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Polymorphism of Pseudocholinesterase in Torpedo marmorata Tissues: Comparative Study of the Catalytic and Molecular Properties of this Enzyme with Acetylcholinesterase

Authors


Address correspondence and reprint requests to Dr. J. P. Toutant, Laboratoire de Neurobiologie, Ecole Normale Supérieure, 46 rue d'Ulm, 75230 Paris Cedex 05, France.

Abstract

Abstract: We report the existence, in Torpedo marmorata tissues, of a cholinesterase species (sensitive to 10−5M eserine) that differs from acetylcholinesterase (AChE, EC 3.1.1.7) in several respects: (a) The enzyme hydrolyzes butyrylthiocholine (BuSCh) at about 30% of the rate at which it hydrolyzes acetylthiocholine (AcSCh), whereas Torpedo AChE does not show any activity on BuSCh. (b) It is not inhibited by 10−5M BW 284C51, but rapidly inactivated by 10−8M diisopropyl-fluorophosphonate. (c) It does not exhibit inhibition by excess substrate up to 5 × 10−3M AcSCh. (d) It does not cross-react with anti-AChE antibodies raised against purified Torpedo AChE. This enzyme is obviously homologous to the “nonspecific” or pseudocholinesterase (pseudo-ChE, EC 3.1.1.8) that exists in other species, although it is closer to “true” AChE than classic pseudo-ChE in several respects. Thus, it shows the highest Vmax with acetyl-, and not propionyl- or butyrylthiocholine, and it is not specifically sensitive to ethopropazine. Pseudo-ChE is apparently absent from the electric organs, but represents the only cholinesterase species in the heart ventricle. Pseudo-ChE and AChE coexist in the spinal cord and in blood plasma, where they contribute to AcSCh hydrolysis in comparable proportions. Pseudo- ChE exists in several molecular forms, including collagen-tailed forms, which can be considered as homologous to those of AChE. In the heart the major component of pseudo-ChE appears to be a soluble monomeric form (G1). This form is inactivated by Triton X-100 within days. In addition, it is converted, on storage of the extracts, into more rapidly sedimenting, Triton X-100-resistant forms (G2 and G4). A G4 form exists in spinal cord and plasma. Collagen-tailed forms, A12 and A8, were characterized in high-salt extracts from spinal cord and heart ventricle. Pseudo-ChE molecular forms sediment slightly slower than the corresponding AChE forms. The two polymorphic enzyme systems, AChE and pseudo-ChE, therefore arose as early as elasmobranchs during the evolution of vertebrates.

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