Cholinesterases from Plant Tissues

Abstract

A cholinesterase was purified 36-fold from mung bean (Phaseolus aureus) roots by a combination of differential extraction media and gel filtration. The enzyme could be effectively extracted only by high salt concentration, indicating that it is probably membrane-bound. Methods used for assaying animal cholinesterases were tested, two of which were adapted for use with the bean cholinesterase. The bean enzyme hydrolyzed choline and noncholine esters but showed its highest affinity for acetylcholine and acetylthiocholine. The pH optimum was 8.5 for acetylthiocholine and 8.7 for acetylcholine. The Michaelis constants were 72 and 84 AM for acetylcholine and acetylthiocholine, respectively. The cholinesterase was relatively insensitive to eserine (half-maximum inhibition at 0.42 mM) but showed high sensitivity to neostigmine (half-maximum inhibition at 0.6 AM). Other animal cholinesterase inhibitors were also found to inhibit the bean enzyme but most of them at higher concentrations than are generally encountered. Choline stimulated enzymatic activity. The molecular weight of the cholinesterase was estimated to be greater than 200,000, but at least one smaller form was observed. It is suggested that the large form of cholinesterase is converted to the smaller form by proteolysis.Acetylcholine has been shown to participate in phytochrome-mediated processes in bean roots (16), and the results obtained in the above work suggested that bean roots might contain a cholinesterase which functions to regulate the content of ACh' in roots. Other workers (8, 9, 13) have also reported that whenever ACh was found to play a role in physiological processes in plant tissues, ChE inhibitors increased the effects of ACh or were essential for getting any effects of ACh. It The chemicals used in this work were obtained from the following sources: choline chloride, acetylcholine chloride, acetylthiocholine iodide, propionylthiocholine iodide, butyrylthiocholine iodide, DTNB, eserine (physostigmine) sulfate, neostigmine (prostigmine) bromide, bovine serum albumin, Triton X-100, and Sephadex G-200 from Sigma Chemical Co.; acetyl-I-'4C-choline chloride (9.2 mc/mmole) and Aquasol

Cholinesterases from Plant Tissue

“…The enzyme also has other properties in common with acetylcholinesterase (EC 3.1.1.7), including inhibition by relatively low concentrations, roughly 1 ,uM, of neostigmine and organophosphates such as paraoxon and inhibition by excess substrate (>1 mM) (12). Furthermore, acetylcholine itself has been identified in extracts of Phaseolus aureus (7).…”

“…Crude preparations of PE from orange hydrolyze acetylcholine, although this ability is nearly lost during further purifixation of the PE (9). On the other hand, PE activity has been reported in a number of plants, cucumber (1), tobacco (2), and Coleus blumei (10), which contain no apparent ChE activity (5 (12). Whereas PE has a broad pH optimum from pH 7 to pH 9 (3,15), ChE activity has a well defined optimum at pH 8.5 (12).…”

“…On the other hand, PE activity has been reported in a number of plants, cucumber (1), tobacco (2), and Coleus blumei (10), which contain no apparent ChE activity (5 (12). Whereas PE has a broad pH optimum from pH 7 to pH 9 (3,15), ChE activity has a well defined optimum at pH 8.5 (12). We report here further experiments which contrast the properties of PE and ChE, and indicate that the two enzymes are different proteins.…”

“…An enzyme with a high affinity for acetylcholine has been identified (12) in extracts prepared from Phaseolus aureus, the mung bean. The enzyme also has other properties in common with acetylcholinesterase (EC 3.1.1.7), including inhibition by relatively low concentrations, roughly 1 ,uM, of neostigmine and organophosphates such as paraoxon and inhibition by excess substrate (>1 mM) (12).…”

“…and purified as described previously (4,12). Briefly, the ChE in a 4% (w/v) (NH4),S04 tissue extract was concentrated by adding solid (NH,).SO to 80% saturation, precipitating the ChE and resulting in a 12-fold purification of the enzyme activity.…”