Multiple molecular forms and lectin interactions of organophosphate-sensitive plasma and liver esterases during...

Liver and plasma acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and carboxylesterase activities of the chick embryo and adult chickens were separated by sucrose density gradient sedimentation and further differentiated by their lectin affinities and organophosphate sensitivities. Changes in plasma cholinesterases during development indicated a characteristic shift in tetrameric (G4) isoforms from a slightly larger G4 AChE in the embryo to G4 BChE in the adult. These changes were not reflected in isoform patterns of liver homogenates, however. Interestingly, the time course of an increase in plasma BChE activity corresponded to the time course of a decrease in liver BChE activity, as if this enzyme was being mobilized and released. The distribution of liver esterases included both monomeric (G1) and G4 BChE and a large p-nitrophenylacetate (p-NPA) esterase activity that was separated into two main peaks by density gradient ultracentrifugation. The effects of organophosphate inhibitors indicated that the two liver p-NPA esterase activities may be regarded as carboxylesterases; however, these enzymes showed very different sensitivities to paraoxon and diisopropylfluorophosphate (DFP), with IC50 values differing by 3 and 4 orders of magnitude. Lectin affinity studies with multiple esterase forms suggested a heterogeneous group of glycoproteins that were packaged at different sites in the liver cell and were consistent with the presence of an intracellular precursor form to plasma BChE.