647 WHERE DOES THE BILIRUBIN GO WHEN YOU TURN ON THE LIGHTS?

Abstract

Phototherapy has been used to reduce bilirubin levels in jaundiced infants for more than 25 years. In vitro and in vivo studies have shown that bilirubin is converted to more polar isomeric forms when exposed to light. There are two types of isomers formed, configurational (E-bilirubins) and structural (lumirubins). The relative contribution of these two photo-products to the response of neonates to phototherapy has not previously been established. The E-bilirubins are formed more rapidly than the lumirubins; however, the decline in plasma bilirubin depends not only upon formation but also upon excretion. In a previous study we found that the excretion of the E-bilirubins in premature infants is too slow (serum half-life = 15 hours) to account for observed reduction in bilirubin levels. We report here evidence that excretion of lumirubins is the principal route of pigment elimination during phototherapy. The study population consisted of nine premature infants with a mean gestational age of 31 ± 2 weeks (± S.D.). Bile samples were obtained during phototherapy and analyzed by high pressure liquid chromatography for bilirubin and its isomers. We found that the major pigment, and in some infants the only pigment, present in the bile was lumirubins. Five serum samples were obtained from each infant during the first 90 minutes after stopping phototherapy, and the measured serum half-life of lumirubins was 120 ± 25 minutes. This is the first demonstration that lumirubins are more important than E-bilirubins to the efficacy of phototherapy.

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