Thyroxine binding affinity

Some hydroxy metabolites of coplanar PCBs, such as 4-OH and 3,3 4,5 -tet-rachlorobiphenyl, act as antagonists of thyroxin (Chapter 6, Section 6.2.4). They have high affinity for the thyroxin-binding site on transthyretin (TTR) in plasma. Toxic effects include vitamin A deficiency. Biomarker assays for this toxic mechanism include percentage of thyroxin-binding sites to which rodenticide is bound, plasma levels of thyroxin, and plasma levels of vitamin A. 

Thyroxine-binding globulin is the least abundant of the three major transport proteins. Nevertheless, it carries about 70% of the circulating T4 and Tj by virtue of its high affinity for the two hormones. Transthyretin, formerly known as thyroxine-binding prealbumin, binds only about 10 to 15% of the hormones. Albumin, a protein that has a binding affinity for a multitude of small molecules, has an even lower affinity for T4 and T3 than... 

Thyroxine binding prealbumin (TBPA) carries about 30% T but no Tj. Its affinity is only of the order of 10 M, but it is much more abundant in semm than is TBG. The amino acid sequence and the structure of this protein are known. Four identical subunits (127 amino acids each) form a prolate ellipsoid. Noncovalent interactions between the subunits form a channel of I nm diameter along the long axis, which has a funnel-shaped opening of 2.5 nm. The T molecule is held in one arm of this channel, binding... 

Thyroid hormones and most steroid hormones are associated with carrier proteins in the serum. The carrier proteins are called, appropriately, thyroxine-binding globulin, transcortin (for cortisol), and sex-steroid-binding protein. These proteins have a high affinity (Kd 10—9— 10 8 m) for their respective hormones. They buffer the concentration of free hormone and retard hormone degradation and excretion. The carrier proteins are distinguishable from the intracellular receptors for these hormones. 

Figure 28-21 Themoleculei-thyroxine. 7. binds to prealbumin, a protein found in blood. Based on x-ray data of the >-thyroxine-prealbumin complex, the binding affinity of novel analogues was prediaed by using a molecular modeling approach.

Figure 28-23 Using molecular modeling methods, four I -thyroxine analogues were predicted to have good binding affinity (8 > 9 = 10 > 11) to prealbumin.

The function of most albumin isoforms is normal, although some have abnormal binding affinities for thyroxine (T4). Binding may be increased, as in familial dysalbu-minemic hyperthyroxinemia, or decreased. Individuals with famihal dysalbuminemic hyperthyroxinemia are euthyroid but have elevated serum T4 and free T4 index the variant albumin comigrates with Alb A. Two glycosylated variants. Alb RedhiU and Alb Casebrook, have altered fatty acid binding properties. 

Tab. 6.1 Relative binding affinity of thyroxine mimetics at the rat liver nuclear receptor.

A comparison of the affinity of the thyroxine-binding globulin for thyroxine and that of the biologically most effective of the triiodothyronines suggests that the specificity of the binding protein is directed toward the biologically active compound [24]. 

It is metabolically more active than thyroxine. It represents 1-2% of the total thyroid hormones in the serum and is carried by thyroxine-binding globulin, although it has a lower affinity for this protein than thyroxine. 

When stimulated to release thyroid hormones, thyroglobulin is degraded through the activity of lysosomes and T3 and T4 are released and rapidly enter the circulation. Iodide freed in this reaction is for the most part recycled and the iodinated tyrosine reused for hormone production. Nearly all of the released hormones are rapidly bound to transport hormones, with 70% bound to thyroxine binding globulin (TBG). Other proteins, such as transthyretin (TTR), albumin, and lipoproteins, bind most of the remainder with significant differences in the strengths of the affinity for the hormones, these proteins transport the hormones to different sites. 

An optical immunosensor for continuous T4 measurement has been described, in which the fluorescent indicator protein is separated from the sample flow chamber by a dialysis membrane.024) The indicator is T4-binding globulin (TBG), the intrinsic fluorescence (ex. 290 nm) of which is quenched by T4binding. Due to the high affinity of the TBG for thyroxine, the immunosensor is not reversible, but multiple measurements can be made until the TBG is saturated. Sensitivity is inadequate for clinically useful concentrations of T4, but suggestions for improvement of the method are made. 

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