Binding of 5α-dihydroprogesterone to proteins in human pregnancy serum

The binding of 5α-dihydroprogesterone (DHP)1 to proteins in pregnancy serum has been investigated and compared with the binding of progesterone. Characteristic properties of the DHP-serum protein interaction were unsaturability, low affinity and poor complex stability. Fractionation of serum using a variety of protein separation techniques, revealed that DHP interacts with several proteins. At low temperature (0–4°C) albumin appeared to be the principal binding component whereas higher temperatures seemed to favour binding to β-lipoproteins and α2-macroglobulin. Binding to specific binding proteins such as the corticosteroid binding globulin (CBG) and the sex hormone-binding globulin, (SHBG) were detectable but appeared to be quantitatively unimportant. Progesterone showed a similar multicomponent interaction but differed from DHP in the extent of binding to CBG. Binding of either hormone to the α1-acid-glycoprotein was negligible. The present study shows that the high endogenous DHP levels present in pregnancy sera are caused by factors other than high affinity protein binding.