alpha-fetoprotein binding specificity for arachidonate, bilirubin, docosahexaenoate, and palmitate. A spin label study.

A dianionic spin label, 1-L-glutamate-5-N-(1-oxyl-2,2,6,6-tetramethyl-4-aminopiperidinyl)-i,4-dinitrobenzene, has been used to probe the relative binding specificity of a single anionic ligand site on bovine alpha-fetoprotein (AFP) to arachidonate, bilirubin, docosahexaenoate, and plamitate. The binding isotherm of the spin label with AFP, as shown by a Scatchard plot, indicates the presence of a single high affinity binding site. The site-site relationship of the four endogenous ligands, arachidonate, bilirubin, docosahexaenoate, and palmitate, was determined by studying their effectiveness in competing for this anionic ligand binding site on AFP. Scatchard plots of the spin label in the presence of 1 to 3 molar equivalents of arachidonate, bilirubin, and docosahexaenoate and up to 6 molar equivalents of palmitate have been determined. The effectiveness of the four endogenous ligands in displacing the spin label from its primary binding site is bilirubin greater than or equal arachidonate approximately equal to docosahexaenoate greater than palmitate. These results indicate that polyunsaturated essential fatty acids and bilirubin share a high affinity binding site on AFP. We propose that the function of this anionic ligand binding site on AFP is for the transport of bilirubin and polyunsaturated fatty acids in fetal serum, as well as for the cross-placental transfer of this metabolite and of essential fatty acids.