Malathion [0,0-dimethyl S-(1,2-bis-carbethoxy) ethyl phosphorodithioate] 
Accumulation of malathion or its metabolites in the tissues of hens 
fed labeled malathion was slow. Two to three per cent was excreted unchange 
or as chloroform-soluble metabolites. Ninety-seven to ninety-eight percent 
is excreted as ionic water-soluble metabolites. Of the tissues examined, 
liver and kidney contained the greatest amount of radioactive compounds. 
The level of radioactivity remained constant for a week after withdrawal 
from the malathion-fortified feed and then slowly decreased. The amount 
of radioactive compounds decreased more rapidly in the white than in the 
yolk of the egg (922). When rats were fed C!4-labeled malathion, the bulk 
(83%) of the activity was eliminated within 24 hours in the urine. A small 
amount of malathion was degraded to respiratory CO. Very little activity 
was found in the tissues or blood (1628). 
Apparently two major degradative pathways occur in mammals: 
that involving carboxy esterases and that involving phosphatases. The 
differences in the activity of these two systems, in mammals as against 
insects, helps account for the difference in toxicity. In the urine of cows, 
the principal metabolite was malathion monoacid; the diacid became more 
important with time. Desmethyl malathion was also found. In fecal matter, 
malaoxon, dimethyl phosphate, and 0,0-dimethyl phosphorothioate were found 
(923, 1093, 1132, 1475, 1485). Other studies with rats and dogs have shown 
the presence of malaoxon in the blood and malathion mono- and di-acids in 
the urine as well as monomethyl phosphate, dimethyl phosphate, and dimethyl 
thiophosphate (807, 945, 1283). 
The hydrolytic effect of various esterases on malathion was investi- 
gated. It was found that malathion was not hydrolyzed at a significant rate 
by horse-serum-cholinesterase, nor by pancreatic lipase. Rat liver ali- 
esterase, however, hydrolyzed malathion to the monoacidic malathion. It was 
not certain which of the two possible acids was formed or whether both were 
formed (921). 
In human liver, malathion was degraded mainly through the action of a 
carboxyesterase. Other hydrolytic actions occurred but at a rather low level: 
Demethylation accounted for about 5%; P-S cleavage, for about 1%; and S-C 
cleavage for about 0.1 to 0.5% (944). 
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