Bilirubin and biliverdin binding to rat Y protein (ligandin).

The binding of bilirubin to poly(L-lysine) produces an optically active complex at pH 10.1. Circular dichroism spectra of these complexes are distinguishable from those generated by binding of bilirubin to the high affinity sites on albumin. Comparison of the circular dichroism spectra of bilirubin bound to the hepatic protein ligandin with those of bilirubin complexed with albumin or polylysine indicates that binding of bilirubin to ligandin occurs at two types of sites. These are distinguishable on the basis of their spectral properties, one resembling the high affinity site of bovine serum albumin and the other resembling polylysine. Complexes of biliverdin with albumin and ligandin bear similarities to the bilirubin-protein complexes. The native protein itself has an ordered structure which consists of 41% alpha-helix and is not altered by the binding of bilirubin.