Isoenzyme composition of hepatic phenylalanine hydroxylase in developing rats after treatment with cortisol, α-methylphenylalanine and p-chlorophenylalanine in vivo

The three isozymes (ν, κ, and π) of phenylalanine hydroxylase, resolved by hydroxyapatite chromatography from the adult rat liver, are already present in the fetus, although isozymes κ and π account for less than 15% of the total. Between the age of 4 and 6 days there is a differential increase, and the ratio of ν:κ:π becomes akin to that in adult rats, 51:47:2. The normal rise during the third postnatal week in the concentration of hepatic phenylalanine hydroxylase can be brought about precociously with cortisol, which, in 24 hr after its injection to 6-day-old rats, raises all three isozymes to their adult level. The administration of either α-methylphenylalanine or p-chlorophenylalanine to adult rats results in total suppression of isozymes κ and π, and almost complete suppression of isozyme ν (87-92%). α-Methylphenylalanine exerts a less striking effect on the three isozymes in suckling rats, especially at the age of 22 days.