PHENYLALANINE HYDROXYLASE ACTIVITY IN HUMAN TERM PLACENTA

Abstract

Phenylketonuria (PKU) is an autosomal recessive aminoacidopathy caused by a deficiency of phenylalanine hydroxylase. PKU is the most common aminoaciduria which responds to dietary restriction of phenylalanine. The enzymic studies of PKU have been difficult to approach since phenylalanine hydroxylase activity has been found primarily in liver. Recently some phenylalanine hydroxylase activity has been reported in cultured human fibroblasts, but the activity seems very low. A search for a new source for phenylalanine hydroxylase was attempted using term human placentas. Slices of fresh placentas were washed with cold 0.15M NaCl and then incubated in serum free medium 199 to which repurified L-U14C-phenylalanine was added. The incubation was carried out for 4 h at 37° with constant stiring. The reaction was terminated by boiling then the material was hydrolyzed with 6 N HCl for 20 h and amino acid analysis was carried out. The conversion of phenylalanine to tyrosine was considered as an indication for phenylalanine hydroxylase activity. In repeated experiments 30%-70% of the labeled phenylalanine was recovered as tyrosine indicating presence of phenylalanine hydroxylase in placenta. Two dimensional thin layer chromatography was performed for the identification of radioactive tyrosine and similar ratios of 30%-70% conversion were observed. These data suggest that placental phenylalanine hydroxylase may be studied in pregnancies at risk for PKU, explore the enzymic differences of PKU variants and possibly used for prenatal detection.