Evaluation of 6,6'-dithionicotinic acid as alternative chromogen in a modified Ellman method--comparison in various species.

For the diagnosis and therapy monitoring of intoxications with organophosphorus compounds, the determination of acetylcholinesterase (AChE) activity in whole blood is crucial. Usually, this testing is done with the colorimetric Ellman test using 412 nm wavelength and 5,5'-dithiobis-nitrobenzoic acid (DTNB) as chromogen. However, it has been described that the assay sensitivity is impaired by the Soret band of hemoglobin. In order to enable more sensitive determination of AChE activity in human, porcine, rat, and guinea pig whole blood dilutions, we tested the alternative chromogen 6,6'-dithionicotinic acid (DTNA) with an optimal absorption wavelength of 340 nm and compared it with a modified Ellman assay using a wavelength of 436 nm. DTNB resulted in a higher background absorption compared with DTNA in human and porcine blood samples, although the saturation of sulfhydryl groups was delayed with DTNA in all species. A slightly higher whole blood AChE activity was recorded in DTNB samples. In conclusion, the results of the present study with human, porcine, guinea pig, and rat whole blood do not provide evidence that using DTNA for the spectrophotometric determination of AChE activity in whole blood is superior to a modified Ellman assay using DTNB.