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 marro w-derived mast cells, the kit Ligand selectively up-regulates GTP cycl ohydrolase 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 en hancement is caused by increased expression of the enzyme. Furthermore we show that GTP cyclohydrolase I is subject to modification at the p ost-translational level. In vivo labeling with [P-32]orthophosphate de monstrates that in primary mast cells and in transfected RBL-2H3 cells overexpressing GTP cyclohydrolase I, the enzyme exists in a phosphory lated form, Antigen binding to the high affinity receptor for IgE trig gers an additional and transient phosphorylation of GTP cyclohydrolase I with a concomitant rise in its activity, and in consequence, cellul ar tetrahydrobiopterin levels increase, These events culminate 8 min a fter stimulation and call be mimicked by phorbol ester. The hyperphosp horylation is greatly reduced by the protein kinase C inhibitor Ro-31- 8220. In vitro phosphorylation studies indicate that GTP cyclohydrolas e I is a substrate for both casein kinase II and protein kinase C.