32
Richard H. Shimabukuro et al.
EEHP
EDHP
Figure 2.6. Apparent mixed function oxidase-catalyzed hydrolysis, O-dealkylation, and desulfuration of etrimfos in beans and com.
example of pesticide metabolism in plants where an mfo system and its reaction
mechanism have been demonstrated (Fig. 2.7) (Frear et ai., 1969, 1972b).
Successive oxidative N-demethylation to monomethyl monuron and p-chlorophenylurea is the major monuron detoxication pathway in plants (Frear and
Swanson, 1972; Frear et ai., 1972b). A minor pathway involved aryl hydroxylation followed by glucosylation of the phenolic intennediates (Frear and Swanson, 1974). The N-dealkylation reaction was catalyzed by an mfo system that
required molecular oxygen and reduced pyridine nucleotides as cofactors (Fig.
2.8). When monuron was the substrate, the reaction products were monomethyl
monuron and fonnaldehyde. An unstable N-hydroxymethyl intennediate was
detected as a reaction product when monomethyl monuron was the substrate.
This intennediate was not isolated, but it degraded stoichiometrically to p-chlorophenylurea and fonnaledhyde (Frear et ai., 1969, 1972b). The N-demethylase
system had limited substrate specificity for dimethyl- and monomethylphenylurea
herbicides and was strongly inhibited by carbamate insecticides (Frear et ai.,
1969, 1972b).
N-Dealkylation may either detoxify or activate a pesticide. Generally, Ndealkylation of herbicides is a detoxication mechanism in contrast to some insecticides. Successive N-demethylation of monuron is a detoxication mechanism
(C. R. Swanson and H. R. Swanson, 1968). Mono-N-dealkylation of atrazine
is only a partial detoxication mechanism (Fig. 2.9) (Shimabukuro, 1967). Sandoz
6706®, a substituted phenyl pyridazinone herbicide, was successively demethylated by susceptible com and soybeans to the more phytotoxic Sandoz 9789®
(norflurazon) and to the nonphytotoxic demethylated metabolite (Fig. 2.9) (Strang
and Rogers, 1974). Diphenamid was successively demethylated in plants, and
the N-methyl-N-hydroxymethyl intennediate was detected in high concentrations
Richard H. Shimabukuro et al.
EEHP
EDHP
Figure 2.6. Apparent mixed function oxidase-catalyzed hydrolysis, O-dealkylation, and desulfuration of etrimfos in beans and com.
example of pesticide metabolism in plants where an mfo system and its reaction
mechanism have been demonstrated (Fig. 2.7) (Frear et ai., 1969, 1972b).
Successive oxidative N-demethylation to monomethyl monuron and p-chlorophenylurea is the major monuron detoxication pathway in plants (Frear and
Swanson, 1972; Frear et ai., 1972b). A minor pathway involved aryl hydroxylation followed by glucosylation of the phenolic intennediates (Frear and Swanson, 1974). The N-dealkylation reaction was catalyzed by an mfo system that
required molecular oxygen and reduced pyridine nucleotides as cofactors (Fig.
2.8). When monuron was the substrate, the reaction products were monomethyl
monuron and fonnaldehyde. An unstable N-hydroxymethyl intennediate was
detected as a reaction product when monomethyl monuron was the substrate.
This intennediate was not isolated, but it degraded stoichiometrically to p-chlorophenylurea and fonnaledhyde (Frear et ai., 1969, 1972b). The N-demethylase
system had limited substrate specificity for dimethyl- and monomethylphenylurea
herbicides and was strongly inhibited by carbamate insecticides (Frear et ai.,
1969, 1972b).
N-Dealkylation may either detoxify or activate a pesticide. Generally, Ndealkylation of herbicides is a detoxication mechanism in contrast to some insecticides. Successive N-demethylation of monuron is a detoxication mechanism
(C. R. Swanson and H. R. Swanson, 1968). Mono-N-dealkylation of atrazine
is only a partial detoxication mechanism (Fig. 2.9) (Shimabukuro, 1967). Sandoz
6706®, a substituted phenyl pyridazinone herbicide, was successively demethylated by susceptible com and soybeans to the more phytotoxic Sandoz 9789®
(norflurazon) and to the nonphytotoxic demethylated metabolite (Fig. 2.9) (Strang
and Rogers, 1974). Diphenamid was successively demethylated in plants, and
the N-methyl-N-hydroxymethyl intennediate was detected in high concentrations