Heterogeneity of pituitary glucocorticoid binding evidence for a transcortin-like compound.

Scatchard analysis, as well as sucrose gradient centrifugation demonstrate that at least two classes of specific glucocorticoid binding sites occur in the adenohypophysis. One, exhibited high affinity for both corticosterone and dexamethasone (receptor D), while the other bound only corticosterone. The latter binder revealed physico-chemical properties closely similar to those of plasma transcortin, but was shown not to be due to simple blood contamination of cytosol. This transcortin-like (TL) component, however, seemed to be carried over from plasma, since the concentration of pituitary binding sites strikingly paralleled physiological variations of plasma transcortin. Furthermore, experiments carried out with isolated pituitary cells suggested an intracellular location of that binder, although sticking to the cell membrane cannot be excluded. Finally, it was shown that the TL compound was unable, unlike the receptor D, to transfer glucocorticoids into the nuclei. One of its possible roles could be to modulate the interaction between steroids and compound D.