Binding of Photobilkirubin to Human Serum Albumin

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Abstract
Results of experiments bases upon circular dichroic spectra suggest that configuratinally (Z → E) isomerized bilirubin (photobilirubin) binds to human serum albumin at the primary bilirubin biding site withan affinity only 2–3‐times lower than that of biliruin. The high affinity of photobiliruin fopr albumin, comparable to that of bilirubin, supports the roles of albumin in the stablization and transport of the isomerized pigment in vivo and stronlgy suggests that albumin also functions to sequester photobilirubin effectively, reducing its toxic potential. The high affinity of photobilirubin for albumin predicts that the isomerized pigment, formed in large amounts during phototherapy ofr neonatal hyperbilirubinemia, should not appear in the fast‐diazo‐reacting (‘dirct’) bilirubin pool nor should it interfere wth nonspectroscopic biliruin binding tests. These predicions were confirmed for the Evelyn and Malloy diazo assay for ‘direct’ bilirubin and a Sephadex chromatography method for assessing ‘loosely bound’ plasma bilirubin.