A specific receptor with high affinity for rat androgen-binding protein (rABP) was identified in isolated adult rat germ cells and in the corresponding plasma membrane-enriched preparations. Binding was reversible and time-dependent, with maximum relative binding after 40 min at 4 °C; it was pH-dependent, with maximum binding at pH 6–8. Unlabelled rABP and human sex steroid-binding protein (hSBP), but not lactotransferrin, serotransferrin, asialofetuin, fetuin or bovine serum albumin, competed with labelled rABP for binding sites on isolated germ cells. Scatchard analysis revealed a single class of binding site with apparent dissociation constant (K d) values of 0·78±0·04 nm and 0·97 ± 0·05 nm in intact germ cells and plasma membrane preparations respectively. A K d of 1·72±0·12 nm for hSBP showed that the receptor binding site was effective for both androgen-carrier molecules. Labelled rABP incubated with solubilized germ cell membrane fractions at pH 7 formed a complex excluded from Superose 6B mini-gels; this complex was not formed at pH 3. The receptor complex was also abolished in the presence of a 100fold excess of either unlabelled rABP or unlabelled hSBP, or in the presence of 20 mm EDTA.
These results suggest that the plasma membrane of rat germ cells contains a receptor which selectively binds rABP and hSBP.