Studies on the interaction between human serum protein fractions and 18O-labeled oxosteroids

The nature of the interaction between progesterone or testosterone and human albumin as well as the interaction between progesterone and partially purified human transcortin has been studied. Modification of lysine residues of albumin with maleic anhydride resulted in a decreased binding of the steroid as judged from equilibrium dialysis experiments. This suggested that lysine residues in albumin interact with the oxosteroids. In order to check this hypothesis, steroids labeled with 18O in their oxo function (testosterone and progesterone) were synthesized for use as probes of the interactions. However, no loss of label was noted when testosterone or progesterone specifically 18O-labeled in their oxo functions were incubated with albumin. This suggested that no covalent interaction between the steroidal oxo group and albumin took place. This was in contrast to the results obtained with 3,20-18O-labeled progesterone and partially purified transcortin, where a complete loss of 18O label in the protein-bound steroid was found. The nonbound steroid showed an almost complete retention of label. These results indicate a participation of steroid oxo groups in the binding of progesterone to transcortin. Of the possible mechanisms discussed, imine bonds between the steroid and transcortin seem most likely although other types of interactions cannot be ruled out.