Abstract
A series of bis-[(m-hydroxyphenyl)trimethylammonium iodide] esters of polymethylenedicarbamic acids and a number of (m-hydroxyphenyl)trimethylammonium iodide esters of straight-chain N-alkylcarbamic acids have been examined as inhibitors of acetylcholinesterase from fly head. Evidence is presented suggesting that inhibition of acetylcholinesterase by the bis-carbamates is due to carbamoylation of the enzyme, as is generally thought to be the case with esters of N-alkylcarbamic acids. Inhibition is irreversible. The (m-hydroxyphenyl)trimethylammonium iodide ester of N-hexylcarbamic acid also inhibits fly head acetylcholinesterase irreversibly. There is therefore no need to implicate a second functional group in bis-carbamate esters to explain the irreversible inhibition of the enzyme. An unusual feature of the inhibition is that inhibition lines do not pass through 100% enzyme activity at t=0, except for rather low concentrations of inhibitor (<10μm for the octamethylene compound). Also, inhibition lines tend towards a maximum slope as inhibitor concentration is increased. The first observation indicates complex-formation, even in the presence of high concentrations of substrate, and by using measurements of inhibition at relatively high inhibitor concentrations, affinity constants K′a have been calculated. K′a varies from 0.1μm for the dodecamethylene compound to 10μm for the tetramethylene compound, in the presence of 3.75mm-acetylthiocholine, indicating high affinity for the enzyme. The second observation shows that, owing to this high affinity, the enzyme becomes saturated with inhibitor under the experimental conditions employed, and from the limiting slope values of the carbamoylation rate constant (k2) have been calculated. k2 varies from 0.15min−1 for the tetramethylene compound to 1min−1 for the decamethylene compound. Variations of potency in this series are therefore mainly due to changes in affinity (100-fold) rather than in carbamoylation rate (sevenfold). The observation that large molecules may acylate the enzyme raises certain problems, which are discussed.
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- DITTERT L. W., HIGUCHI T. RATES OF HYDROLYSIS OF CARBAMATE AND CARBONATE ESTERS IN ALKALINE SOLUTION. J Pharm Sci. 1963 Sep;52:852–857. doi: 10.1002/jps.2600520908. [DOI] [PubMed] [Google Scholar]
- KLUPP H., KRAUPP O., SCHWARZACHER H. G., STUMPF C. Cholinesterasehemmwirkung und pharmakologische Eigenschaften von Octamethylen-Biscarbaminoyl-m Trimethylammoniumphenol. Arch Int Pharmacodyn Ther. 1955 Mar 1;101(2):205–227. [PubMed] [Google Scholar]
- KRAUPP O., SCHWARZACHER H. G., STUMPF C. Uber die pharmakologischen Eigenschaften einiger Polymethylen-Biscarbaminoyl-m-Trimethylammoniumphenole. Naunyn Schmiedebergs Arch Exp Pathol Pharmakol. 1955;225(1-2):117–119. [PubMed] [Google Scholar]
- MAIN A. R. AFFINITY AND PHOSPHORYLATION CONSTANTS FOR THE INHIBITION OF ESTERASES BY ORGANOPHOSPHATES. Science. 1964 May 22;144(3621):992–993. doi: 10.1126/science.144.3621.992. [DOI] [PubMed] [Google Scholar]
- Main A. R., Hastings F. L. Carbamylation and binding constants for the inhibition of acetylcholinesterase by physostigmine (eserine). Science. 1966 Oct 21;154(3747):400–402. doi: 10.1126/science.154.3747.400. [DOI] [PubMed] [Google Scholar]
- O'Brien R. D., Hilton B. D., Gilmour L. The reaction of carbamates with cholinesterase. Mol Pharmacol. 1966 Nov;2(6):593–605. [PubMed] [Google Scholar]
- O'Brien R. D. Kinetics of the carbamylation of cholinesterase. Mol Pharmacol. 1968 Mar;4(2):121–130. [PubMed] [Google Scholar]
- WILSON I. B., HARRISON M. A., GINSBURG S. Carbamyl derivatives of acetylcholinesterase. J Biol Chem. 1961 May;236:1498–1500. [PubMed] [Google Scholar]
- WILSON I. B., HATCH M. A., GINSBURG S. Carbamylation of acetvlcholinesterase. J Biol Chem. 1960 Aug;235:2312–2315. [PubMed] [Google Scholar]