Acetylcholinesterase in the brain of the marine gastropod Murex trunculus L. (Prosobranchia). Isolation and properties

Abstract

1. 1. Cholinesterase (ChE) of two types—acetylcholinesterase (AChE, acetylcholine hydrolase, EC 3.1.1.7) and propionylcholinesterase (PrChE, acylcholine hydrolase, EC 3.1.1.8)—was found in the brain of the marine gastropod Murex trunculus L. (Prosobranchia). 2. 2. PrChE is a soluble enzyme which can be easily extracted by salt solution after freezing-thawing of the untreated brain. 3. 3. AChE is membrane-bound. It was solubilized by a 0.2% solution of Triton X-100. 4. 4. Some part of AChE (up to 36%) can spontaneously pass into solution. 5. 5. The specific activity of AChE in Triton X-100 extracts is 100 ± 10 nmol acetylcholine/mg protein/min. 6. 6. Enzyme hydrolysis of acetylcholine (ACh), acetylthiocholine (ATCh) and propionylthiocholine (PrTCh) is suppressed by excess of substrate. Michaelis constants ( K m ) for their hydrolysis by AChE are 0.33, 0.017 and 0.018 mM, respectively. 7. 7. Bimolecular rate constants with organophosphorus inhibitors of different structure points to a similarity of the gastropod brain AChE to the typical enzyme of vertebrates in the structure of the active surface.