28 II. DIGESTION AND ABSORPTION OF FATS 



destroyed by serum, a fact which had been reported earlier by Plattner. 180 

 The rate of destruction of acetylcholine by blood obtained from different 

 species of mammals was found to decrease in the following order: man, 

 pig, cattle, dog, horse, rabbit, and cat. 181 On the basis of inhibition ex- 

 periments, Stedman et al. m were led to the conclusion that the acetyl- 

 choline-destroying principle is an esterase; hence they gave this enzyme 

 the name "choline esterase." In the more recent literature the term has 

 been abbreviated to "cholinesterase," as used throughout this volume, 

 orto"ChE." 



It is obvious that the cholinesterase mechanism is one which limits the 

 duration of life of acetylcholine, and hence of the nerve impulse. Were it 

 not for the presence of cholinesterase in the tissues, acetylcholine would be 

 a dangerous poison. Agents which inhibit the activity of cholinesterases, 

 therefore, are toxic, since they prevent the degradation of the acetyl- 

 choline which is normally being produced. Torda and Wolff 182 called 

 attention to the fact that most of the convulsion-inducing agents cause a 

 rapid accumulation, or sudden increase, in acetylcholine, either by aug- 

 menting the rate of synthesis of acetylcholine (pentamethylene tetrazol 

 and picrotoxin) or by decreasing the extent of hydrolysis of acetylcholine 

 (strychnine, morphine, etc.). When acetylcholine is simultaneously 

 administered, the effect is further potentiated. 



(c) Types of Cholinesterases. In their original report on the enzymes of 

 horse serum, Stedman and co-workers 179 noted that not only acetylcholine 

 but also ethyl acetate and tributyrin were hydrolyzed by these sera. They 

 question "whether one enzyme is responsible for the hydrolysis of all three 

 types of substrate or whether different enzymes are involved." However, 

 Easson and Stedman 183 subsequently concluded, on the basis of studies on 

 human sera, that cholinesterase exerts a specific action, even though they 

 admitted the possibility that it may likewise act to a slight extent on other 

 esters such as tributyrin. On the other hand, the cholinesterase obtained 

 from the blood of guinea pigs differs from that present in human sera in 

 that it is insensitive to the inhibiting action of eserine. This finding 

 led Easson and Stedman 183 to the conclusion that a "second esterase" 

 might exist which had the capacity to inactivate acetylcholine. 



At least three cholinesterases are now recognized, which are as follows: 



1. "True" or "specific" cholinesterase which hydrolyzes only choline 



180 F. Plattner, Arch. ges. Physiol. (Pflilger's), 214, 112-129 (1926). 



181 O. Galehr and F. Plattner, Arch. ges. Physiol. {Pfliiger's), 218, 506-513 (1928). 



182 C. Torda and H. G. Wolff, Am. J. Physiol, 151, 345-354 (1947). 



183 L. H. Easson and E. Stedman, Biochem. J., 31, 1723-1729 (1937). 



