BINDING OF BILIRUBIN BY BOVINE AND HUMAN ALPHA-FETOPROTEIN

α-Fetoprotein, a fetal protein associated with certain tumors, was found to bind bilirubin. Addition of human or bovine α-fetoprotein to bilirubin solutions enhanced the light absorbance of bilirubin and shifted its maximum. Bovine α-fetoprotein caused a marked shift towards shorter wavelengths, while human α-fetoprotein gave a slight red shift. The spectral changes were used to study the characteristics of the binding of bilirubin by bovine α-fetoprotein. These studies indicated the presence of one binding site/molecule of α-fetoprotein with an association constant of about 1.1 · 106 M-1. A difference between the spectral changes brought about by α-fetoprotein and albumin allowed comparison of their relative affinities for bilirubin. The spectrum approximated the average between the spectra induced by the two proteins when the ratio of bovine α-fetoprotein to bovine albumin was 6.3 : 1, and of the human proteins 21 : 1, respectively. These results show that α-fetoprotein from two species binds bilirubin with an affinity somewhat lower than that of albumin. Binding of bilirubin by α-fetoprotein is in agreement with the recent demonstration of structural homology between α-fetoprotein and albumin. Whether α-fetoprotein plays a role in the metabolism of bilirubin or other degradation products of heme remains to be investigated. © 1979.