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Environmental iodine deficiency: a challenge to the evolution of terrestial life?
Environmental iodine deficiency: a challenge to the evolution of terrestial life?
Sebastiano Venturi, Francesco M. Donati, Alessandro Venturi and Mattia Venturi
Published in "Thyroid", 2000 Aug;10 (8):727-9.

Iodine is the heaviest and richest in electrons among essential elements required in the diet. Inorganic iodides are necessary for all living vegetal and animal cells, but only the Vertebrates have the thyroid gland and its iodinated hormones. In humans the total amount of iodine is about 30-50 mg and less than 30 % is present in thyroid gland and in its hormones. About 60-80 % of total iodine is non-hormonal and it is concentrated in extrathyroidal tissues, but its biological role is still unknown. Recently we have hypothesized that iodide might have an ancestral antioxidant function in all iodide-concentrating cells from primitive Algae to more recent Vertebrates (1-7). Into these cells iodide acts as an electron donor in the presence of H2O2 and peroxidase (8), the remaining iodine readily iodinates the tyrosine and (more slowly) the histidine or some specific lipid (9), and so, neutralizes its own high oxidant power.

IODINE, THYROXINE AND THYROID EVOLUTION

Over three billion years ago, blue-green algae (Cyanobacteria), which are the most primitive oxygenic photosynthetic organisms, ancestors of multicellular eukaryotic algae, that contain the highest amount of iodine, were the first living cells to produce oxygen, which was toxic at that time, in the terrestrial atmosphere. So, algal cells required a protective antioxidant action in which iodides might have had this specific role. In fact iodides are greatly present and available in sea-waters, where algal phytoplankton acts as a biological accumulator of iodides. Recently our hypothesis of the ancestral antioxidant action of iodides has experimentally been confirmed in some algae by an important study carried out by Kupper et al.(10). Since about 700 million years ago thyroxine (T4) is present in fibrous exoskeletal scleroproteins of the lowest invertebrates (Porifera and Anthozoa)(11), without showing any hormonal action. When some primitive marine vertebrates started to emerge from the iodine-rich sea and transferred to iodine-deficient fresh water and finally land, their diet became iodine deficient and also harboured vegetal iodide-competitors such as nitrates, nitrites, thiocyanates and some glycosides (12). Hence these animals needed an efficient thyroid gland as reservoir of iodine. Therefore....

Keywords: antioxidant, evolution, iodide, iodine deficiency, selenium, thyroid

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