Effect of phototherapy on the urinary excretion of tryptophan metabolites in neonatal hyperbilirubinemia.

The 24 hours urinary excretion of tryptophan metabolites after an aminoacid loading (50 mg/Kg body-weight) was studied in a group of hyperbilirubinemic infants with or without light exposure treatment, in comparison to normal newborn babies. Kynurenine was the main metabolite in the urine of the control subjects. In the hyperbilirubinemic infants a high urinary excretion of kynurenine and 3-hydroxyanthranilic acid was observed. In the ones treated with phototherapy these two substances were markedly decreased. Besides 3-hydroxykynurenine was almost always absent. The effect of the light exposure was studied on kynurenine and 3-hydroxykynurenine "in vitro". Kynurenine did not present any photodecomposition. On the contrary, 3-hydroxykynurenine was easily decomposed. This fact can explain the decreased excretion of 3-hydroxykynurenine and 3-hydroxyanthranilic acid observed in urine of hyperbilirubinemic newborn infants treated with phototherapy. The decrease of kynurenine in these infants probably is a consequence of the photodecomposition of 3-hydroxykynurenine.