Steroid Receptors and Hormone Receptivity

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Abstract
HORMONES are informational molecules. These chemical messengers must find, in the target cells, specific recognition mechanisms that will select them appropriately from all other components of the "milieu intérieur." These recognition mechanisms, which involve binding sites (r on Fig 1) of very high affinity (dissociation constant, KD, between 0.1 and 10 nanomolar [nM], adjusted to the low concentration of hormones in the plasma) and strict (stereo) specificity, constitute the primary function of receptors. We know that the catalytic sites of enzymes also display specificity, but the affinity is 104times lower than that of the hormone-receptor binding sites, since substrates such as sugars, amino acids, or lipids are found there in higher concentration and then transformed into products. In contrast, the interaction of hormones with their receptors does not alter their chemical composition per se; it is a purely "physical" phenomenon. Receptors The capacity for specific binding, even