Iodide transport of thyroid plasma membranes

Plasma membranes consisting of closed vesicles were isolated from hog thyroid homogenate. The membrane vesicles showed uphill transport of iodide from an external medium containing monovalent cations, of which K+ induced iodide transport more potently than Na+. The activity of the iodide transport expressed as T/M[I-] was as little as 3 to 11 in the presence of K+, but was invariably present. The ratio reached a maximum within about 10 min and then decreased fairly rapidly to unity. The addition of SCN- or CIO-4 to the external medium inhibited iodide transport. The transport activity was found to be maximum at pH 7.0 to 7.5 in the external medium. A kinetic study showed that the transport rate was saturated with respect to the iodide concentration. These observations suggested the presence of a carrier-mediated iodide transport system which was coupled with K+ flux across the plasma membrane.