Wetherell_1986_Biochem.Pharmacol_35_939

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 Km (Km1 11.4 +/- 3.3 microM) with a Vmax of 0.06 mumol/min/ml and a site with a high Km (Km2 132.4 +/- 14.8 microM) and a Vmax of 0.107 mumol/min/ml. The Km2 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 Km2 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 microM). Whereas high concentrations of SDTCh (200 micron) 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 less than 20) all three substrates were hydrolysed at a single site with a higher Km than the Km2 site present in normal plasma. The clinical implications of these results are discussed.