body. The comparative toxicities and anti- 
cholinesterase activities of malathion and 
mala-oxon, which are much less selective, 
are shown in table 5. 
In addition to the activation oxidase system, 
malathion and mala-oxon are degraded in both 
mammals and insects by carboxyesterase, 
which removes one of the ethyl ester groups, 
leaving compounds that are not anticholines- 
terases because the phosphorus atom is no 
longer highly electrophilic. Further degrada- 
tion of the malathion-type molecules also 
occurs. The complete metabolic scheme in 
insects and mammals is shown in figure 1. 
Table 5.--Comparison of malathion and mala-oxon (31) 



Insecticide 

Maitathion....2..c<.0 
Malia ORON se. se oes 
S 
MeO <ul 
LDs5g mg. per kg. 
Musca Periplaneta 
Mouse, i.p. Alas Periplaneta, 



I50 ChE M. 
Fly brain 
oxidase MeO . | 
“ pscHCOOE+ —————> Z PSCHCOOE+ 
a” 
MeO | MeO | 
CH»COOE+ CH» COOEt 
ye es 
MeO ; Meo. 9 
PSCHCOOE+ \ bscHCOoE+ ; PS: CHCOOE+ 
Vaptaedl | 
MeO 
ee CH»COOE+ MeO CHyCOOH CH2COOH 
Meo S MeO F 
i 
PSCHCOOH PSCHCOOH 
ia | | 
10) MeO 
Meo GH, COOH | CH, COOH 
S \ S (8) 
POH PSH POH 
/ / / 
MeO MeO MeO 
+ HOCHCOOH + HSCH-COOH 
| | 
CH2COOH CH,COOH 
Figure 1,--Metabolic pathways of malathion in insects and vertebrates, 
