Differential effects of methylmercuric chloride and mercuric chloride on the histochemistry of rat thyroid peroxidase and the thyroid peroxidase activity of isolated pig thyroid cells

This study was designed to characterize the interaction of CH3HgCl or HgCl2 with thyroid peroxidase (TPO). Two types of experiments were performed. First, the thyroids from rats that were given 5.6 mg/kg/day of either CH3HgCl or HgCl2 for 2 weeks by intubation were subjected to histochemical treatment and then to electron microscopy. TPO activities in all cell compartments were inhibited by HgCl2 but not by CH3HgCl. Morphological observation showed that taller epithelia were induced by HgCl2, whereas flattened epithelia forming large follicles were induced by CH3HgCl. The serum thyrotropin level was substantially lowered by CH3HgCl but was unchanged by HgCl2. Second, the guaiacol oxidation by TPO in isolated and ruptured pig thyroid cells was spectrophotometrically monitored in the presence of either CH3HgCl or HgCl2. The TPO was not inhibited by CH3HgCl but was inhibited by HgCl2. These results indicated that CH3HgCl induced a hypothyroid state without affecting TPO, whereas HgCl2 inhibited TPO and induced a hypertropic state owing to compensation for loss of enzyme activity, and that the lack of inhibitory activity of CH3HgCl was not due to the inability to penetrate the cells. Therefore, there appeared to be a differential interaction of organic and inorganic forms of mercurials with the thyroid.