Circular dichroism analysis of the secondary structure of Z protein and its complexes with bilirubin and other organic anions.

Circular dichroism (CD) methods were employed to study the conformation of Z protein and characterize its complexes with bilirubin and other organic anions. Z protein-bilirubin complexes exhibited a spectrum with overlapping ellipticity bands of opposite sign in the bilirubin absorption region. These results were compared with those obtained with ligandin, the other major organic anion binding protein of liver. Secondary structural differences between the two proteins were easily demonstrated since ligandin is predominantly an alpha-helical protein and Z features mainly beta-structure. Furthermore, the optical activity pattern generated by bilirubin binding to Z was virtually a mirror image of that of the ligandin bilirubin system. CD experiments were designed to study the direct transfer of bilirubin between Z protein and ligandin, and it was shown that both proteins have almost equal affinities for bilirubin. The bilirubin on Z was readily displaced by oleic acid and displaced to a lesser extent by sulfobromophthalein,