Development of Phenylalanine Hydroxylase in Liver of the Rat. ∗

Summary

Development of phenylalanine hydroxylase in livers of fetal, newborn, and adult rats has been studied. The results indicate that the enzyme system develops shortly after birth. Activity of the enzyme is negligible in livers of fetal rats near term and rats less than 24 hours old; it approaches the adult level in livers of animals only a few days old. The virtually inactive preparations obtained from livers of fetal and newborn animals could not be activated by adding increasing amounts of pyridine nucleotide to the reaction mixtures in either oxidized or reduced form. The activity of soluble fractions of adult liver was partially lost during short dialysis; the initially low activity of preparations of livers from fetal and newborn animals was particularly sensitive to this procedure.