Degradation of Pesticides by Animals
15
are frequently formed from aromatic and aliphatic compounds containing a hydroxyl group. In other instances, sulfate conjugates are formed with amino
groups. The resulting conjugates of hydroxyl and amino groups are referred to
as sulfate ester conjugates or ethereal sulfates. Ethereal sulfate conjugation occurs
in all mammals, and in birds, reptiles, mollusks, and insects. It does not occur
in some microorganisms or in plants due to a lack of sulfotransferase enzymes
(Dodgson and Rose, 1970). Most animal species conjugate phenolic compounds
as both sulfates and glucuronides, and the ratio of the conjugates will vary
according to the substrate and species involved (Berenbom and Young, 1951;
Capel et al., 1974; Usui et al., 1977).
1.3.3. Other Conjugation Reactions
Glutathione (GSH) conjugation or mercapturic acid formation is an important metabolic pathway for some pesticides in plants and animals. GSH conjugation of pesticides was reviewed by Hutson with an emphasis on herbicides
(1976).
GSH is a tripeptide containing glutamic acid, cysteine, and glycine. GSH
conjugation is catalyzed by enzymes called GSH-S-transferases, which are located in the soluble fraction of tissue homogenates. The general reaction is
outlined in Fig. 1.14.
Many types of chemicals undergo GSH conjugation because of the various
types of substrate-specific GSH-S-transferases that mediate this reaction (Boyland
and Chasseaud, 1969). These include GSH-S-aryl, -aralkyl, and -alkenetransferases. Organophosphorous triesters, S-triazines, diphenyl ethers, organothiocyanates, and thiocarbamates (sulfoxide) are among the pesticides known to be
substrates of GSH-S-transferases (Usui et al., 1977). Mercapturic acid formation
GSH - S-trons'erose
1.
RX + G5H
t-glutamyl transferase
2. R5G
3. R-Cys-Gly
R5G + HX
R-Cys-Gly + glutamate
R-SCH 2 CHCOOH + glycine
I
NH2
occtyl4. R-5CH 2 C I HCOOH + acetyl-CoA - - - - - - . R-SCH 2 C I HCOOH + CoA
transferase
NH2
NHCOCH a
Figure 1.14. General reaction scheme for glutathione conjugation.
15
are frequently formed from aromatic and aliphatic compounds containing a hydroxyl group. In other instances, sulfate conjugates are formed with amino
groups. The resulting conjugates of hydroxyl and amino groups are referred to
as sulfate ester conjugates or ethereal sulfates. Ethereal sulfate conjugation occurs
in all mammals, and in birds, reptiles, mollusks, and insects. It does not occur
in some microorganisms or in plants due to a lack of sulfotransferase enzymes
(Dodgson and Rose, 1970). Most animal species conjugate phenolic compounds
as both sulfates and glucuronides, and the ratio of the conjugates will vary
according to the substrate and species involved (Berenbom and Young, 1951;
Capel et al., 1974; Usui et al., 1977).
1.3.3. Other Conjugation Reactions
Glutathione (GSH) conjugation or mercapturic acid formation is an important metabolic pathway for some pesticides in plants and animals. GSH conjugation of pesticides was reviewed by Hutson with an emphasis on herbicides
(1976).
GSH is a tripeptide containing glutamic acid, cysteine, and glycine. GSH
conjugation is catalyzed by enzymes called GSH-S-transferases, which are located in the soluble fraction of tissue homogenates. The general reaction is
outlined in Fig. 1.14.
Many types of chemicals undergo GSH conjugation because of the various
types of substrate-specific GSH-S-transferases that mediate this reaction (Boyland
and Chasseaud, 1969). These include GSH-S-aryl, -aralkyl, and -alkenetransferases. Organophosphorous triesters, S-triazines, diphenyl ethers, organothiocyanates, and thiocarbamates (sulfoxide) are among the pesticides known to be
substrates of GSH-S-transferases (Usui et al., 1977). Mercapturic acid formation
GSH - S-trons'erose
1.
RX + G5H
t-glutamyl transferase
2. R5G
3. R-Cys-Gly
R5G + HX
R-Cys-Gly + glutamate
R-SCH 2 CHCOOH + glycine
I
NH2
occtyl4. R-5CH 2 C I HCOOH + acetyl-CoA - - - - - - . R-SCH 2 C I HCOOH + CoA
transferase
NH2
NHCOCH a
Figure 1.14. General reaction scheme for glutathione conjugation.