DEVELOPMENTAL ASPECT OF PHENYLALANINE-HYDROXYLASE IN THE RAT - HORMONAL INFLUENCES

Interpretations of the development of phenylalanine hydroxylase (PAH) in rat liver are controversial, and the mechanism of ontogenic changes is not yet elucidated. Fetal PAH activity at a gestational age of 21 days appeared to reach 32% that of adult male level at birth. The in vivo effectiveness of fetal PAH activity was correlated with enhancement of blood tyrosine, while tyrosine amino-transferase activity appeared only after birth. No sex difference was noted in weaning rats, whereas in adult females, PAH activity was only 42% that of males. Investigating hormonal influences on liver PAH activity, no change of enzyme activity following hydrocortisone, ACTH and estradiol treatment was noted. Testosterone treatment for 4 days in weaning female rats increased PAH activity (.times. 1.7). Testosterone could explain increased PAH activity in adult males.