Photoisomerized bilirubin in blood from infants receiving phototherapy.

A pigment different from (Z,Z)bilirubin-IX alpha was detected by fluorometric methods in blood specimens from newborn infants undergoing blue-light therapy for unconjugated hyperbilirubinemia; it was not detected in specimens from infants not under therapy. The phototherapy-associated pigment has fluorescence, solubility, and photochemical properties that are identical to those exhibited by what are thought to be configurational (Z leads to E) isomers of bilirubin. It is concluded that isomerized bilirubin in the blood of neonates under phototherapy can reach as high as 15% of the total.