The effect of methimazole (MMI) and 2-mercaptoethanol (ME) on Itransport was studied using phospholipid vesicles (P-vesicles) made from porcine thyroid plasma membranes and soybean phospholipids by sonication.
1. When buffer solutions contained either 1mM MMI or 2mM ME, I-uptake by P-vesicles in the presence of external Na+ was apparently higher than that in the absence of external Na+. Na+-dependent I- uptake was inhibited by both C104- and SCN- added externally.
2. When PM was treated with 4mM N-ethylmaleimide prior topreparation of P-vesicles, the activity of Na+-dependent I- transport was completely lost even when P-vesicles were incubated in the presence of ME.
3. When neither MMI nor ME was added to buffers, I- uptake in the presence of external Na+ was not at all higher than that in the absence of external Na+. In these instances, however, I- uptake was much higher compared than the baseline uptake in the presence of MMI or ME, and was inhibited by external SCN- and not by C104- without relation to external Na+.
These data indicate that MMI or ME has two distinct effects on our model system of I- transport. The one is preservation of the Na+-dependent I- transport acitivity by protecting a sulfhydryl group, and the other is reduction of nonspecific I- binding to P-vesicles. In addition, C104- is a more specific inhibitor of thyroid I- transport than SCN-, when non-specific I- oxidation is imperfectly prevented.