Biochemical characterization of histamine-sensitive adenylate cyclase in mammalian brain

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Abstract

Certain biochemical characteristics of an adenylate cyclase that is activated by low concentrations of histamine (Ka, 8 μm) and that is present in cell-free preparations from the dorsal hippocampus of guinea pig brain have been studied. Histamine increased the maximal reaction velocity of adenylate cyclase without altering the Km (0.18 mm) for its substrate, MgATP. Increasing concentrations of free Mg2+ stimulated enzymatic activity; the kinetic properties of this activation by Mg2+ suggest the existence of a Mg2+ allosteric site on the enzyme. Histamine increased the affinity of this apparent site for free Mg2+. Free ATP was a competitive inhibitor with respect to the MgATP substrate. The apparent potency of free ATP as an inhibitor increased in the presence of histamine. In the presence of Mg2+, low concentrations of Ca2+ markedly inhibited adenylate cyclase activity; half-maximal inhibition of both basal and histamine-stimulated enzyme activity occurred at 40 μm Ca2+. Other divalent cations, including Zn2+, Cu2+, and Cd2+, were also inhibitory. Of the divalent cations tested, only Co2+ and Mn2+ could replace Mg2+ in supporting histamine-stimulated adenylate cyclase activity. The nucleoside triphosphates GTP and ITP increased basal adenylate cyclase activity and markedly potentiated the stimulation by histamine. Preincubation of adenylate cyclase with 5′-guanylylimidodiphosphate dramatically increased enzyme activity; in this activated state, the adenylate cyclase was relatively refractory to further stimulation by histamine or F. The subcellular distribution of histamine-sensitive adenylate cyclase activity was studied in subfractions from guinea pig cerebral cortex. The highest total and specific activities were observed in those fractions enriched in nerve endings, while adenylate cyclase activity was not detectable in the brain cytosol fraction. A possible physiological role for this histamine-sensitive adenylate cyclase in neuronal function is discussed.

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    This study was supported by United States Public Health Service Grants MH-17387 and NS-08440.

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    Recipient of NIH Postdoctoral Fellowship No. 1-F22-NS-03016.

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