Noninvasive monitoring of bilirubin photoisomer excretion during phototherapy

Lumirubin is the most prevalently excreted hydrophilic bilirubin photoisomer in phototherapy for neonatal jaundice caused by excess hydrophobic unconjugated bilirubin (ZZ-bilirubin). We developed a simple method to estimate the amount of lumirubin by monitoring the reverse photoisomerization of lumirubin to ZZ-bilirubin. Although lumirubin formation was long considered irreversible, exposure to blue light in the presence of the fluorescent protein UnaG, which binds specifically and tightly to ZZ-bilirubin, enables the reverse photoisomerization of lumirubin. This reaction was first detected using a fluorescence assay of neonatal urine sampled during phototherapy and purified lumirubin. The phenomenon of reverse photoisomerization of lumirubin was validated using liquid chromatography–mass spectrometry, which confirmed that lumirubin is reconverted to ZZ-bilirubin in the presence of UnaG. Analyses of 20 urine samples from 17 neonates revealed a significant correlation (correlation coefficient [ r ] = 0.978; 95% confidence interval 0.867–0.979; P < .001) between lumirubin and ZZ-bilirubin concentration before and after reverse photoisomerization. In general, the rate of photo-reconversion of lumirubin to ZZ-bilirubin is approximately 40%. In conclusion, we demonstrate here that lumirubin can be photo-reconverted to ZZ-bilirubin via exposure to blue light in the presence of UnaG. Utilizing this approach, urinary lumirubin levels can be estimated using an easy-to-perform fluorescence assay.