Comparison Between Thiocholinesters and o-Nitrophenylbutyrate as Substrates in the Assay of Serum Cholinesterase

The technical and clinical usefulnessof three substratesfor serum cholinesterasewere compared. The diagnosticvalue of acetylthiocholine was equal to that of butyrylthio-choline and superior to that of o-nitrophenylbutyrate. Technically, butyryithiocholine is superiorto the other two substrateschiefly becauseof its greater affinity for serum cholinesterase. A kinetic method using o-nitrophenylbutyrate is described that is capable of carrying out 60 tests per hour with good results. N UMEROUS METHODS have been described for the determination of cholinesterase (ChE)t activity ill serum based on widely differing principles and using the most varied substrates (1, 2). Of the many substrates, thiocholinesters (3, 4) and nitrophenylesters (5, 6) seemed to us very useful, particularly because both types of substrates lend themselves to kinetic determinations using visible light. ChE liberates thiocholine from thiocholinesters. Thiocholine reacts spontaneously with 5:5’-dithiobis-(2-nitrobenzoate), DTNB, forming the yellow dye, 5-mercapto-2-nitrobenzoate (7, 8). In nitrophenylesters the liberated riitrophenol already possesses a yellow color (5). The substrates have zero absorbance in visible light; the reaction products, on the other hand, have an absorption maximum at 410 um. Of the thiocholinesters, either acetyl- (9, 10) or butyryithiocholille (ii) can be used. Of the nitrophenylesters, the 0-isomer 5 the most suitable. Its chemical stability and affinity for ChE is superior to th