Acetylcholinesterase, choline acetyltransferase, and the postulated acetylcholine receptor of canine platelets

Abstract

Choline acetyltransferase (acetyl-CoA: choline O-acetyltransferase, EC 2.3.1.6) activities of canine and human platelets were found to be surprisingly similar, in striking contrast to cholinesterase activities which differed markedly. Choline acetyltransferase activities were present predominantly in the soluble fraction of the platelet homogenate, while cholinesterase (acetylcholine acylhydrolase, EC 3.1.1.8) activities were distributed in the soluble fraction, in plasma membranes, and possibly in storage granules. In canine platelets, part of the cholinesterase activities appeared in soluble form along with adenine nucleotides and serotonin when the platelet release reaction was initiated by agents such as aeetylcholine, acetyl-β-methylcholine, or thrombin; this soluble cholinesterase activity was shown to be an acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7). However, choline acetyltransferase activities were not released from platelets by these agents. Aggregation of canine platelets by thrombin resulted in a decrease of choline acetyltransferase activity in platelets by 50 per cent. Although previous results strongly suggested the presence of an acetylcholine receptor in canine platelets, demonstration of the binding of isotopically labeled acetylcholine and atropine to this postulated receptor was unsuccessful. The significance of these data and possible relationships of acetylcholinesterase, choline acetyltransferase, and acetylcholine receptor are discussed.