Melittin potentiates guanylate cyclase activation stimulated by atrial natriuretic factor and ATP

The biologically relevant receptor for atrial natriuretic factor (ANF) has been shown to be membrane-bound guanylate cyclase. While guanylate cyclase is known to be activated by ANF and ATP, the molecular mechanism of the enzyme activation remains unclear. We now show that melittin, the main peptide toxin of bee venom, activates membrane-bound guanylate cyclase and potentiates ANF- and ATP-stimulated guanylate cyclase activity in rat lung membranes. Melittin stimulated basal guanylate cyclase activity by increasing the Vmax without significantly affecting the Km of the substrate, GTP. However, melittin enhances ANF- and ATP-stimulated enzyme activity by altering both the Vmax and the EC50 of ANF and ATP. Although melittin activates guanylate cyclase in crude membranes, it has little effect on the activity of the purified enzyme. The effect of melittin on guanylate cyclase activation in rat lung membranes is attenuated by the Ca2+ chelator, EGTA. These results suggest that the effects of melittin on guanylate cyclase activation may require the participation of accessory proteins or nonprotein factors. Therefore, melittin would be a valuable tool for exploring the molecular mechanisms of ANF-mediated guanylate cyclase activation.