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alpha-Foetoprotein:immunoreactivity of the major oestrogen-binding component in mouse amniotic fluid.

Competitive protein binding studies, using oestrone as the inhibitor, demonstrated that mouse amniotic fluid contains a high affinity oestradiol-binding component precipitable by monospecific rabbit anti-alpha-foetoprotein (AFP) in a double-antibody radio-immunoassay. On disc gel radio-electrophoresis the oestrogen-binding entity migrated as an alpha-1 protein which was immunoreactive to anti-AFP serum. Immunoprecipitation of protein-bound [3H]-oestradiol from solution was proportional to anti-AFP immunoglobulin G concentration. Rabbit antisera directed against other major proteins in amniotic fluid did not produce significant precipitation of radioactivity. Only when anti-AFP antiserum was the complexing protein and AFP, either crude or purified, was the binding protein did a substantial [3H]-oestradiol precipitate form. Our data suggest that oestradiol and anti-AFP do not bind at the same sites on the AFP molecule and that estradiol does not serve as a hapten in the production of anti-AFP serum in rabbits.