Endogenous activation of guanylate cyclase in synaptosomal soluble fraction from rat brain

Guanylate cyclase in crude mitochrondrial (P2) soluble fraction prepared from rat brain, obtained by a hypo-osmotic treatment of P2, showed extremely higher activity than that in the same fraction from other organs. In addition the soluble fraction obtained from synaptosomes (P2 - B) contained the highest enzyme activity among other subfractions of the cerebral P2 examined. Guanylate cyclase activity in the synaptosomal soluble fraction was, however, markedly suppressed by various compounds reacting with free radicals. These results suggest that guanylate cyclase in the synaptosomal soluble fraction may be activated endogenously be a free radical and involved in the regulatory mechanisms for cyclic guanosine monophosphate (cyclic GMP) at presynaptic terminals.