Interaction of bilirubin with human serum albumin and cationic detergents.

In biological systems, once the unconjugated bili­ rubin is released into the plasma, it is transported rather firmly bound to albumin1_3. The binding equilibria of bilirubin to albumin and tissues are of great interest in order to be able to understand the development of hyperbilirubinemia3. It is well established that while the first mole of bilirubin is tightly bound to albumin, the subsequent moles of bilirubin show a less affinity4’5. Free bilirubin at pH about 7 exhibits in the visual spectra an absorp­ tion maximum at approx. 440 nm 5_7. Its spectra show a red-shift after binding to human serum albu­ min, at a bilirubin : albumin molar ratio of 1 : 1 , where the maximum shifts to around 457 nm, de­ pending on the ionic strength, pH and method of pre­ paration 8; 9. Recent studies by various investigators have indicated that a variety of non-covalent and hydrophobic interactions may contribute to the stabi­ lity of the humanor bovine serum albumin-bilirubin complex 6’10. It was therefore considered of interest to compare the spectral changes of bilirubin on binding to human serum albumin with spectral changes of bilirubin caused by a. cationic detergents and b. addition of ethanol.