J. Biol. Chem., Vol. 263, Issue 6, 2824-2829, 02, 1988
Studies on the interaction between human serum protein fractions and 18O-labeled oxosteroids
CG Eriksson and P Eneroth
Department of Obstetrics and Gynecology, Karolinska Hospital, Stockholm, Sweden.
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.
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