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Some studies on adenylate cyclase in brain Ma, Yvonne Suk-Fong

Abstract

The Gilman's cyclic AMP binding assay was used to examine the possibility of adopting this method for adenylate cyclase determinations. Cyclic AMP determinations were not invalidated by the reagents used in the adenylate cyclase reaction. Cyclic AMP measured by the binding assay was directly proportional to adenylate cyclase activity. Although variability in recovery of cyclic AMP was obtained, it could be reduced by performing triplicate assays. Thus, the cyclic AMP binding assay, with some reservations, would appear applicable for measuring adenylate cyclase activity. Adenylate cyclase in rat brain was studied by using the cyclic AMP binding method for determination of product formed. Rat brain cortex was fractionated by the method of Whittaker. The highest adenylate cyclase activity was found in the fraction containing the highest acetylcholinesterase activity, and this fraction was shown by electronmicroscopic studies to be rich in synaptosomes. A modified sucrose gradient was used for isolating satisfactory synaptosomal fractions (the layer between 1.0 M and 1.1 M sucrose). Properties of synaptosomal adenylate cyclase were examined. The enzyme was dependent on the concentrations of ATP and Mg²⁺ or Mn²⁺ ion. The enzyme was stimulated by fluoride and inhibited by calcium ion. Synaptosomal adenylate cyclase was not sensitive to catecholamines or adenosine. No hormonal stimulation was obtained in the presence of GTP. In experiments where the effects of endogenous catecholamines were reduced by the addition of α and β adrenergic blocking agents or by prior treatment of the animals with reserpine, hormonal stimulation of adenylate cyclase in particulate preparations could not be demonstrated.

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