Cortisol-protein binding

Radioassay is a competitive protein binding assay which employs a natural binding protein instead of an antibody. For example, transcortin is the binding protein for cortisol in nature and also in the radioassay procedure for cortisol. 

M12. Morey, K. S., and Litwack, G., Isolation and properties of cortisol metabolite binding proteins of rat liver cytosol. Biochemistry 8, 4813-4821 (1969). 

Transcortin acts as a reservoir from which a constant supply of unbound cortisol may be provided to target cells. In addition, when serum albumin levels are low, less circulating cortisol becomes bound, which yields a greater physiological effect. Not only does protein binding control the amount of biologically active cortisol available, but it also reduces the rate at which steroids are cleared from the blood and thus limits steroid suppression of corticotrophin release from the pituitary gland. 

Lung maturation in the fetus is regulated by the fetal secretion of cortisol. Treatment of the mother with large doses of glucocorticoid reduces the incidence of respiratory distress syndrome in infants delivered prematurely. When delivery is anticipated before 34 weeks of gestation, intramuscular betamethasone, 12 mg, followed by an additional dose of 12 mg 18-24 hours later, is commonly used. Betamethasone is chosen because maternal protein binding and placental metabolism of this corticosteroid is less than that of cortisol, allowing increased transfer across the placenta to the fetus. 

Albumin is a major transport facilitator of hydrophobic compounds which would otherwise disrupt cellular membranes. These compounds include free fatty acids and bilirubin as well as hormones such as cortisol, aldosterone, and thyroxine when these materials have exceeded the capacity of proteins normally associated with them. Albumin also binds ions, including toxic heavy metals and metals such as copper and zinc which are essential for normal physiological functioning but may be toxic in quantities in excess of their binding capacity for their carrier proteins. Binding of protons is the basis for the buffering capacity of albumin. 

Phenytoin decreases the urinary excretion of 17-ketosteroids and 17-hydroxycorticosteroids by stimulating the conversion of cortisol to 6-P-hydroxycortisol it also diminishes serum FSH and the sperm count in semen, and thereby reduces fertility, Phenytoin also lowers the serum thyroxine concentration, probably by competitive displacement of thyroxine from its protein-binding sites free thyroxine also tends to be low. Serum triiodothyronine is low, probably as a result of stimulated metabohsm in the liver, but the concentration of TSH is unaffected by the altered thyroxine metabolism. 

Fig. 2. Xenobiotic impact on glucocorticoid signaling. Xenobiotics impact on GR signaling could include either a direct effect on the GR signaling pathway and/or an indirect effect mediated by changes in other proteins/pathways. The direct effect encompasses impact on cortisol (F) binding to GR, their translocation, binding to GRE and initiation of transcription, whereas the indirect effect may include impact on kinases and phosphatases (K), accessory proteins (AP), the cytoskeletal network (C) and the proteasomal pathways (P), all of which are involved in GR signaling (see text for details).

FI. Farese, R. V., and Plager, J. E., The in vitro red blood cell uptake of C -cortisol studies of plasma protein binding of cortisol in normal and abnormal states. J. Clin. Invest. 41, 53-60 (1962). 

M7. Murphy, B. E. P., Clinical evaluation of urinary cortisol determinations by competitive protein-binding radioassay. J. Clin. Endocrinol. Metab. 28, 343-348 (1968). 

Gutenberger, S.K. Olson, D.P. Kagel, R.A. Comparison of reversed phase high-performance liquid chromatography and competitive protein binding assay in the quantification of cortisol in bovine plasma. J.Liq.Chromatogr., 1985, 8, 107-124 [plasma cow dexamethasone (IS)]... 

In the serum, progesterone is bound to either cortisol binding globulin or albumin, with only a small fraction being freely available. The metabolic clearance rate for progesterone is 2,100 to 2,500 L/day, for which protein binding has no role. 

Transcortin can be used in competitive protein binding assays for the estimation of cortisol. 

Estrogens are bound to SHBG and progestins to CBG. SHBG binds estradiol about five times less avidly than it binds testosterone or DHT, while progesterone and cortisol have little affinity for this protein (Table 42-8). In contrast, progesterone and cortisol bind with nearly equal affinity to CBG, which in turn has little avidity for estradiol and even less for testosterone, DHT, or estrone. 

Adrenal Tumours The assay-method is entirely based on the Schwartz-Mann Kit. According to this method, cortisol is first extracted from the plasma using CH2C12 (methylene chloride). In the actual radioimmunoassay the cortisol present in the extract competes with Cortisol-H3 i.e., the radioactive tracer) for the common binding sites on transcortin, which is incidently not an antibody but a cortisol-binding protein. Now, the free cortisol is quantitatively removed by adsorption on dextran-coated charcoal from the one bound to the transcortin. Finally, the bound radioactivity (due to Cortisol-H3) is measured which is then employed to calculate exactly the amount of cortisol present in the sample by the help of a Standard Curve (or Calibration Curve). 

Cortisol secreted in response to stress, is permissive for glucagon in hypoglycemia and acts through an intracellular receptor, which, like other steroid receptors, is a zinc-finger DNA binding protein,... 

An account of the principles which help to understand how hormones achieve their roles in the body is given in Chapter 12. The understanding is based on separation of the effects of hormones into three components the action, the effects (biochemical and physiological) and the function. A steroid hormone binds to a cytosolic intracellular receptor, which then moves into the nucleus where it binds to DNA at a specific site (the steroid response element) and activates genes which result in the formation of proteins that elicit biochemical and physiological effects. This is discussed for cortisol in Chapter 12 and aldosterone in Chapter 22. Much of the interest in the reproductive steroid hormones is in the physiological effects and how these account for their functions. 

Oral contraceptives have their most significant effect on endocrine parameters. Blood cortisol, thyroxine, protein-bound iodine, T3 uptake, and urinary free cortisol are elevated. Urinary 17,21-dihydroxy steroids, 17-ketosteroids, and estrogens are decreased. There is no effect on urinary catecholamines or VMA (Table 10) (LIO). The effect of thyroid functions tests is due to the administered hormone stimulating an increase in the production of thyroid-binding globulin which in turn binds 1-thyroxine. The lowering of free thyroxine stimulates the anterior pituitary to produce thyrotropin, which in turn stimulates the thyroid to produce more thyroxine. Since the additional thyroxine is bound to the extra protein, there is an equilibrium and the patient remains clinically euthyroid, but the protein-bound iodine and the thyroxine are elevated. 

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