Binding of bilirubin by bovine and human alpha-fetoprotein.
alpha-Fetoprotein, a fetal protein associated with certain tumors, was found to bind bilirubin. Addition of human or bovine alpha-fetoprotein to bilirubin solutions enhanced the light absorbance of bilirubin and shifted its  maximum. Bovine alpha-fetoprotein caused a marked shift towards shorter wavelengths, while human alpha-fetoprotein gave a slight red shift. The spectral changes were used to study the characteristics of the binding of bilirubin by bovine alpha-fetoprotein. These studies indicated the presence of one binding site/molecule of alpha-fetoprotein with an association constant of about 1.1 . 10(6) M-1. A difference between the spectral changes brought about by alpha-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 alpha-fetoprotein to bovine albumin was 6.3 : 1, and of the human proteins 21 : 1, respectively. These results show that alpha-fetoprotein from two species binds bilirubin with an affinity somewhat lower than that of albumin. Binding of bilirubin by alpha-fetoprotein is in agreement with the recent demonstration of structural homology between alpha-fetoprotein and albumin. Whether alpha-fetoprotein plays a role in the metabolism of bilirubin or other degradation products of heme remains to be investigated.
D000818:Animals; D001663:Bilirubin / Q000378:metabolism*; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D006801:Humans; D011485:Protein Binding; D012710:Serum Albumin, Bovine / Q000378:metabolism; D013045:Species Specificity; D013053:Spectrophotometry; D000509:alpha-Fetoproteins / Q000378:metabolism*
D000509:alpha-Fetoproteins; D012710:Serum Albumin, Bovine; D001663:Bilirubin