An inhibitor-free assay of acetylcholinesterase and butyrylcholinesterase in the cerebrospinal fluid.

An inhibitor-free assay for the simultaneous determination of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in the cerebrospinal fluid (CSF) is described. It is based on our finding that the individual activity ratios of BChE on both its substrates acetylthiocholine (ACh) and butyrylthiocholine (BCh) in the CSF and in the parallel serum are identical under conditions of at least 5 mmol/l substrate concentration (Q(BChE)SE = Q(BChe)CSF). Considering that AChE only reacts with ACh as substrate and occurs with negligible activities in the serum, the measured individual activity ratio of BChE in the serum (Q(BChE)SE) and the total hydrolysis rate of ACh and BCh in the CSF do allow a precise calculation of the AChE activity in the cerebrospinal fluid. The derivation of the corresponding formula is demonstrated in detail. The inhibitor-free assay was compared with procedures using cholinesterase inhibitors (BW284c51 for AChE and/or iso-OMPA for BChE). Achieving widely identical results in particular between the procedure using the AChE inhibitor and the inhibitor-free test, the latter has decisive advantages: (1) it avoids the use of highly toxic inhibitors, (2) it minimizes the test volume needed, (3) it characterizes additionally the status of the blood-CSF barrier by means of the BChE activity ratio in the CSF and in the parallel serum.