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Sepiapterin reductaae, the terminal enzyme in the biosynthetic pathway of tetrahydrobiopterin, was stoichiometrically phosphorylated by Ca*+I cahnodulin-dependent protein kinase II and protein kinase C (Ca'+/phospholipid-dependent protein kinase) in vitro. Maximal incorporation of phosphate into the enzyme subunit by these was 3.05 + 0.05 (n = 4) and 0.74 f 0.03 (n = 5) 32P mol per mol enzyme subunit, respectively. The enzyme was not phosphorylated by cyclic nucleotide-dependent protein kinase of either the CAMP-dependent or cGMP-dependent type in this study. Dihydropteridine reductase, another enzyme working in direct supply of tetrahydrobiopterin, was also a good substrate for Ca%almodulin-dependent protein kinase II. Phosphorylation of sepiapterin reductase by these protein kinases modified the kinetic properties of the enzyme. It is likely that these multifunctional Ca'+-activated protein kinases may play a role in the regulation of the physiological function of the BH4-generating enzymes in vivo, as was previously found in the case of BH4-requiring enzymes.

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