On the binding of steroid sulfates to albumin

3H-Labeled steroid sulfates, sulfate of estrone (E1S) or dehydroepiandrosterone (DHAS), were dialyzed against delipidated human serum albumin or human plasma in the presence of increasing amounts of competing non-labeled sulfates (DHAS or E1S). The apparent equilibrium constants (K) of the tracers did not measurably change at concentrations of the non-radioactive sulfates below 10(-5) mol/l. At higher concentrations, K decreased gradually. The apparent equilibrium constant of 3H-E1S was diminished by plasma in a similar fashion. It may be concluded that albumin possesses one strong, non-specific binding site. This site, however, does not seem to be utilized for the binding of E1S in vivo, because of its preferential occupation by other ligands. This may be true for other steroid sulfates as well, depending on their relative abundance in plasma.