Evidence for the Presence of a GTP‐Dependent Regulatory Component of Adenylate Cyclase in Myelin from Rat Brain

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Abstract
A GTP‐dependent regulatory component of adenylate cyclase was found in myelin from rat brain. The fraction solubilized from myelin contained a component that reconstituted guanine nucleotide‐responsive adenylate cyclase activity when combined with the catalytic unit of adenylate cyclase prepared from rat brain. Purified myelin demonstrated little adenylate cyclase activity, even in the presence of F− or Mn2+. The reconstituted activity was dependent on the amount of the solubilized myelin fraction and required the presence of 5′‐guanylylimidodiphosphate, a hydrolysis‐resistant analog of GTP. The elution pattern of the component solubilized from myelin in gel filtration was very similar to that of a GTP‐dependent regulatory component from synaptic plasma membranes. The content of the regulatory component‐like activity in myelin was estimated to be 50–60% of that in synaptic plasma membranes. Cholera toxin ADP‐ribosylated proteins having molecular weights of 48,000, 38,000, 23,000, 20,000 and 15,000 and other minor peptides in myelin, some of which were also present in synaptic plasma membranes. We conclude that myelin contained a GTP‐dependent regulatory component of adenylate cyclase despite the apparent lack of adenylate cylcase activity in myelin.