Hepatic phenylalanine 4-monooxygenase is a phosphoprotein.

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Abstract
Phenylalanine hydroxylase [phenylalanine 4-monooxygenase; EC 1.14.16.1; L-phenylalanine, tetrahydropteridine:oxygen oxidoreductase(4-hydroxylating)] isolated from rat liver is a phosphoprotein containing approximately 0.31 .mu.mol of protein-bound phosphate per .mu.mol of subunit (50,000 MW). When the enzyme is further phosphorylated in the presence of ATP and a 3'':5''-cyclic-AMP-dependent protein kinase (EC 2.7.1.37; ATP:protein phosphotransferase), an additional 0.7 .mu.mol of phosphate per .mu.mol of subunit is introduced, bringing the total phosphate content up to about 1 .mu.mol/.mu.mol of subunit. This phosphorylation of the enzyme in vitro is accompanied by a 2.6-fold increase in hydroxylase activity when the activity is assayed in the presence of tetrahydrobiopterin. Partial proteolytic digestion of phenylalanine hydroxylase, which previously had been shown to activate the enzyme 20- to 50-fold, removes almost all of the phosphate from the enzyme.