36
Richard H. Shimabukuro et al.
DDT was highly resistant to metabolism by marine phytoplankton (Bowes,
1972). DDE was the only metabolite of DDT in seven marine phytoplankters
with a conversion rate of 0-7.4% after 9 days of growth with DDT. Apparently,
oxidation to DCBP did not occur. Metabolism by dehydrochlorination is not a
detoxication mechanism as far as the effect on the noncyclic electron transport
reaction in photosynthesis is concerned. Both DDT and DDE inhibited noncyclic
electron transport in isolated chloroplasts with an Iso concentration of 20 J1.M
(Bowes, 1972).
Examples of in vitro pesticide metabolism by plant mfo systems are limited
(Lamoureux and Frear, 1979). The mixed function oxidases from plants may
have narrow substrate specificities in contrast to animal mfo systems. Plants
typically show levels of mfo activity with specific activities between 1.0 to 10
nmole productlmg protein per hour. Concentrations of P-450 in plants ranged
from 0.007 to 0.02 nmole P-450/mg microsomal protein as compared to 0.8-1.1
nmole P-450/mg microsomal protein in rat liver (Markham et al., 1972).
2.3.2. Reduction
Reduction is less prevalent than oxidation in the metabolism of pesticides
by plants. Reduction of the sulfoxide of fensulfothion to the sulfide and the
reductive dechlorination of tetrachlorvinphos in plants have been described (Fig.
2.12) (Eto, 1974). Aryl nitroreduction of fluorodifen was a minor metabolic
pathway in the peanut (Eastin, 1971). However, aryl nitroreduction of the fungicide PCNB was a significant reaction in peanut root in the absence of root
aeration (Fig. 2.12) (Lamoureux and Rusness, 1976). About 28% of the absorbed
PCNB was converted to pentachloroaniline.
Aryl nitroreductase from roots and hypocotyls of peanut seedlings reduced
PCNB under a nitrogen atmosphere with both FAD and NADPH as cofactors
(Lamoureux and Rusness, 1976). The herbicide Dinoben® was also reduced to
another herbicide, chloramben, by an aryl nitroreductase from soybean roots
(Fig. 2.12) (Frear, 1975). This enzymatic reaction occurred under nitrogen with
either NADPH or NADH as cofactors, but the more purified enzyme required
FAD or FMN in addition to NADPH or NADH. The physiological significance
of Dinoben® reduction is not clear, but it may be an activation mechanism
influencing herbicidal selectivity of Dinoben®.
The reduction of the nitro groups in substituted dinitroaniline herbicides
occurs in plants (Biswas and Hamilton, 1969; Probst et al., 1976). The biotransformation of these herbicides in plant tissues has been questioned (Probst
et al., 1976), but published reports indicate that plants metabolize these compounds rapidly and extensively by reactions including nitroreduction, N-dealkylation, and cyclization (Biswas and Hamilton, 1969; Marquis et al., 1979; Sumner et al., 1976; Wright et al., 1975).
Richard H. Shimabukuro et al.
DDT was highly resistant to metabolism by marine phytoplankton (Bowes,
1972). DDE was the only metabolite of DDT in seven marine phytoplankters
with a conversion rate of 0-7.4% after 9 days of growth with DDT. Apparently,
oxidation to DCBP did not occur. Metabolism by dehydrochlorination is not a
detoxication mechanism as far as the effect on the noncyclic electron transport
reaction in photosynthesis is concerned. Both DDT and DDE inhibited noncyclic
electron transport in isolated chloroplasts with an Iso concentration of 20 J1.M
(Bowes, 1972).
Examples of in vitro pesticide metabolism by plant mfo systems are limited
(Lamoureux and Frear, 1979). The mixed function oxidases from plants may
have narrow substrate specificities in contrast to animal mfo systems. Plants
typically show levels of mfo activity with specific activities between 1.0 to 10
nmole productlmg protein per hour. Concentrations of P-450 in plants ranged
from 0.007 to 0.02 nmole P-450/mg microsomal protein as compared to 0.8-1.1
nmole P-450/mg microsomal protein in rat liver (Markham et al., 1972).
2.3.2. Reduction
Reduction is less prevalent than oxidation in the metabolism of pesticides
by plants. Reduction of the sulfoxide of fensulfothion to the sulfide and the
reductive dechlorination of tetrachlorvinphos in plants have been described (Fig.
2.12) (Eto, 1974). Aryl nitroreduction of fluorodifen was a minor metabolic
pathway in the peanut (Eastin, 1971). However, aryl nitroreduction of the fungicide PCNB was a significant reaction in peanut root in the absence of root
aeration (Fig. 2.12) (Lamoureux and Rusness, 1976). About 28% of the absorbed
PCNB was converted to pentachloroaniline.
Aryl nitroreductase from roots and hypocotyls of peanut seedlings reduced
PCNB under a nitrogen atmosphere with both FAD and NADPH as cofactors
(Lamoureux and Rusness, 1976). The herbicide Dinoben® was also reduced to
another herbicide, chloramben, by an aryl nitroreductase from soybean roots
(Fig. 2.12) (Frear, 1975). This enzymatic reaction occurred under nitrogen with
either NADPH or NADH as cofactors, but the more purified enzyme required
FAD or FMN in addition to NADPH or NADH. The physiological significance
of Dinoben® reduction is not clear, but it may be an activation mechanism
influencing herbicidal selectivity of Dinoben®.
The reduction of the nitro groups in substituted dinitroaniline herbicides
occurs in plants (Biswas and Hamilton, 1969; Probst et al., 1976). The biotransformation of these herbicides in plant tissues has been questioned (Probst
et al., 1976), but published reports indicate that plants metabolize these compounds rapidly and extensively by reactions including nitroreduction, N-dealkylation, and cyclization (Biswas and Hamilton, 1969; Marquis et al., 1979; Sumner et al., 1976; Wright et al., 1975).