Endocrine Abstracts

Seaweeds share properties with the mammalian thyroid and other iodide concentrating organs in that they take up iodide from surrounding seawater and internalise it within cells. The previously demonstrated high iodine concentration within seaweeds prompted a study of accumulation and retention of ¹²⁵I and its location within the plants. Seaweed discs (1 cm³) cut from members of the red, green and brown phyla which were incubated at 4C in seawater spiked with ¹²⁵I and uptake of radioactivity counted. To measure ¹²⁵I retention discs were then transferred to fresh seawater and efflux of radioactivity measured. Dose dependent blockade of ¹²⁵I uptake, analogous to that seen in the human thyroid, was observed when the brown kelp L. digitata was incubated with perchlorate(100 uM). Seaweed discs from a variety of species accumulated ¹²⁵I with maximum uptake converging at 62.8%–91.0% after 20 hrs. Rates of ¹²⁵I uptake and efflux are also species specific. Rate of uptake of iodide by seaweed is of the same order of magnitude as the human thyroid although accumulation in Laminaria is considerably higher. The most iodide rich seaweeds are also the most iodide avid (L. Digitata>U. Lactuca). Interestingly ¹²⁵I retention was highly effective in seaweeds with 85–90% of accumulated being retained at 30 mins compared to only ∼10% retention in rat thyroid FRTL-5 cells. The location of iodide transport within L. digitata was studied by enzymatic removal of the cell wall and subsequent harvesting and purification of protoplasts. Transport of ¹²⁵I into cells, rather than attachment to the cell wall, was demonstrated by protoplasts’ uptake (∼50%) and retention (∼85%) of that seen in intact plants. Understanding the mechanisms by which seaweeds take up and in particular retain iodide may assist in providing a template with potential applications in radioiodide ablative therapy of human cancers.