Submitted on November 23, 2004
Accepted on January 5, 2005
Developmentally Regulated Thyroid Hormone Distributor Proteins in Marsupials, a Reptile and Fishes
Samantha J Richardson1*, Julie A Monk1, Caroline A Shepherdley1, Lars O. E Ebbesson2, Frank Sin3, Deborah M Power4, Peter B Frappell5, Josef Kohrle6, and Marilyn B Renfree7
1 Biochemistry and Molecular Biology, The University of Melbourne, Parkville, Victoria, Australia
2 Biology, University of Bergen, Bergen, Norway
3 Zoology, University of Canterbury, Christchurch, New Zealand
4 Centro de Ciencias de MAR, Universidade do Algarve, Faro, Portugal
5 Zoology, La Trobe University, Bundoora, Victoria, Australia
6 Institut fur Experimentelle Endokrinologie und Endokrinologisches Forschungs-Centrum der Charite Berlin, Berlin, Germany
7 Zoology, The University of Melbourne, Parkville, Victoria, Australia
* To whom correspondence should be addressed. E-mail: sjrich{at}unimelb.edu.au.
Thyroid hormones are essential for vertebrate development. There is a characteristic rise in thyroid hormone levels in blood during critical periods of thyroid hormone-regulated development. Thyroid hormones are lipophilic compounds, which readily partition from an aqueous environment into a lipid environment. Thyroid hormone distributor proteins are required to ensure adequate distribution of thyroid hormones, throughout the aqueous
environment of the blood, and to counteract the avid partitioning of thyroid hormones into the lipid environment of cell membranes. In human blood, these proteins are albumin, transthyretin and thyroxine-binding globulin. We analysed the developmental profile of thyroid hormone distributor proteins in serum from a epresentative of each order of marsupials (M.eugenii; S.crassicaudata), a reptile (C.porosus), in two species of salmonoid fishes (S.salar; O.tshawytsch), and throughout a calendar year for sea bream (S.aurata). We demonstrated that during development, these animals have a thyroid hormone distributor protein present in their blood which is not present in the adult blood. At least in mammals, this additional protein has higher affinity for thyroid hormones than the thyroid hormone distributor proteins in the blood of the adult. In fish, reptile and polyprotodont marsupial, this protein was transthyretin. In a diprotodont marsupial, it was thyroxine-binding globulin. We propose an hypothesis that an augmented thyroid hormone distributor protein network contributes to the rise in total thyroid hormone levels in the blood during development.
