Ellman Method
Definition
Standard spectrophotometric cholinesterase assay. Acetylthiocholine substrate plus 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) yields a yellow 5-thio-2-nitrobenzoate anion read at 412 nm. The rate of absorbance change is proportional to cholinesterase activity.
- Substrate
- Acetylthiocholine
- Reagent
- DTNB (Ellman's reagent)
- Detection wavelength
- 412 nm
- Coloured product
- 5-thio-2-nitrobenzoate anion
- Measures
- Cholinesterase enzyme activity
Common questions
Why is the Ellman method used in forensic toxicology?+
It measures blood or plasma cholinesterase activity after suspected organophosphate or carbamate poisoning. Inhibited enzyme reacts more slowly, giving a lower rate of absorbance change than a normal sample.
Does a low reading identify the specific poison?+
No. A reduced rate shows cholinesterase inhibition consistent with an organophosphate or carbamate, but the assay does not name the compound. Confirmatory chromatography is still required to identify the agent.
Related terms
- AChE / BChE
- Acetylcholinesterase (RBC membrane bound, true cholinesterase) and butyrylcholinesterase (plasma, pseudo-cholinesterase). Both depressed in OP and carbamate exposure; plasma BChE falls first and...
- Aluminium Phosphide (AlP)
- Solid grain-fumigant rodenticide sold as Celphos, Quickphos, Sulfas tablets. On contact with moisture or stomach acid, releases phosphine (PH3) gas, which blocks...
- Carbamate
- Carbamic-acid ester insecticides (carbaryl, carbofuran, propoxur, aldicarb). Reversibly inhibit acetylcholinesterase by carbamylating the active-site serine; spontaneous decarbamylation makes oximes unnecessary and sometimes...
- Organochlorine (OC)
- Chlorinated hydrocarbon insecticides (DDT, BHC/lindane, endosulfan, dicofol, aldrin, dieldrin). Highly lipophilic, neurotoxic via voltage-gated sodium channel disruption, and persistent in fat and...
- Organophosphate (OP)
- A class of insecticides (monocrotophos, chlorpyrifos, methyl parathion, dichlorvos, phorate) that inhibit acetylcholinesterase, producing the cholinergic toxidrome. The dominant suicidal poison in...
- Pyrethroid
- Synthetic analogues of natural pyrethrins (deltamethrin, cypermethrin, permethrin, fenvalerate). Act on voltage-gated sodium channels like OC but break down quickly in mammals,...
- QuEChERS
- Quick, Easy, Cheap, Effective, Rugged and Safe sample-prep protocol for pesticide residues. Acetonitrile extraction, MgSO4 + NaCl partition, dispersive SPE clean-up with...