Some aspects of the phosphorylation of phenylalanine 4‐monooxygenase by a calcium‐dependent and calmodulin‐dependent protein kinase

A calmodulin‐dependent protein kinase purified from liver catalyzed the incorporation of up to 0.7 mol of phosphate per mol subunit of phenylalanine 4‐monooxygenase. The phosphorylation was accompanied by a proportional increase in the hydroxylase activity. The reaction was Ca²⁺ ‐dependent and was inhibited by physiological concentrations of phenylalanine. Phenylalanine 4‐monooxygenase was also a substrate for the cGMP ‐dependent protein kinase, but in this system phenylalanine stimulated the rate of phosphorylation to a similar extent as that observed in the reaction catalyzed by cAMP ‐dependent protein kinase. The hydroxylase was not a substrate for phosphorylase kinase. The calmodulin‐dependent reversal of the kinase reaction in the presence of MgADP, was also inhibited by phenylalanine. Since the kinetics of the reverse reaction was the same using ³²P‐hydroxylase phosphorylated by calmodulin‐dependent and cAMP‐dependent kinases, it is likely that both kinases phosphorylate the same site on the enzyme. This conclusion was further supported by peptide mapping of tryptic and peptic digests of ³²P‐hydroxylase, which revealed one major phosphopeptide with enzyme phosphorylated by either kinase. The Ca²⁺ ‐dependent and calmodulin‐dependent phosphorylation described above may mediate the increased phosphorylation of the hydroxylase [Garrison, J. C., Johnsen, D. E., and Campanile, C. P. (1984) J. Biol. Chem. 259, 3283–3292] and its increased activity [Fisher, M. J., Santana, M. A., and Pogson, C. I. (1984) Biochem. J. 219, 87–90] recently observed in hepatocytes exposed to Ca²⁺ ‐elevating agents.