The characterization of the esterases of human plasma

Abstract

Using human plasma as a source of a single nonspecific esterase and the cholinesterase inhibitors diisopropyl fluorophosphonate (DFP) and di-(2-chloroethyl) methylamine (DDM), a method was described for characterizing the enzymic activity of the unpurified plasma towards representative choline and non-choline esters. This involved the detn. for each substrate of the ratio of the concns. of each inhibitor which produced 50% inhibition of hydrolysis (the DDM:DFP I50 ratio). The method enabled the most probable value of the ratio and the range of probable values to be estimated. The values for the I50 ratio for the different esters were closely similar, ranging from 2.28 to 2.44 x 105. Evidence was obtained for the existence in human plasma of a small amt. of a second enzyme, DFP- and DDM-insensitive, which accounted for 5-20% (depending on the substrate) of the aliphatic esterase activity of the plasma, and which also hydrolyzed triolein. When the I50 ratios for tributyrin and triacetin were corrected for the contribution of the DFP-insensitive esterase they became identical with the value (2.28 x 105) for acetylcholine.