Butyrylcholinesterase activity assays

Exposure to a toxic dose of OP results in inhibition of acetylcholinesterase and butyrylcholinesterase activities. The most common method to measure OP exposure is to assay acetylcholinesterase and butyrylcholinesterase activities in blood using a spectrophotometric method (EUman et al, 1961 Wilson et al, 2005 Worek et al, 1999). The drawbacks of activity assays are that they do not identily the OP. They show that the poison is a cholinesterase inhibitor but do not distinguish between nerve agents, OP pesticides, carbamate pesticides, and tightly bound, noncovalent inhibitors like tacrine and other anti-Alzheimer drugs. In addition, low-dose exposure, which inhibits less than 20% of the cholinesterase, carmot be determined by measuring acetylcholinesterase and butyrylcholinesterase activity because individual variability in activity levels is higher than the percent inhibition. 

The activities of two enzymes have been used as biomarkers of effects for OPs, namely acetylcholinesterase (EC 3.1.1.7) and butyrylcholinesterase, sometimes known as pseudocholinesterase (EC 3.1.1.8). The structure and function of these enzymes has been reviewed. " In humans the former is present in red blood cells and the latter in plasma, but such distribution is not true of all species. In dogs, both enzymes are present in plasma with a ratio of butyrylcholinesterase to acetylcholinesterase of 7 1, while in the rat, plasma cholinesterase activity comprises more acetylcholinesterase with a butyrylcholinesterase to acetylcholinesterase activity of 1 3 in males and 2 1 in females in neither blood compartment are the functions of the enzymes fully understood.Because of the possibility of confusion, the terms plasma cholinesterase and erythrocyte cholinesterase as synonyms for butyrylcholinesterase and acetylcholinesterase are to be deprecated, especially when used of enzymes in animals where serious confusion may result. It is often necessary to look in detail at animal studies to see what activity has been measured in each matrix. In particular, it is necessary to look at the substrate(s) used in the assay together with any inhibitors used. Methods for measuring acetylcholinesterase have been reviewed and acetylcholinesterase and butyrylcholinesterase activities can be measured separately. In almost all cases it is the enzyme activity, rather than protein concentration, that is measured and many of the procedures used are variants of the Ellman method. Correct storage of blood samples is important as reactivation of inhibited enzymes ex vivo can occur. 

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