The hydrolysis of succinyldithiocholine and related thiocholine esters by human plasma and purified cholinesterase

Abstract

The kinetic properties of cholinesterase (ChE) present in plasma were compared with those of purified human ChE using the substrates succinyldithiocholine (SDTCh), acetylthiocholine (AcTCh) and butyrylthiocholine (BuTCh). SDTCh was hydrolysed at two sites; a site with a low K m ( K m1 11.4 ± 3.3 μM) with a V max of 0.06 μmol/min/ml and a site with a high K m ( K m2 132.4 ± 14.8 μM) and a V max of 0.107 μmol/min/ml. The K m2 site was absent in the sample of purified ChE. The related thiocholine esters, AcTCh and BuTCh were hydrolysed at two sites by both plasma and purified ChE. This indicated that the K m2 site which hydrolysed SDTCh was not ChE. The identity of this component in plasma remains unknown but it was shown not to be albumin. The anticholinesterase agents soman and pyridostigmine were used to demonstrate the direct relationship between inhibition of plasma ChE and hydrolysis of SDTCh at the low concentrations present clinically (20 μM). Whereas high concentrations of SDTCh (200 μm) could be partly hydrolysed by an enzyme present in plasma which is insensitive to ChE inhibitors. In a limited study on the plasma from two “atypical” individuals (Dibucaine number < 20) all three substrates were hydrolysed at a single site with a higher K m than the K m2 site present in normal plasma. The clinical implications of these results are discussed.