Relations between fatty acids and oestrogen binding properties of pure rat alpha1-foetoprotein

A delipidation procedures based on treatment with charcoal at pH 3 has been applied to highly purified rat alpha₁-foetoprotein preparations. The oestrogen binding properties of the delipidated proteins have been studied with an equilibrium dialysis technique, and compared with the properties of the untreated foetal protein, as well as those of preparations reconstituted from the defatted α₁-foetoprotein and the removed lipids. An important increase has been evidenced for the binding levels of oestrone, oestradiol-17β and diethyl-stilboestrol by the delipidated α₁-foetoprotein. A reversal of this effect has been obtained by incubating the delipidated protein either with the lipids extracted from the purified α₁-foetoprotein or with a potent competitor of the rat α₁-foetoprotein-oestrogen interaction, designated as ‘L’, previously demonstrated and isolated from whole rat sera, and tentatively characterized as a mixture of fatty acids. Scatchard analysis of the oestrone and oestradiol-17β binding parameters show that the enhanced fixation of the hormones after defatting is primarily due to a two-fold increase of the apparent number of binding sites/ mol α₁-foetoprotein. The results are interpreted in terms of the probable, at least partial, identity between the lipids closely associated with the pure α₁-foetoprotein and the fatty acid mixture ‘L’ isolated from whole sera. The possible biological role of a complex interplay between oestrophilic α₁-foetoproteins, phenolsteroids and fatty acids in the control of eostrogen levels during development is discussed brieftly.