(EZ)-Cyclobilirubin formation from bilirubin in complex with serum albumin derived from various species.

We determined the specificity of photochemical changes of bilirubin in complex with serum albumin from various species. There were no general trends in the configurational photoisomerizations of (ZE)-bilirubin/(ZZ)-bilirubin and (EZ)-bilirubin/(ZZ)-bilirubin associated with the albumins from various species as compared to those associated with human serum albumin. The absorbance spectra of bilirubin in complex with albumins from various species differed, indicating that the three-dimensional structures of (ZZ)-bilirubin bound to the various serum albumins, which are substrates of (ZE)- and (EZ)-bilirubin, differ among species. The rates of conversion of the (EZ)-bilirubin isomer into the structural cyclobilirubin isomer were similar for the albumins of chicken, rat, rabbit, dog, bovine, and pig, and were significantly slower than the rate for human serum albumin. This suggests that the three-domain human albumin has evolved to allow ready conversion of (EZ)-bilirubin to (EZ)-cyclobilirubin. Cyclobilirubin formation in a bilirubin-alpha-fetoprotein solution was much lower than that in a bilirubin-human serum albumin solution. It is believed that the ability of human serum albumin to facilitate the photochemical change of bilirubin was evolutionarily selected in response to neonatal jaundice in humans.