Hormone Displacement from Binding Proteins

Using any of the carrier proteins available in highly purified form, eg, TBG or TBPA, a convenient and accurate quantitative determination of and is possible by displacement of radioiodinated or T. This procedure enables their quick determination at low concentrations even in the presence of coundess other substances that occur in body duids (31). In a similar fashion, intact cell nuclei or solubilized proteins from rat fiver cell nuclei, which display high affinities for thyroid hormones, especially T, have been used to establish relative binding affinities of many thyromimetic compounds (7). 

Activation of the phosphodiesterase by light requires the presence of GTP and is associated with the binding of GTP to another protein, transducin. Transducin is a member of the family of G proteins that participate in the activation or inhibition of adenylate cyclase by hormones in other tissues (see chapter 24). Like other G proteins, transducin consists of three subunits, a, (3, and y (fig. S2.12). In the resting state, the a subunit contains a molecule of bound GDP. When rhodopsin is transformed to metarhodopsin II by light, it interacts with transducin, causing GTP to displace the bound GDP. Once GTP is attached, the a subunit probably separates from the (3 and y subunits and binds to an inhibitory subunit of the phosphodiesterase. The removal of the inhibitory component activates the phosphodiesterase. 

Thyroid hormones, levothyroxine + phenytoin Rise in thyroxine blood levels with reported tachycardia Displacement of thyroxine by phenytoin from protein binding Great care should be taken in administering parenteral phenytoin to people on thyroid therapy... 

Another form of subtle regulation could be the displacement by conjugates of free hormones from their binding sites on plasma transport proteins. The conjugates could thus modulate the circulating concentration of free hormone. This could be the case for corticosterone as corticosterone sulfate is bound to human transcortine with one-half the affinity of free corticosterone (Lebeau and Baulieu, 1970). 

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