ENZYMES CONCERNED WITH DIGESTION OF LIPIDS 33 



esterase was inhibited by eserine and by diisopropyl fluorophosphate 

 (usually designated as DFP), 205 as well as by caffeine and morphine. 206 



The turnover number (or turnover rate) of cholinesterase gives some indi- 

 cation as to the rapidity of its action. Since the enzyme was not available 

 in pure form, it was impossible to express the turnover rate in terms of 

 molecules; consequently, Berry 207 employed the active centers for the evalu- 

 ation. In the case of partially purified c-cholinesterase (obtained from hu- 

 man red blood cells), the value was reported as 162,000 molecules of 

 acetylcholine per minute. In a later report, Berry 208 observed that the 

 turnover rate of cholinesterase varied, not only with species, but also with 

 individuals of any one species. 



b'. Specificity of Cholinesterases : The e-type of cholinesterase is 

 generally considered to be extremely specific, in that it will not act on non- 

 choline esters. 193 On the other hand, the s-type of cholinesterase acts not 

 only on acetylcholine, but also on methyl butyrate and tributyrin. Richter 

 and Croft 193 have described ali-esterases in the sera of several species, 

 which act on methyl butyrate or tributyrin but not on acetylcholine. 

 Adams and Whittaker 209 have ascribed the specificity of the erythrocyte (e) 

 cholinesterase to at least two factors, i.e., the acyl group and the alcohol 

 group of the substrate. In contradistinction to the behavior of the e- 

 cholinesterase, the plasma (s) cholinesterase accommodates preferentially 

 the larger acyl groups such as propionyl and butyryl. Chain-branching 

 causes variations in the action of the two types of cholinesterase. 



Although it is generally agreed that the cholinesterases prepared from 

 the sera of the several species examined are non-specific, it has been reported 

 that preparations from erythrocytes of several species other than man 

 were able to split non-choline esters. However, Mendel and Rudney 186 

 did find that purified erythrocyte cholinesterase preparations, regardless 

 of the source of the cells, were unable to hydrolyze methyl butyrate or 

 tributyrin. Mounter and Whittaker 210 reported that a partially purified 

 preparation of horse erythrocyte cholinesterase had the capacity to hydro- 

 lyze a number of aliphatic esters; the rates of hydrolysis increased as the 

 structure of the substrate approached that of acetylcholine. Thus, it was 

 found that the carbon analogue of acetylcholine (3,3-dimethylbutylacetate, 



205 F. Bovet-Nitti, Experientia, 3, 283-286 (1947). 



206 E. A. Zeller, unpublished data cited in E. A. Zeller, in Advances in Enzymology, 

 Vol. VIII, Interscience, New York and London, 1948, pp. 459-495. 



207 W. K. Berry, Biochem. J., 47, xxi (1950). 



208 W. K. Berry, Biochem. J., 49, 615-620 (1951). 



209 D. H. Adams and V. P. Whittaker, Biochem. J., 43, xiv-xv (1948). 

 110 L. A. Mounter and V. P. Whittaker, Biochem. J., 47, 525-530 (1950). 



