Phosphorylation of GTP cyclohydrolase I and modulation of its activity in rodent mast cells -: GTP cyclohydrolase I hyperphosphorylation is coupled to high affinity IgE receptor signaling and involves protein kinase C

GTP cyclohydrolase I controls the de novo pathway for the synthesis of tetrahydrobiopterin, which is the essential cofactor for tryptophan 5-monooxygenase and thus, for serotonin production. In mouse bone marrow-derived mast cells, the kit Ligand selectively up-regulates GTP cyclohydrolase I activity (Ziegler, I., Hultner, L., Egger, D., Kempkes, B., Mailhammer, R., Gillis, S., and Rodl, W. (1993) J. Biol. Chem. 268, 12544-12551), Immunoblot analysis now confirms that this long term enhancement is caused by increased expression of the enzyme. Furthermore we show that GTP cyclohydrolase I is subject to modification at the post-translational level. In vivo labeling with [P-32]orthophosphate demonstrates that in primary mast cells and in transfected RBL-2H3 cells overexpressing GTP cyclohydrolase I, the enzyme exists in a phosphorylated form, Antigen binding to the high affinity receptor for IgE triggers an additional and transient phosphorylation of GTP cyclohydrolase I with a concomitant rise in its activity, and in consequence, cellular tetrahydrobiopterin levels increase, These events culminate 8 min after stimulation and call be mimicked by phorbol ester. The hyperphosphorylation is greatly reduced by the protein kinase C inhibitor Ro-31-8220. In vitro phosphorylation studies indicate that GTP cyclohydrolase I is a substrate for both casein kinase II and protein kinase C.