Spontaneous activation of phenylalanine hydroxylase in rat liver extracts.

A wide variety of modifications of rat liver phenylalanine hydroxylase, such as limited proteolysis and phosphorylation, leads to a selective increase in the tetrahydrobiopterin-dependent hydroxylase activity without affecting the 6-methyltetrahydropterin or the 6,7-dimethyltetrahydropterin-dependent activity. The ratio of hydroxylase activity in the presence of tetrahydrobiopterin and one of these synthetic pterins, therefore, can serve as an indicator of the state of activation of the enzyme. Comparisons of this ratio for the crude and the pure liver enzyme, indicated that the purified enzyme has undergone activation. We have studied this activation under controlled conditions and have found that it is a Mn2+-sensitive process that occurs in liver extracts that have been freed of small molecules. The activation appears to be different from any of those that have been described previously.