- 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. Hesslinger, C., Kremmer, E., Hültner, L., Ueffing, M., Ziegler, I. J. Biol. Chem. (1998)