Phenylalanine Hydroxylase and Tyrosine Aminotransferase in Human Fetal and Adult Liver

Abstract

The mean value of phenylalanine hydroxylase activity in adult human liver was almost twice as high as that in the fetal ones; no consistent variations were seen with sex, fetal age (between the 11th-22nd wk of gestation) or with hours between delivery and death. The tyrosine aminotransferase levels did not correlate with sex, age or method of abortion; however, they were 5 times higher in the 2 fetuses which survived for more than 1.5 h after delivery. The mean concentration of tyrosine aminotransferase (excluding these 2 fetuses) was about 15 times lower than in the adult liver. Phenylalanine hydroxylase was known to appear in rat liver on the 20th-21st day of gestation. Tyrosine aminotransferase emerged on the 1st postnatal day and can be evoked by premature delivery. In man, as in the rat, phenylalanine hydroxylase approached physiologically significant levels at an earlier developmental stage than did tyrosine aminotransferase, and extrauterinization stimulates the synthesis of tyrosine aminotransferase but not of phenylalanine hydroxylase.