A new approach to determining cholinesterase activities in samples of whole blood.

A sensitive method, especially suitable for clinical laboratories, for the routine determination of cholinesterase activities in whole blood is presented. This method is based on the hydrolysis of propionylthiocholine and the spectrophotometric determination of the thiocholine produced by reaction with 4,4'-dithiodipyridine. The reaction product 4-thiopyridone has an absorption maximum at 324 nm, so that measurement in the presence of hemoglobin is possible. Propionylthiocholine is used at the substrate for both plasma butyrylcholinesterase and erythrocyte acetylcholinesterase. These two enzymes, in the relative amounts at which they are present in human blood, split this ester at about the same rate. Consequently, a first determination gives the total activity of which each individual activity is about 50%. A second determination in the presence of a selective inhibitor ("Astra 1397") for plasma butyrylcholinesterase gives the activity of the erythrocyte acetylcholinesterase. The difference between the two values represents the activity of the plasma enzyme. The validity of the method and the reliability of the results were checked with each blood sample in two ways: (1) by determining the activities of whole blood with an earlier gasometric technique which uses blood sample dried on filter paper; and (2) by measuring the activities in separated plasma and erythrocyte hemolysate eith propionylthiocholine as the substrate.