Evidence for an intrinsic factor bound to rat alpha 1 fetoprotein. Fluorescence investigation.

An endogenous chromophore regularly associated with rat alpha 1 fetoprotein was detected by a systematic study of the fluorescence spectra of the protein purified by immunoabsorption. This fluorescent ligand, of low molecular weight, was dissociated from rat alpha 1 fetoprotein by exhaustive dialysis of the protein at low concentration. A phase partition method with aqueous preparations of rat alpha 1 fetoprotein allowed extraction of this chromophore into n-butanol and determination of its spectrofluorimetric characteristics: an excitation maximum at 305 nm and an emission maximum at 365 nm. We have also shown that this endogenous chromophore competed with estradiol-17 beta for the same binding sites in rat alpha 1 fetoprotein. In conclusion, the implication of a specific transport role of this protein for small ligands in fetal development is discussed.