Enhanced Phosphorylation of Tyrosine Hydroxylase at More Than One Site Is Induced by 56 mM K+ in Rat Pheochromocytoma PC 12 Cells in Culture

Abstract

Incubation of rat pheochromocytoma PC12 cells with dibutyryl cyclic AMP or 56 mM K⁺ is associated with increased activity and enhanced phosphorylation of tyrosine hydroxylase in situ. Following incubation of the PC12 cells with ³²P_i, rapid isolation of the tyrosine hydroxylase, and tryptic digestion of the enzyme, two distinct ³²P-peptides can be identified after paper electro-phoresis. 56 mM K⁺ increases ³²P_i incorporation into both of these peptides, whereas dibutyryl cyclic AMP increases ³²P_i incorporation into only one of these peptides. The rate of increase in the incorporation of ³²P_i into these two peptides in cells treated with 56 mM K⁺ is similar. The phosphorylation of tyrosine hydroxylase in PC12 cells occurs exclusively on serine residues. These results suggest that tyrosine hydroxylase in PC12 cells is phosphorylated on serine residues at two or more distinct sites after 56 mM K⁺-induced depolarization. Since only one of these sites is phosphorylated by cyclic AMP-dependent protein kinase, activation of tyrosine hydroxylase by 56 mM K⁺ may involve phosphorylation by multiple protein kinases in rat pheochromocytoma PC12 cells.