Guanylate cyclase in synaptosomal soluble fraction from rat brain: possible involvement of free radical acceptor in endogenous activation mechanisms.
Guanylate cyclase (GTP pyrophosphate-lyase (cyclizing); EC 4.6.1.2), in the crude synaptosomal soluble (P(2)-soluble) fraction obtained by hypo-osmotic treatment and subsequent centrifugation of crude synaptosomal (P(2)) fraction from the rat brain, showed a significantly high activity when assayed in the presence of high concentration of Mn(2+). The basal activity was markedly inhibited by various compounds reacting with free radicals. Guanylate cyclase in the P(2)-soluble fraction treated with acetone exhibited a low activity and was not activated by nitrosoguanidine. The responsiveness to nitrosoguanidine was restored, however, by the addition of low concentration hemin. On the other hand, the preincubation (37 degrees C, 10 min) of P(2)-soluble fraction with membranous fraction of P(2) similarly induced a marked decrease of guanylate cyclase activity, but this enzyme was strongly stimulated by nitrosoguanidine. The responsiveness of the enzyme to nitrosoguanidine was also eliminated by acetone-treatment or gel-filtration on Sepharose 6B, but was partially restored by the addition of hemin. These results suggest that guanylate cyclase in the P(2)-soluble fraction from rat brain may be activated endogenously by free radical(s) through a free radical acceptor which is replaceable by hemin.
Yoshikawa
K
K
Department of Pharmacology, Kyoto Prefectural University of Medicine, Kamikyo-Ku, Kyoto 602, Japan.
Kuriyama
K
K