Iodine content, chemical composition and mechanisms of control of iodide uptake and efflux by Laminaria.

(A Marine Institute funded post-doctoral fellowship)

The Irish Seaweed Centre has started a project in co-operation with the endocrine laboratory, Department of Medicine and Therapeutics, UCD on studying the iodine content, chemical composition and mechanisms of control of iodide uptake and efflux by Laminaria, and to utilise the seaweed model to provide information for therapeutic potential in utilising radio iodide in the treatment of breast and other cancers. 

This project runs for 3 years an finishes in 2005. Dr Robert Wilkes (robert.wilkes[at]nuigalway.ie) is the responsible person in the Irish Seaweed Centre for this project and takes care of all the sampling of the different types of seaweed.  Dr Emma Burbridge is responsible for the iodine and DNA analysis side of this project. Dr Peter Smith, head of the Endocrine Laboratory of the Conway Institute of Biomolecular and Biomedical Research is the overall responsible person for this project.

Background and Justification

During the 17 ^th century seaweeds (mainly kelps) were used as raw material for extraction of Iodine. The Chinese for centuries have treated goitre, caused by an iodine deficiency, by means of iodine obtained from Laminaria (kelp) species. Brown algae in general are very high in iodine content up to 0.7 % of the wet weight. A doses of 1 g of kelp daily would provide the 0.1 to 0.2 mg of iodine required by a normal adult (Arasaki & Arasaki, 1983; Indergaard & Minsaas, 1990).

Translocation of Iodine as in Laminaria has shown that distribution is unidirectional, basipetal and directed towards the meristematic region, although storage as water-soluble iodine or via organification is not understood. Diffusion, as a mechanism of transport, is unlikely as velocities of transport of ¹²⁵I of 3.5 cm.h ^-1 were recorded (Amat & Srivastava, 1985). These authors suggested that transport most probably takes place via the sieve elements in Laminaria. No results are present for other seaweeds.

A study on Pacific seaweeds showed that Laminaria contained over 90% of the iodine in a water-soluble form, mainly as I ^-. In other Pacific algae 5.5% to 37.4% was found as organic iodine. Of the organic iodine 50% is in the form of iodo-amino acids (Hou et al., 1997). In Sargassum and Laminaria sp. thyroxin and triiodothyronine as well as monoidotyrosine and diiodotyrosine have been found of which monoidotyrosine reached a level of 0.1% in Sargassum (Nisizawa, 1979). The results of the study on Pacific seaweeds suggested that the mechanism of iodine enrichment is different for various algae and that its bioavailability varies as well. The protein bound iodine compounds have been shown to lower blood cholesterol levels in rats and weight reduction and antilipemic effects in humans (Arasaki & Arasaki, 1983).

About 20% of all iodine in humans and animals is found in the thyroid gland. Free iodine and iodate are reduced to iodide in the intestinal wall. In the thyroid, iodide is oxidized, converted to organic iodine by coupling to the amino acid L-tyrosine, and incorporated into hormones like thyroxin (T4) and triiodothyronine (T3). The thyroid influences many functions of an organism: intensity of the metabolism, Physic and mental development, differentiation and maturation of tissues, muscle functions, cardiovascular system, reproduction and fertility performance, and metabolism of a range of nutrients (Indergaard & Minsaas, 1990)

In the thyroid gland of humans and animals a key plasma membrane protein, the Na /I symporter (NIS), catalyses the active accumulation of iodide. The thyroid is capable of concentrating I by a factor of 20-40 with respect to the concentration of the anion in the plasma. Recently the primary sequence of the NIS molecule has been discovered and screening for genetic defects by absence of thyroidal I transport in patients to ascertain whether the symporter molecule bears mutations is possible. RNA is isolated and subjected to RT-PCR with NIS specific primers to amplify the NIS cDNA after which it can be sequenced and compared (Dai et al., 1996).

Iodine in marine algae may reach 0.9% of the dry weight, while the iodine concentration in seawater is only 6 x 10 ^-8 g/l. The enrichment factor for iodine therefore reaches 10 ⁶  (Hou et al., 1997) and is much higher compared to the thyroid gland. Therefore iodine is taken up actively although it is not known via what kind of uptake system.

Objectives and expected deliverables

• Objective 1: To determine iodine contents of 10 commercial viable seaweed species (total iodine, water soluble iodine and soluble organic iodine).
• Deliverables: The outcome will provide a list for end users (sea-vegetable producers and agriculture) consisting of iodine concentrations and bioavailability in commercial available seaweeds. Generation of new data on the commercial applications of iodine in seaweeds.
• Objective 2: To measure uptake rates and translocation of ¹²⁵I in different species of seaweed and different types of seaweed tissue.
• Deliverables: Iodine distribution maps for 10 commercially seaweed species, and advanced understanding of distribution patterns of Iodine in seaweeds.  
• Objective 3: To search for the presence of a mechanism for active absorption of iodine analogous to the plasma membrane protein, the Na /I symporter (NIS) using molecular techniques.
• Deliverables: Advanced understanding of active absorption mechanisms for Iodine in seaweeds

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