Hsia J C, Er S S, Tan C T, Estes T, Ruoslahti E
J Biol Chem. 1980 May 10;255(9):4224-7.
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.
一种双阴离子自旋标记物，1-L-谷氨酸-5-N-(1-氧基-2,2,6,6-四甲基-4-氨基哌啶基)-1,4-二硝基苯，已被用于探究牛甲胎蛋白（AFP）上单个阴离子配体位点对花生四烯酸、胆红素、二十二碳六烯酸和棕榈酸的相对结合特异性。如Scatchard图所示，自旋标记物与AFP的结合等温线表明存在一个单一的高亲和力结合位点。通过研究花生四烯酸、胆红素、二十二碳六烯酸和棕榈酸这四种内源性配体竞争AFP上该阴离子配体结合位点的有效性，确定了它们之间的位点关系。已测定了在存在1至3摩尔当量的花生四烯酸、胆红素和二十二碳六烯酸以及高达6摩尔当量的棕榈酸时自旋标记物的Scatchard图。这四种内源性配体将自旋标记物从其主要结合位点置换出来的有效性为：胆红素大于或等于花生四烯酸，约等于二十二碳六烯酸，大于棕榈酸。这些结果表明，多不饱和必需脂肪酸和胆红素在AFP上共享一个高亲和力结合位点。我们提出，AFP上这个阴离子配体结合位点的功能是在胎儿血清中运输胆红素和多不饱和脂肪酸，以及这种代谢物和必需脂肪酸的跨胎盘转运。
