Changes in the Composition of Bilirubin‐IX Isomers During Human Prenatal Development

We analyzed the isomeric composition of bilirubin-IX in human fetal bile using HPLC. The approximate ratio of the bilirubin-IX isomers obtained from the fetal bile at 20 weeks of gestation was IX alpha, 6%; IX beta, 87%; IX gamma, 0.5%; and IX delta, 6%. From 15 to 22 weeks, bilirubin-IX beta was predominant and bilirubin-IX delta and bilirubin-IX alpha were also present in the bile as minor components. By 28 weeks, bilirubin-IX alpha constituted about 50% of the total bilirubin. There was a general correlation between fetal age and the proportion of bilirubin-IX alpha to bilirubin-IX beta in the bile and the small intestinal contents of fetuses. As development proceeded from mid-gestation to near term, the isomeric composition dramatically changed, with a decrease in the IX beta isomer and a subsequent increment of the IX alpha isomer. In contrast, the IX delta isomer changes little. Recently, we identified four forms of biliverdin reductase including two biliverdin-IX alpha reductases and two biliverdin-IX beta reductases in human liver cytosolic fractions [Yamaguchi, T., Komoda, Y. & Nakajima. H. (1994) J. Biol. Chem. 269, 24,343-24,348]. The proportion of the total activity of biliverdin-IX beta reductases to that of biliverdin-IX alpha reductases was considerably higher in the fetal, than in the adult liver.