MONOCLONAL-ANTIBODIES AGAINST PROGESTERONE - EFFECT OF STEROID-CARRIER COUPLING POSITION ON ANTIBODY SPECIFICITY

Monoclonal anti-progesterone antibodies were raised by immunizing mice with progesterone coupled through either the C3, C6 or C11 positions to protein carrier (bovine serum albumin, BSA). The specificity of fou r antibodies for a range of steroids related to progesterone, some car rying substitutions at various ring positions, was studied by competit ive inhibition in an ELISA system. The results demonstrated that the r ing coupling position has a determining effect on the cross-reactivity of the antibodies obtained. The patterns of cross-reaction were inter preted in the light of the structure of the combining site of an anti- progesterone antibody (DB3) recently determined by X-ray crystallograp hy, and inferences drawn about the orientation of steroid in the combi ning sites of the antibodies studied. Specifically, in two antibodies raised against progesterone-11-BSA, the orientation of steroid resembl ed that of the progesterone-DB3 complex, with positions C11 and C3 exp osed and C6 and C20 buried; an antibody raised against progesterone-6- BSA bound steroid in an apparently similar disposition, except that C6 was exposed and C11 buried; finally, in an antibody raised against pr ogesterone-3-BSA, all steroid positions other than C3 were apparently buried in the steroid-antibody complex.