Determination of Residues of Sepiapterin Reductase Phosphorylated by Ca²⁺/Calmodulin-Dependent Protein Kinase II

Abstract

Tetrahydrobiopterin (BH₄) is an essential cofactor for the aromatic amino acid hydroxylases, i.e., phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase, which enzymes catalyze the rate-limiting steps in the biosynthesis of neurotransmitters in the central nervous system. Sepiapterin reductase (SPR) is essential for BH4 biosynthesis by catalyzing the last step of the synthesis, and is important for controlling the level of BH4 in the brain. It has been reported that the BH₄-requiring enzymes phenylalanine, tyrosine, and tryptophan hydroxylases are phosphorylated by Ca²⁺/calmodulin-dependent protein kinase II (CaM KII). We have also found the phosphorylation of a BH4-recycling enzyme (dihydropteridine reductase) and SPR (sepiapterin reductase) by CaM KII in vitro (1). CaM KII, which is a Ser/Thr protein kinase, is known to occur abundantly in the brain, and is thought to play important roles in a variety of neuronal functions mediated by calcium ions (2). Recently, we reported that SPR phosphorylated by CaM KII was more sensitive to Ca²⁺-activated protease (calpain) than the non-phosphorylated form of SPR (3). This information suggests that the functions of both enzymes, i.e., those generating and those requiring BH₄, may be regulated by CaM KII-dependent phosphorylation. In this study, we constructed various mutant SPRs and determined sites of SPR phosphorylated by CaM KII.