ELIMINATION OF GTP BIPHASIC REGULATION OF SYNAPTOSOMAL ADENYLATE CYCLASE BY MANGANESE AND SOLUBILIZATION

Abstract

Effects of divalent cations and solubilization with Lubrol-PX were studied on guanine nucleotide regulation of synaptosomal adenylate cyclase activity of the rat caudate nucleus. In the presence of Mg²⁺, both GTP and Gpp(NH)p exerted biphasic actions on the membrane-bound adenylate cyclase activity. The K_0.5 value for the GTP stimulation of the cyclase was 47 nM, and the value for the GTP inhibition was 4.5 μM. One hundred μM dopamine selectively enhanced the stimulatory phase of the GTP action, whereas 10μM morphine selectively enhanced the inhibitory phase of the GTP action. When Mg²⁺ was replaced by Mn²⁺, the inhibition of the membrane-bound adenylate cyclase by these nucleotides and morphine was completely abolished; but the catalytic activity of adenylate cyclase was not impaired. These results suggest that the inhibitory action of GTP is responsible for the morphine inhibition of synaptosomal adenylate cyclase. Lubrol-solubilized adenylate cyclase prefered Mn²⁺ to Mg²⁺ for its activity. The stimulation of adenylate cyciase by either GTP or Gpp (NH)p was eliminated in the Sepharose 6B-fractionated solubilized preparation in the presence of either Mg²⁺ or Mn²⁺. Ten mM NaF also failed to activate the fractionated adenylate cyclase. In the fractionated solubilized preparation, GTP and Gpp(NH)p failed to inhibit adenylate cyclase. These results indicate that GTP and Gpp(NH)p are unable to inhibit the resolved catalytic unit of the synaptosomal adenylate cyclase.