New thiocholine ester substrates for the assay of human serum cholinesterase.

BACKGROUND Several thiocholine alkanoyl esters were newly synthesized and explored as substrates for the assay of human serum cholinesterase after being subjected to the Ellman reaction (Arch Biochem Biophys 1958;74:443-50 and Arch Biochem Biophys 1959;82:70-7). METHODS We synthesized thiocholine ester iodides by the method of Renshow et al. (J Am Chem Soc 1938;60:1765-70). We examined solubility in H(2)O, substrate specificity serum for cholinesterase, (spontaneous) self-hydrolysis, storage stability, and reaction conditions for measurement of the activity of the enzyme. RESULTS Isobutyryl and cyclohexane-carboxyl esters showed the best efficiency for the specific and stable assay of human serum cholinesterase. Aqueous solubility of each was >10 mmol/L, and the reactivity with acetylcholinesterase was negligible. For isobutyryl and cyclohexane-carboxyl esters, respectively, spontaneous hydrolysis in the aqueous phase was approximately 1/25 and approximately 1/175 slower than the enzymatic hydrolysis, and assays with these substrates were linear to 1800 and 3000 U/L, respectively. The K(m) values of these acylthiocholines with human cholinesterase were almost equivalent (6.9 x 10(-3) mmol/L). The substrates were stable in aqueous solution and in the solid state as the iodides for at least 5 years at 5 degrees C. CONCLUSIONS The isobutyrate and cyclohexane-carboxylate of thiocholine are suitable for the specific assay of human serum cholinesterase.