CONFIGURATIONAL ISOMERIZATION OF BILIRUBIN AND THE MECHANISM OF JAUNDICE PHOTOTHERAPY

Bilirubin-IXα and its dimethyl ester, which have the Z configuration at the meso bridge double bonds, undergo rapid and reversible photoisomerization on irradiation with visible light giving Z-E, E-Z and E-E isomers. At photoequilibrium [Z-Z] ≫ [E-Z] ≈ [Z-E] > [E-E]. The non-Z-Z isomers of bilirubin-IXα are less lipophilic and more polar than the natural Z-Z isomer. The reaction probably is important in vivo during phototherapy of neonatal jaundice and responsible for the biliary excretion of unconjugated pigment associated with it. © 1979.