Carrier Proteins in Human Fetal Serum : Bilirubin-Binding Abilities of Albumin, α-Fetoprotein and Ligandin

The bilirubin-binding abilities of human serum albumin, α-fetoprotein and ligandin were investigated by employing absorbance spectral measurement, peroxidation and fluorescence measurement techniques. The binding constants of the proteins from adult serum and cord serum were very similar. However, those of α-fetoprotein were slightly smaller than those of albumin. Ligandin had two cooperative binding sites for bilirubin, and the binding constants were of the same order as those of the weaker binding sites of albumin. A study of the effect of linolic acid revealed that the bilirubin bound to α-fetoprotein was more easily liberated in the presence of linolic acid than that bound to albumin binding. The drug-binding abilities of these proteins were also examined, and no significant difference was found between adult serum and cord serum albumins. However, α-fetoprotein appeared not to exhibit drug-binding ability when the peroxidation method was employed. The biological role of α-fetoprotein in fetal plasma may be similar to that of albumin.