Bilirubin photoisomers in rhesus monkey serum

As rhesus monkeys exhibit physiological jaundice during the neonatal period, we used rhesus monkey serum to examine changes in bilirubin photoisomers. Bilirubin-rhesus monkey serum solution was irradiated with blue light-emitting diode, and changes in the absorbance and bilirubin fraction were compared with those in bilirubin- human serum albumin (HSA) and bilirubin-rat albumin solutions. The λ_max decreased with light irradiation. The mean production rate of cyclobilirubin IXα was 1.98, 199 and 0.76 × 10^-2/min in rhesus monkey serum, HSA and rat albumin, respectively. There was no significant difference between rhesus monkey serum and HSA. The (ZE)-bilirubin IXα/(ZZ)-bilirubin IXα ratio was 0.33, 0.45, and 0.10, respectively, differing significantly among the groups. The (EZ)-bilirubin IXα/(ZZ)-bilirubin IXα ratio was 0.020, 0.010, and 0.062, respectively, with no significant difference between rhesus monkey serum and HSA. The production rate of (EZ)-cyclobilirubin XIIIα(= (ZE)-cyclobilirubin XIIIα) was 0.73, 1.60, and 0.51 × 10^-2/min, respectively, with differing significantly among the groups. The (EZ)-bilirubin IIIα/(ZZ)-bilirubin IIIα ratio was significantly different among the groups at 0.20, 0.38, and 0.15, respectively. This is the first report demonstrating the photoisomerization of bilirubin in rhesus monkey serum and the animal with the same cyclobilirubin production rate as HSA.Rhesus monkeys may be used as an animal model for neonatal hyperbilirubinemia in humans to evaluate the efficacy of phototherapy.