Thyroxine-binding prealbumin

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... 

McKinney JD, Chae K, Oatley SJ, et al. 1985a. Molecular interactions of toxic chlorinated dibenzo-p-dioxins and dibenzofurans with thyroxine binding prealbumin. J Med Chem 28 375-381. 

J. D. McKinney, K. Chae, S. J. Oatley, and C. C. F. Blake, /. Med. Chem., 28, 375 (1985). Molecular Interactions of Toxic Chlorinated Dibenzo-p-dioxins and Dibenzofurans with Thyroxine Binding Prealbumin. 

RBP forms a 1 1 complex with the tetrameric thyroxine-binding prealbumin, transthyretin. This is important to prevent urinary loss of retinol bound to the relatively small RBP (Mr 21,000), which would be filtered by the glomerulus transthyretin has an Mr of 54,000 hence, the complex will not normally be filtered, ffowever, moderate renal damage, or the increased permeability of the glomerulus in infection, may result in considerable loss of vitamin A bound to RBP-transthyretin. 

Kelleher, P. C., Phinney, S. D., Sims, E. A., Bogardus, C., Hortan, E. S., Bistrian, B. R., Ama-truda, J. M., and Lockwood, D. H. (1983). Effects of carbohydrate-containing and carbohydrate-restricted h3rpocaloric and eucaloric diets on serum concentrations of retinol-binding protein, thyroxine-binding prealbumin, and transferrin. Metabolism 32, 95-101. 

Figure 10. Model of thyroxine binding prealbumin-receptor interaction based on crystallographic protein data (39)...

LBM includes skeletal muscle, somatic protein, and functional proteins such as circulating proteins and the visceral proteins. Biochemically, LBM can be assessed by measuring the serum visceral proteins, albumin (ALB), transferrin (TFN), and prealbumin (thyroxine-binding prealbumin or transthyretin). Other serum proteins, such as retinol-binding protein, flbronectin (an opsonic protein), and somatomedin-C (insulin-like growth factor-1), that have a very short half-life (less than 12 to 24 hours), have been suggested as... 

Oppenheimer, J. H., Martinez, M., and Bernstein, G., Determination of maximal binding capacity and protein concentration of thyroxine-binding prealbumin in human serum. J. Lab. Clin. Med. 67, 500-509 (1966). 

Sterling, K., and Tabachnick, M., Paper electrophoretic demonstration of thyroxine-binding prealbumin fraction in serum. Endocrinology 68, 1073-1075 (1961). 

Surks, M. I., and Oppenheimer, J. H., Postoperative changes in concentration of thyroxine-binding prealbumin and serum free thyroxine. J. Clin. Endocrinol. Metab. 24, 794-802 (1964). 

W3. Woeber, K. A., and Ingbar, 8. H., The contribution of thyroxine-binding prealbumin to the binding of thyroxine in human serum as assessed by immuno-adsorption. J. Clin. Inveot. 47, 1710-1721 (1968). 

Savu, L., R. Vranckx, M. Maya, D. Gripois, M. F. Blouquit, and E. A. Nunez. 1989. Thyroxinebinding globulin and thyroxine-binding prealbumin in hypothyroid and hyperthyroid developing rats. Biochimica et Biophysica Acta 992 379-384. 

L3. Larsen, P. R., Salicylate-induced increases in free triiodothyronine in human serum Evidence of inhibition of triiodothyronine binding to thyroxine-binding globulin and thyroxine-binding prealbumin. /. Clin. Invest. 51, 1125-1134... 

Vergani, C., Stabilini, R., and Agostoni, -A., Thyroxine-binding prealbumin in thyrotoxicosis and hypothyroidism. Clin. Chem. 15, 216-218 (1969). 

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