Metabolism of Bilirubin and Its Photoisomers in Newborn Infants during Phototherapy1

Bilirubin and its photoisomers in the biological fluids of a hyperbilirubinaemic newborn infant before and during phototherapy were analyzed by a recently improved HPLC method. In the serum, the percentages of ( EZ )- and ( ZE )-bilirubin in the total bilirubin concentration before phototherapy were approximately 10% and on average increased over 1.5-fold at 2 h after initiation of phototherapy. The percentage of the ( EZ )-cyclobilirubin in the serum biirubin was under 1 %. In the bile, the mean concentration of ( ZZ )-biirubin, derived mainly from ( ZE )-biirubin, nearly tripled during phototherapy. The ( EZ )-cyclobiirubin concentration in the bile was very low before phototherapy, increased nearly ten-fold at 3 h after initiation of phototherapy, and was 5- to 6-fold as high as that of ( ZZ )-biirubin. In the urine, upon exposure to light, the urinary concentration of ( EZ )-cyclobilirubin is apparently equivalent to half of the biliary concentration of ( ZZ )-bilirubin and one-fifth of that of ( EZ )-cyclobilirubin. It was concluded that during phototherapy of neonatal hyperbilirubinaemia the structural photoisomer [( EZ )-cyclobilirubin] predominates considerably over the geometric photoisomer [( ZE )-bilirubin].