Tetrahydrobiopterin-dependent stabilization of neuronal nitric oxide synthase dimer reduces susceptibility to phosphorylation by protein kinase C in vitro.

Binding of (6R)-5,6,7,8-tetrahydro-L-biopterin (H4B) stabilizes the homodimeric structure of neuronal nitric oxide synthase (nNOS). In the present study, low-temperature sodium dodecylsulfate-polyacrylamide gel electrophoresis revealed differential susceptibility of stabilized and non-stabilized dimers to in vitro phosphorylation by protein kinase C. Protein kinase C preferentially phosphorylated the non-stabilized dimer. Although a low extent of phosphorylation was detected in the stabilized dimer, most of it was estimated to be due to phosphorylation of the dimer before its stabilization. Phosphorylation did not affect the stabilizing effect of H4B. These results indicate that H4B-dependent dimer stabilization prevents nNOS from protein kinase C-dependent phosphorylation in vitro.