New concepts in phototherapy: photoisomerization of bilirubin IX alpha and potential toxic effects of light.

New information is summarized, indicating that configurational photoisomerization of bilirubin at the 5 and 15 carbon bridges is the major mechanism of bilirubin photocatabolism in vivo, and that singlet oxygen photooxidation plays only a minor role. The literature is reviewed concerning potentially damaging photodynamic reactions that are observed in vitro with vitamins, proteins, lipids, and nucleic acids, and their possible relationships to the limited number of toxic side-effects that have been detected with clinical phototherapy of neonatal jaundice. Secondary toxic effects, mediated by bilirubin photoderivatives or by retina-neuroendocrine pathways are also considered. Areas requiring further investigations are delineated.