Steroid hormones receptor properties

Amine hormones include the thyroid hormones and the catecholamines. The thyroid hormones tend to be biologically similar to the steroid hormones. They are mainly insoluble in the blood and are transported predominantly (>99%) bound to proteins. As such, these hormones have longer half-lives (triiodothyronine, t3, = 24 h thyroxine, T4, = 7 days). Furthermore, thyroid hormones cross cell membranes to bind with intracellular receptors and may be administered orally (e.g., synthryoid). In contrast to steroid hormones, however, thyroid hormones have the unique property of being stored extra-cellularly in the thyroid gland as part of the thyroglobulin molecule. 

The l,25(OH)2D3 receptor of the chick intestine has been extensively characterized by our laboratory and several others, and its biochemical properties reveal many similarities to classical steroid hormone receptors [5,6], The l,25(OH)2D3 receptor is a protein with a molecular mass of approximately 67000 Da when occupied, unoccupied or in the absence of protease inhibitors. The receptor has a high affinity for l,25(OH)2D3 with a Kd in the range of 1-50 x 10 n M. The specificity of the receptor for binding of l,25(OH)2D3 metabolites and analogues, as determined by competitive binding studies, parallels the biological activity of these compounds. 

The steroid hormones are hydrophobic, this property enabling them to readily cross the plasma membrane to bind their respective cytosolic receptors. However, transport of such hydrophobic hormones through the blood stream requires hormone-binding proteins such as the steroid-binding globulins and corticosteroid-binding globulins. 

Properties of human steroid hormone receptors (SHRs)... 

In Table 11.2 the properties of two families of nuclear receptors, A and B, are compared. To family A belong the steroid hormone receptors and to family B, the non-steroidal receptors,25 the thyroid hormone receptors, the retinoic acid receptors, the vitamin D3 receptor, the peroxysome proliferator activating receptor, and several orphan receptors. 

Table 11.2 The two families of nuclear receptors and their properties are compared. Group A are the steroid hormone receptors and group B the non-steroidal nuclear receptors...

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