl in rats and mice: no correlation with formation of methylsulfonyl metabolites.

A relationship between formation of methylsulfonyl (MeSO2) metabolites of 2,2',4,5,5'-pentachlorobiphenyl (PentaCB) and decrease in serum thyroxine (T4) level was examined in the rats and mice after a single i.p. injection of PentaCB (342 micromol/kg body weight). In either rats or mice, levels of the 3- and 4-MeSO2 metabolites of PentaCB in the liver and feces increased in a time-dependent fashion up to 8 days after PentaCB-treatment. However, there was a marked difference between rats and mice in the amount of the metabolites formed, and the cumulative amount of the either MeSO2 metabolite for 8 days after PentaCB treatment in the liver was 4 to 15 times higher in mice than in rats. On the other hand, a 40 to 60% decrease in level of serum total T4 occurred in both rats and mice at 1 day after PentaCB treatment, and the decrease was retained up to 8 days after PentaCB treatment. Thus, there was a marked difference between rats and mice in the formation of MeSO2 metabolites from PentaCB but not a significant difference between rats and mice in PentaCB-induced decrease in the level of serum total T4, indicating that PentaCB-induced decrease in the level of serum total T4 is not necessarily dependent on the MeSO2 metabolites formed.