Hemin is a catalyst of the formation of reactive oxygen species. We proposed that hematophagous insects are exposed to intense oxidative stress because of hemoglobin hydrolysis in their midgut (Petretsky, M. D., Ribeiro, J. M. C., Atella, G. C., Masuda, H., and Oliveira, P. L. (1995) J. Biol. Chem. 270, 10893-10896). We have shown that hemin stimulates urate synthesis in the blood-sucking insect Rhodnius prolixus (Graça-Souza, A. V., Petretsky, J. H., Demasi, M., Bechara, E. J. H., and Oliveira, P. L. (1997) Free Radical Biol. Med. 22, 209-214). Once released by fat body cells, urate accumulates in the hemolymph, where this radical scavenger constitutes an important defense against blood-feeding derived oxidative stress. Incubation of Rhodnius fat bodies with okadaic acid raises the level of urate synthesis, suggesting that urate production can be controlled by protein phosphorylation/dephosphorylation. Urate synthesis is stimulated by dibutyryl cAMP and inhibited by N(2((p-bromocinnamil)amino)ethyl)-5-isoquinolinesulfonamide (H-89), an inhibitor of protein kinase A, as well as activated by the protein kinase C activator phorbol 12-myristate 13-acetate. In the presence of hemin, however, inhibition of urate synthesis by H-89 does not occur, suggesting that the hemin stimulatory effect is not mediated by protein kinase A. Calphostin C completely inhibits the hemin-induced urate production, suggesting that the triggering of urate antioxidant response depends on protein kinase C activation. This conclusion is reinforced by the observation that in fat bodies exposed to hemin, both protein kinase C activity and phosphorylation of specific endogenous polypeptides are significantly increased.