TEMPERATURE DEPENDENCE OF 

 CHOLINESTERASE ACTIVITY 



Leigh E. Chadwick, Physiology Division, Chemical Warfare 

 Laboratories, Army Chemical Center, Alaryland 



HE HYDROLYSIS of acetylcholine (ACh) by cholinesterases (ChE's), be- 

 cause of its recognized physiological importance, has been investigated in- 

 tensively for 30 years. Recently, an added stimulus has been the fact that 

 certain insecticides and chemical warfare agents owe their potency to inter- 

 ference with this reaction ; but, considering the volume of study devoted to 

 the system, some of its fundamental properties have received rather cursory 

 attention. Among them is its temperature dependence, a subject with nu- 

 merous theoretical and practical implications, the latter especially among 

 poikilothemic organisms. Though work of the past few years has added 

 to our knowledge of this phase, there are still many unanswered questions. 



Existing information may be summarized conveniently under three main 

 headings. 1} temperature and rate of hydrolysis, 2) inactivation of ChE 

 by heat, and 3} effect of temperature on rate of reaction with inhibitors. 

 Since data on all three aspects are obtained from study of hydrolysis rates, 

 there is inevitably some overlap in this classification, as will be apparent 

 in the discussion below. 



TEMPERATURE AND RATE OF HYDROLYSIS 



The ability of ChE's to split ACh and some other esters has been ex- 

 amined as a function of temperature with enzymes from: mammalian blood 

 {2, 8, 12-15, 17, 18, 21, 26, 34, 39, 40, 45, 46) ; ox cortex and caudate 

 nucleus (35) ; intestine of the fish, Ameiurus (29) ; developing eggs of the 

 grasshopper (44) ; arthropod nerve cord: lobster (7), cockroach (37) ; fly 

 heads: Dacus (38) , Musca (10) . In addition, it is possible that the enzyme 

 from developing eggs of Melanoplus, investigated by Carlson (9) with 

 methyl butyrate as substrate, is the same as the ChE of Tahmisian (44) , 

 since the method of preparation and the temperature response were very 

 similar. Smallman and Wolfe (43) have produced data, still unpublished, 

 that supplement those cited above for the head of the house fly. 



Despite the number and variety of references, the exact nature of the 

 relationship between temperature and ChE activity is still in doubt. Some 

 of the reports deal with specific, others with nonspecific esterases, and in 

 many the temperature data are only incidental. There seem also to be 



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