Cyclic nucleotides, cyclic nucleotide phosphodiesterase, protein kinase, and phosphoprotein phosphatase in the central nervous sytem of Manduca sexta Public Deposited

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  • Adenosine 3', 5' -cyclic monophosphate (cyclic AMP, cAMP) and guanosine 3', 5' -cyclic monophosphate (cyclic GMP, cGMP) were quantitated in the central nervous system (CNS) of the insect Manduca sexta by competitive protein binding techniques. The M. sexta CNS was found to contain a strikingly high level of cGMP, about 100-fold greater than that of mammalian brain. The level of cAMP, however, was estimated to be about one-sixth that of mammalian brain. The basal ratio of cGMP/cAMP in the insect CNS was approximately ten, whereas this ratio has been reported to be less than one in a variety of vertebrate and invertebrate tissues. Acetylcholine caused a great elevation of cGMP, but not cAMP, in the M. sexta CNS. Short-term incubation with ecdysterone (insect metamorphosis hormone) promoted the accumulation of both cyclic nucleotides. The existence of cyclic nucleotide- stimulable protein kinases (ATP: protein phosphotransferase, EC 2. 7. 1. 37), a system of enzymes postulated to be instrumental in the biochemical expression of cAMP and cGMP, was demonstrated in the CNS of both larval and adult M. sexta. At low concentrations, cAMP was a much more effective activator of kinase activity than cGMP, Cyclic AMP lowered the Km of the CNS kinase for ATP, a phenomenon which has been reported to be unique to nervous tissue in mammals. A number of the enzymological properties of the insect kinase were similar to those reported in the literature for this enzyme in mammalian tissues. This insect CNS was also shown to possess a potent enzyme system, viz. phosphoprotein phosphatase, for the dephosphorylation of the phosphorylated products of kinase. Phosphatase activity was gauged using phosphoprotamine as substrate. Both kinase and phosphatase activities were found to be enriched in particulate fractions of the CNS. An enzyme system for the destruction of cAMP and cGMP, cyclic nucleotide phosphodiesterase (nucleoside 3', 5' -cyclic phosphate nucleoside 5' -phosphate 31 -phosphohydrolase, EC 3.1. 4< c), was examined in the M. sexta larval and adult CNS. Phosphodiesterase (PDE) was found in both soluble and particulate fractions of the CNS, but highest specific activity PDE was noted in a crude mitochondrial preparation, a fraction presumably containing synaptic elements.PDE was greatly enriched in the brain relative to the other CNS ganglia, and was present at higher levels in nervous relative to nonnervous tissues. The hydrolysis of both cAMP and cGMP appeared to be the function of a single enzyme (or similar isozymes) in the larval CNS. Kinetic evidence suggested that PDE in the insect is a cooperative enzyme and is characterized by non-linear, biphasic double-reciprocal plots. PDE could be effectively inhibited by presumed physiological levels of ATP.
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