Effect of age, sex, and nutrition on liver phenylalanine hydroxylase activity in rats

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Abstract
This study provides data on a number of factors that influence phenylalanine hydroxylase activity in rat liver. By means of a method which utilizes crude liver homogenates, the enzyme activity was determined in rat liver obtained from early embryos to 150-day-old animals. The activity increases with age to a maximum at the young adult age of 50–60 days and then declines. The most rapid increase in activity was found between 18–21 days. The female rat livers show a rapid decline in activity with increasing age, then stabilize at a lower activity than males after maturity is reached. Fasting causes a marked drop in activity, and diets deficient in phenylalanine cause a decrease in the enzyme activity. High casein diets, which increase the phenylalanine and total amino acid intake, had no apparent effect on the level of enzyme activity.