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In the human fetus, as in the adult, biliverdin-IXα and any small amounts of non-IXα isomers that are formed are reduced to the corresponding bilirubins. Of these, bilirubin-IXα is uniquely hydrophobic and lipophilic, and ready to cross the placenta for elimination by the mother. In utero, residual non-IXα isomers too polar to cross the placenta, particularly the IXβ isomer, accumulate and are detectable in bile and meconium by 15 weeks gestation.1 This observation has led some to conclude erroneously that heme catabolism in the fetus yields predominantly the IXβ isomer. The major form of bilirubin generated in infants and adults alike is bilirubin-IXα, and can be measured in various forms (Table 3-1).

Table 3-1. Bilirubin Nomenclature and Chemical Behavior

The poor solubility of bilirubin can be explained by considering its chemical three-dimensional structure.2 Although often represented as a linear structure for convenience (Figure 3-1, structure 5), bilirubin has a folded flexible structure in which the weakly acidic propionic acid side chains can stretch and form internal hydrogen bonds with spatially proximate nitrogen and oxygen groups. This results in a compact structure in which the surface is lipophilic and the polar parts of the molecule are protected from interactions with solvent water. The stereochemical configuration of the two double bonds between the rings in bilirubin is the same as in heme from which the bilirubin was derived and is designated unambiguously in current organic chemistry nomenclature as Z (from zusammen, German: together) (in contradistinction to E [entgegen: opposite], the other possible configuration). Because of its low solubility in water at physiologic pH, bilirubin requires a carrier molecule for transport from the reticuloendothelial system to the liver for excretion.3 In blood and extravascular fluid, ...

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