Iodine in biology
It is a component of biochemical pathways in organisms from all biological kingdoms, suggesting its fundamental significance throughout the evolutionary history of life.
An adequate intake of iodine-containing compounds is important at all stages of development, especially during the fetal and neonatal periods, and diets deficient in iodine can present serious consequences for growth and metabolism.
These molecules are made from addition-condensation products of the amino acid tyrosine, and are stored prior to release in an iodine-containing protein called thyroglobulin.
Thyroid hormones play a fundamental role in biology, acting upon gene transcription mechanisms to regulate the basal metabolic rate.
T3 acts on small intestine cells and adipocytes to increase carbohydrate absorption and fatty acid release, respectively.
A family of selenium-dependent enzymes called deiodinases converts T4 to T3 (the active hormone) by removing an iodine atom from the outer tyrosine ring.
The higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of other body systems, including lactating breasts, gastric mucosa, salivary glands, oral mucosa, arterial walls, thymus, epidermis, choroid plexus and cerebrospinal fluid, among others.
[10][11][12] Iodine and thyroxine have also been shown to stimulate the spectacular apoptosis of the cells of the larval gills, tail and fins during metamorphosis in amphibians, as well as the transformation of their nervous system from that of the aquatic, herbivorous tadpole into that of the terrestrial, carnivorous adult.
The frog species Xenopus laevis has proven to be an ideal model organism for experimental study of the mechanisms of apoptosis and the role of iodine in developmental biology.
[23] Plants, insects, zooplankton and algae store iodine as mono-iodotyrosine (MIT), di-iodotyrosine (DIT), iodocarbons, or iodoproteins.
[23] They form spontaneously without need for enzymatic catalysts which may have contributed to their early adoption by organisms,[32][33] although enzymes make the yields significantly higher.
[45][46] Molecular iodine (I2) is toxic to most single-celled organisms by disrupting the cell membrane[47] however Alphaproteobacteria and Choanoflagellates are resistant.
[23] The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for iodine in 2000.
[49] As a safety consideration, the IOM sets tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient.
[53] For U.S. food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV).
Natural sources of iodine include many marine organisms, such as kelp and certain seafood products, as well as plants grown on iodine-rich soil.
[citation needed] Worldwide, iodine deficiency affects two billion people and is the leading preventable cause of intellectual disability.
In areas where there is little iodine in the diet, typically remote inland areas and semi-arid equatorial climates where no marine foods are eaten, iodine deficiency also gives rise to hypothyroidism, the most serious symptoms of which are epidemic goitre (swelling of the thyroid gland), extreme fatigue, mental slowing, depression, weight gain, and low basal body temperatures.
[citation needed] Only a small amount of elemental iodine will dissolve in water, but triiodides are highly soluble; potassium iodide thus serves as a phase transfer catalyst in the tincture.
Elemental iodine (I2) is poisonous if taken orally in large amounts; 2–3 grams is a lethal dose for an adult human.
When mixed with ammonia and water, elemental iodine forms nitrogen triiodide, which is extremely shock-sensitive and can explode unexpectedly.
Compared to the elemental form, potassium iodide has a median lethal dose (LD50) that is relatively high in several animals: in rabbits, it is 10 g/kg; in rats, 14 g/kg, and in mice, 22 g/kg.
[8] Selenocysteine (abbreviated as Sec or U, in older publications also as Se-Cys)[80] is the 21st proteinogenic amino acid, and is the root of iodide ion toxicity when there is a simultaneous insufficiency of biologically available selenium.
