The role of non-critical binding proteins in the sensitivity of acetylcholinesterase from different species to diisopropyl fluorophosphate (DFP), in vitro.

In vitro studies showed that organophosphate insecticides have different IC50 values (i.e., concentration of inhibitor required to inhibit 50% enzyme activity) for acetylcholinesterases (AChE's) from different species. Since previous reports indicated that the binding of active cholinesterase inhibitors to non-critical binding proteins is an important mechanism of detoxification of organophosphate insecticides in vivo, we investigated the role of non-critical binding proteins in synaptosomal membrane preparations from monkey, rat and chicken brain tissues as a possible explanation of the differences in the IC50's. The amount of non-critical binding proteins as well as critical binding sites, AChE, were determined by 3H-DFP binding in vitro. The affinity constants (Ka) and phosphorylation constants (Kp) of DFP for AChE's from these preparations were also determined by kinetic studies. In IC50 studies, monkey brain AChE was 1.5X and 3.2X more sensitive to DFP inhibition than chicken and rat brain AChE. The observed differences in IC50's cannot be explained on the basis of differences in the amount of non-critical binding proteins, since only small amount of non-critical binding proteins were found in these preparations. However, in the kinetic studies, monkey brain AChE had 3.7X and 2.1X higher affinity than chicken and rat brain AChE, respectively; and chicken brain AChE had about 2.7X faster rate of phosphorylation than the other two. It is therefore concluded that non-critical binding proteins are relatively unimportant in terms of affecting IC50's. The differences observed in IC50's are primarily due to different affinity and/or rate of phosphorylation of AChE active sites by DFP.