Developmental aspect of phenylalanine hydroxylase in the rat -- hormonal influences

Interpretations of the development of phenylalanine hydroxylase (PAH) in rat liver have been controversial, and the mechanism of ontogenic changes have not yet been 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 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 we noted no change of enzyme activity following hydrocortisone, ACTH and estradiol treatment. However, 4 days of testosterone treatment in weaning female rats increased PAH activity (X1.7). Therefore, testosterone could explain increased PAH activity in adult males.