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Abstract
Quantitative determination of bilirubin (unconjugated, by chloroform extraction), bilirubin diglucuronide (by the isotope derivative method) and albumin was done in 26 specimens of amniotic fluid from cases suspected of hemolytic disease in utero.
A hypothetical scheme of fetal bilirubin metabolism is proposed, presuming unidirectional transplacental elimination of unconjugated bilirubin. On the basis of theoretical considerations of the mechanism of conjugation it is thought possible that bilirubin in the fetal liver is in chemical equilibrium with its diglucuronide. Both pigments are in equilibrium with the pigments in the amniotic fluid. Equilibration takes place through the fetal plasma. The ratio of concentrations of bilirubin and bilirubin diglucuronide in the amniotic fluid is determined by the following three factors: the relative affinities of the binding of the two pigments to albumin, the ratio of concentrations of UDPGA to UDP in the liver, and the standard free energy of conjugation. The slow glycogen synthesis in the fetal liver may be responsible for a high ratio of UDPGA to UDP and hence for a high proportion of conjugated bilirubin in amniotic fluid.
The experimental findings are in agreement with this hypothesis, offering an alternative to the generally accepted conception of low transferase activity and impaired hepatic excretion of conjugate before and shortly after birth.