Isolation and partial purification of a clonidine‐displacing endogenous brain substance

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Abstract
A new compound, designated clonidine-displacing substance (CDS), has been isolated from calf brain by ion-exchange chromatography, zone electrophoresis and high-performance liquid chromatography. CDS binds specifically to α2-adrenergic receptors in rat brain and human platelet membranes, as measured in direct binding experiments using [3H]clonidine and [3H]yohimbine respectively. Unlike clonidine or other α2-agonists, CDS does not affect basal levels of adenylate cyclase in human platelets at the highest concentrations obtainable. The apparent molecular mass of the compound is estimated to be 500 ± 50 Da, as determined by gel-filtration chromatography on Sephadex G-15. The new compound is thermostable, not affected by proteolytic enzymes, such as trypsin. chymotrypsin, pronase, papain and pyroglutamase, or by boiling in 0.2 M HCl for 5min. It does not bind to α1receptors in rat brain or to β-adrenergic receptors in turkey erythrocytes, since it is unable to displace [3H]prazosin and [125I]cyanopindolol from α1 and β-receptors respectively.