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Regulatory mechanisms of guanylate cyclase ec 4.6.1.2 activity in nervous tissues



Regulatory mechanisms of guanylate cyclase ec 4.6.1.2 activity in nervous tissues



Journal of Kyoto Prefectural University of Medicine 89(11): 841-868



Regulatory mechanisms of guanylate cyclase (GTP pyrophosphate-lyase (cyclizing) EC 4.6.1.2) activity in nervous tissues were analyzed on the basis of responsiveness to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Although the soluble guanylate cyclases in 100,000 .times. g supernatant fractions from frog retina and rat brain were markedly activated by MNNG, the membrane-bound enzymes prepared from frog rod outer segments and cerebral crude synaptosomes (P2) of rat had no responsiveness to MNNG. The soluble fraction of rat cerebral P2 (P2-soluble), mostly derived from synaptosomal soluble fraction, contained significantly high guanylate cyclase activity. This enzyme was hardly activated by MNNG, and the basal activity was markedly inhibited by various compounds reacting with free radicals. Guanylate cyclase in the P2-soluble fraction treated with acetone or gel filtration on Sepharose 6B exhibited a low activity and had no responsiveness to MNNG (MNNG-insensitive). The responsiveness to MNNG was restored by the addition of low concentration of hemin. Preincubation (37.degree. C, 10 min) of the P2-soluble fraction with membranous fraction of P2 elicited a marked decrease of guanylate cyclase activity, but this enzyme was strongly stimulated by MNNG (MNNG-sensitive). The MNNG-sensitive guanylate cyclase present in pre-treated P2-soluble fraction was markedly stimulated by superoxide dismutase (SOD). This activation was completely suppressed by various compounds reacting with free radicals. Although gel filtration of MNNG-sensitive guanylate cyclase on Sepharose 6B induced the loss of responsiveness to SOD, low concentration of hemin was capable of restoring the responsiveness to SOD. Membrane-bound guanylate cyclase in rod outer segments had no sensitivity to SOD, and hemin had no effect on the responsiveness of this enzyme to SOD. There are apparently 2 different types of guanylate cyclase, i.e., soluble and membrane-bound types, in the nervous tissues in terms of the responsiveness to MNNG. The enzyme in synaptosomal soluble fraction from the rat brain seems to be activated endogenously by free radical(s). MNNG, SOD and the endogenous activating factor may act on guanylate cyclase molecule through a common acceptor which is functionally replaceable by hemin.

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