PHOSPHORYLATION BY CA2+ CALMODULIN-DEPENDENT PROTEIN-KINASE-II AND PROTEIN-KINASE-C OF SEPIAPTERIN REDUCTASE, THE TERMINAL ENZYME IN THE BIOSYNTHETIC-PATHWAY OF TETRAHYDROBIOPTERIN/

Sepiapterin reductase, the terminal enzyme in the biosynthetic pathway of tetrahydrobiopterin, was stoichiometrically phosphorylated by Ca2/calmodulin-dependent protein kinase II and protein kinase C (Ca2+/pho spholipid-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 +/- 0.03 (n = 5) 32p mol per mol enzyme subunit, respectively . The enzyme was not phosphorylated by cyclic nucleotide-dependent pro tein kinase of either the cAMP-dependent or cGMP-dependent type in thi s study. Dihydropteridine reductase, another enzyme working in direct supply of tetrahydrobiopterin, was also a good substrate for Ca2+/calm odulin-dependent protein kinase II. Phosphorylation of sepiapterin red uctase by these protein kinases modified the kinetic properties of the enzyme. It is likely that these multifunctional Ca2+-activated protei n kinases may play a role in the regulation of the physiological funct ion of the BH4-generating enzymes in vivo, as was previously found in the case of BH4-requiring enzymes.