Substrate-dependent kinetic behavior of horse plasma cholinesterase: evidence for kinetically distinct populations of active sites.
The inhibition of horse plasma cholinesterase by propranolol showed characteristics which depended upon the identity of the substrate used. With butyrylthiocholine as substrate, the inhibition showed a first-order dependence on inhibitor concentration, and was characterized by a Ki of 8 microM (pH 7.4, 20 degrees C). With p-nitrophenylbutyrate as substrate, a biphasic v-1 versus [I] relationship was obtained. The biphasic curve could be resolved into two components, with apparent Ki's of 9 microM and 1.3 mM. Use of butyrylthiocholine as alternative substrate resulted in partial inhibition of p-nitrophenylbutyrate hydrolysis. Inhibition of butyrylthiocholine hydrolysis by p-nitrophenylbutyrate could be accounted for by pure competitive inhibition at two sites. The results were interpreted in terms of a four-site, low-symmetry model, in which two active sites could process both substrates, and the remaining sites could process only p-nitrophenylbutyrate.
D000495:Allosteric Site; D000818:Animals; D001667:Binding, Competitive; D002092:Butyrylthiocholine / Q000097:blood / Q000494:pharmacology; D055598:Chemical Phenomena; D002621:Chemistry; D002800:Cholinesterase Inhibitors; D002802:Cholinesterases / Q000097:blood*; D006736:Horses; D006868:Hydrolysis; D007700:Kinetics; D010654:Phenylbutyrates / Q000097:blood / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D013379:Substrate Specificity
D002800:Cholinesterase Inhibitors; D010654:Phenylbutyrates; C016013:2-nitrophenyl butyrate; D002092:Butyrylthiocholine; D011433:Propranolol; D002802:Cholinesterases