carpio and the bream, Abramis brama . The latter contains an enzyme of the 

 acetyl cholinesterase type (Kozlovskaya and Tshuyko 1979). 



As intoxication by organophosphorus pesticides advances, the animals ex- 

 hibit a progressive decline in the level of cholinesterase, although in 

 dying animals the enzyme may not be entirely inhibited. Such facts are 

 cited in a number of reviews (0' Brian 1964; Rosengart and Sherstobitov 

 1978). 



After acute exposure of perch ( Perca f luviatilis ) to Dylox (48-hour 

 LCioo 0^ 5 mg/ ; 48-hour LC50 of 0.62 mg/£) fish were assayed immediately 

 after death, 8 and 33 hours of the experiment, respectively. The cholin- 

 esterase activity in these cases was partially retained (up to 25 percent). 

 In fish which were left in the toxic environment after death for a few 

 hours, the enzyme was inhibited to a greater extent (Figures 3a and 3b). 

 Similar results were obtained in experiments with carp ( Carassius carassius ) 

 and pond snails ( Limnaea stagnalis ) . Densitometry of electrophorograms 

 showed that not all molecular forms of the enzyme were completely inacti- 

 vated (Figures 4a and 4b). It appears that the toxicant interacts with 

 vitally important forms of the enzymes. 



The inhibitation of AChE in the brain of perch has been also observed at 

 sublethal concentrations, although the external symptoms of poisoning were 

 absent (Table 6). Upon placing the animals in freshwater, the gradual re- 

 activation of enzymes took place. 



TABLE 6. CHANGES IN THE ACETYL CHOLINESTERASE ACTIVITY OF THE PERCH BRAIN 



IN THE MINIMUM TOLERABLE CONCENTRATIONS OF DYLOX (0.12 mg/J) WITH 



SUBSEQUENT WASHING IN FRESHWATER 



Enzyme of Activity 

 Number of % of the 

 Exposure samples y M AChE g/h control 



Exposure in the Dylox 1-10 427.9 + 0.84 87.2 

 Solution 5-10 339.3+0.79 67.6 



One day exposure in fresh- 1-15 358.6 + 1.03 73.5 

 water after 5-days expo- 5-11 507.1 +0.43 97.4 

 sure in Dylox 



The periodic addition of Dylox to the test system causes increases in 

 inhibition of AChE with each dose. Fish mortality occurs at a total concen- 

 tration of 0.36 mg/l, considerably below the minimum lethal concentration 

 (Table 7). 



Similar results have been obtained with experiments on roach. Daily ex- 

 posure to one-tenth of the 48-hour LCiqo ^^^ to a greater toxic effect 

 than the exposure in concentrations equal to the full 48-hour LC]oo. 



90 



