Ligand effects on the phosphorylation state of hepatic phenylalanine hydroxylase

The effects of substrate and cofactors on the phosphorylation of hepatic phenylalanine hydroxylase by cAMP-dependent protein kinase and on dephosphorylation by phosphoprotein phosphatase have been examined. The presence of the natural cofactor (6R)-tetrahydrobiopterin strongly inhibits the activation observed under phosphorylating conditions; in contrast, this activation is enhanced approximately 20 to 50% by phenylalanine. The phosphorylation of the hydroxylase is strongly inhibited (approximately 80%) by (6R)-tetrahydrobiopterin, while phosphorylation is modestly stimulated by phenylalanine. High concentrations of phenylalanine (1 mM), however, can substantially reverse the inhibition of phosphorylation by (6R)-tetrahydrobiopterin. Neither (6R)-tetrahydrobiopterin nor phenylalanine affect the phosphorylation of a synthetic peptide substrate of cAMP-dependent protein kinase. The inhibition is specific for (6R)-tetrahydrobiopterin; the diastereoisomer (6S)-tetrahydrobiopterin has a much smaller effect, and 6-methyltetrahydropterin and 6,7-dimethyltetrahydropterin have no effect. Both phenylalanine and (6R)-tetrahydrobiopterin inhibit to a small extent the dephosphorylation of phosphorylated phenylalanine hydroxylase catalyzed by phosphoprotein phosphatase. Neither phenylalanine nor (6R)-tetrahydrobiopterin inhibit the dephosphorylation of phosphorylated histones by phosphoprotein phosphatase. These results suggest that the phosphorylation state, and thus the activation state, of phenylalanine hydroxylase in vivo may be modulated, in part, by the availability of substrate.