Relative binding affinity, RBA

Fig. 3.3. Ligands and their relative binding affinity (RBA) to estrogen receptors a and p (25).

Most of the knowledge that we have regarding the interaction of chemicals with the endocrine system comes from in vitro receptor binding data. These receptor interaction data form the basis for many of the structure-activity relationships that have been derived for potential endocrine disruptors. It is important to note that potency measured in in vitro studies may or may not be relevant to the in vivo situation. The relative binding affinity (RBA) and potency of binding vary significantly within a given class of substances. For example, Branham et al. [11] reported... 

Table 2 Relative Binding Affinities (RBA) of Commercially Available Glucocorticoids to the Glucocorticoid Receptor1...

Relative binding affinity (RBA) assay in serum-free medium determined by Nagel et al. [110]. 

Table 7. Chemical structure, molecular formula, molecular mass, relative binding affinity (RBA) to the androgen receptor (AR), n-octanol/water partition coefficient (log Kqw)> and bioconcentration factors on a wet weight basis (BCF ) and on a lipid basis (BCFl) of Steroidal Androgens and Nonsteroidal Chemicals with Antiandrogenic Activity... 

Table 1 Relative binding affinity (RBA) of some mononuclear organometallic estradiol derivatives to the ERa ...

It is generally understood that in order for a compound to act via the estrogen receptor, it must be recognized by this receptor. Recognition is measured by the relative binding affinity (RBA) value of the compound for the ER, the reference value being that of estradiol (RBA = 100% by definition). It should also be noted that a second form of the receptor, ERfi, was recently discovered in various tissues and has been found in breast cancers classified as hormone-independent However, the exact role of this receptor in the cell is not yet well understood and remains a subject of debate [54, 55]. 

The relative binding affinity(RBA) of each chemical was calculated as the ratio of concentrations of 176-estradiol and chemical required to reduce the specific radioligand binding by 50%. RBA value for 176-estradiol was arbitrarily set at 100(2). 

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