Studies on cholinesterase. 9. Species variation in the specificity pattern of the pseudo cholinesterases

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Abstract
Pseudo cholinesterases from the sera of 10 mammalian and 3 avian species were studied. Eight pseudo cholinesterases can be classified as butyro and 4 as propiono-cholinesterases, while the pseudo cholinesterase of pig serum could not be classified readily. Substrate specificity patterns of the 2 groups can be arranged into a series which shows a gradual shift in the opt. acyl group. Similar specificity patterns were found for the pseudo cholinesterase in 12 different rat tissues, indicating that the enzyme is probably the same in all tissues. The pseudo cholinesterases appear to be sp. specific but not organ specific. The harderian glands of the rat and hamster also contain an eserine-resistant esterase capable of hydrolysing butyrylcholine. The true cholinesterase of chicken brain hydrolyzes propionylcholine more rapidly than acetylcholine and resembles other true cholinesterases in its remaining enzymo-logical properties, which differ greatly from those of the pseudo cholinesterases. A rigid classification of the cholinesterases on the basis of substrate specificity patterns would characterize both the true and pseudo cholinesterase of the chicken as propiono-cholinesterases. Acetyl-beta-methylcholine cannot be used as a specific substrate for measurement of true cholinesterase activity in the chicken and other avian spp., whereas butyrylcholine is suitable (Earl and Thompson, 1952). The proposed classification of cholinesterases on the basis of substrate specificity patterns is criticized, and the importance of designating the pseudo cholinesterases according to their source is stressed.