Characteristics of the Thyroid Iodide Translocator and of Iodide-Accumulating Phospholipid Vesicles*

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Abstract
Some biochemical characteristics of the porcine thyroid I- translocator and of I--accumulating phospholipid vesicles (P-vesicles) were studied. P-vesicles were made from thyroid plasma membranes (PM) and soybean phospholipids by sonication. The optimal incubation temperatures for Na+-dependent I- accumulation in P-vesicles was from 18-26.degree. C. Only a small amount of Na+-independent I- accumulation was observed at various incubation temperatures, but it increased in proportion with the temperature up to 36.degree. C. The optimal incubation pH (7.0-7.5) was near the physiological extracellular pH. When PM were heated at 55.degree. C for 30 min before preparation of P-vesicles, Na+-dependent I- accumulation in the vesicles decreased by 35%. When they were heated at 65.degree. C for 30 min, the I--accumulating activity was almost completely lost. The translocator was also inactivated when PM were sonicated at 37.degree. C in the presence of trypsin. The internal and external administration of ouabain to the vesicles did not affect the activity of Na+-dependent I- accumulation. When PM were treated with sodium dodecyl sulfate at a final concentration of 0.2-0.6 mg/ml, the I- translocator was inactivated or detached from PM, whereas the ouabain-sensitive Na+,K+-ATPase activity was preserved in the PM fragments. The thyroid I- translocator consists of a protein component that is bound to PM at a site separate from Na+,K+-ATPase.