ALPHA-FETOPROTEIN AS A CARRIER PROTEIN IN PLASMA AND ITS BILIRUBIN-BINDING ABILITY

The bilirubin-binding ability of human .alpha.-fetoproteins, which were purified from fetal cord serum and from ascites fluid of a hepatoma-bearing patient, was examined by the difference spectrum and the Jacobsen peroxidase methods. The difference spectrum observed as a result of the specific binding of bilirubin to .alpha.-fetoprotein had a maximum at 482 nm, and this pattern was similar to that observed for serum albumin. The result obtained by the difference spectrum method showed that 1 mol of .alpha.-fetoprotein bound 1 mol of bilirubin at pH 8.3, and the dissociation constants of the complexes of bilirubin with fetal .alpha.-fetoprotein and hepatoma-derived .alpha.-fetoprotein were 2.6 .times. 10-7 and 5.0 .times. 10-7 M, respectively. The Jacobsen enzymatic method using horseradish peroxidase gave the same values for molar binding ratios and similar dissociation constants, 7.1 .times. 10-7 M for fetal .alpha.-fetoprotein and 7.4 .times. 10-7 M for hepatoma-derived .alpha.-fetoprotein. .alpha.-Fetoprotein may function as a carrier protein for bilirubin as was shown for serum albumin.