Phenylalanine Hydroxylase Stimulator Protein Is a 4α-Carbinolamine Dehydratase

Phenylalanine hydroxylase stimulator protein is shown to be an enzyme that catalyzes the dehydration of the 4α-carbinolamine, derived from the turnover of 6-methyltetrahydropterin with phenylalanine hydroxylase, to form p-quinonoid-6-methyldihydropterin. This conclusion is based on the facts that: 1) a linear dependence on phenylalanine hydroxylase stimulator concentration of the first order rate of decay at low levels of 4α-carbinolamine is observed; 2) the physical separation of phenylalanine hydroxylase from phenylalanine hydroxylase stimulator and 4α-carbinolamine gives identical kinetic parameters with those obtained in its presence, thus indicating no catalytically significant protein-protein interaction; and 3) the 4α-carbinolamine analog, 5-deaza-4α-hydroxy-6-methyltetrahydropterin is an inhibitor of phenylalanine hydroxylase stimulator activity.