The serum competitor of oestrogen--rat alpha 1-foetoprotein interactions. Identification as a mixture of non-esterified fatty acids.

The novel endogenous serum ligands of rat alpha 1-foetoprotein previously demonstrated in different mammalian sera were identified by g.l.c.--mass-spectrometric methods as a mixture of non-esterified long-chain and predominantly unsaturated fatty acids. Detailed comparative analyses of these ligands extracted from foetal- and pregnant-rat sera, rat amniotic fluid and foetal human sera are presented. We also show that an important fraction of these ligands remains associated with the rat alpha 1-foetoprotein after purification; analyses are given for the composition of this lipid moiety of the foetoprotein. The physiological relevance of these results is discussed.