Atrazine metabolism in sorghum. Chloroform-soluble intermediates in the N-dealkylation and glutathione conjugation pathways

Abstract

Two unrelated metabolites of atrazine were isolated from sorghum. Complete N-dealkylation yielded 2-chloro-4,6-diamino-s-triazine, which no longer inhibited the Hill reaction and cyclic and noncyclic photophosphorylation in isolated pea chloroplasts. The isolation and identification of the metabolite N,Ai'-bis(4-ethylamino-6isopropylamino-s-triazinyl-2)cystine gave further support to a previous report that atrazine was metabolized by the glutathione conjugation pathway to its lanthionine conjugate. The isolation of the dimer does not necessarily indicate that this is the form of the metabolite in plants. The Ncysteine monomer may dimerize in vivo during the sequence of reactions leading to the lanthionine conjugate.The metabolism and detoxication of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) in higher plants occurs via 2-hydroxylation, N-dealkylation, and glutathione conjugation, as discussed in a recent review (Shimabukuro et al., 1971b). Glutathione conjugation seems to be the key pathway responsible for the mechanism of resistance in plants such as corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], and sugarcane (Saccharum officinarum L.) (Lamoureux et al., 1970(Lamoureux et al., , 1972Shimabukuro and Swanson, 1969;Shimabukuro et al., 1971a). N-Dealkylation is significantly less active than glutathione conjugation in corn and sorghum (Shimabukuro, 1967;Shimabukuro et al., 1970). In sorghum, Ndealkylation yielded the metabolites 2-chloro-4-amino-6isopropylamino-s-triazine and 2-chloro-4-amino-6-ethylamino-s-triazine (Shimabukuro, 1967). Chloroform-soluble metabolites other than the two mono-N-dealkylated derivatives were also present in sorghum (Shimabukuro, 1967). Plimmer et al. (1971) isolated and identified 2chloro-4,6-diamino-s-triazine as one of the reaction products resulting from a free-radical oxidation of atrazine. This metabolite has not been isolated and identified in higher plants, although a metabolite with chromatographic properties similar to those of 2-chloro-4,6-diamino-s-triazine was reported to be present in resistant grasses (Hurter, 1967). Another unknown chloroform-soluble metabolite, amounting to 4.1% of the total radioactivity in sorghum shoots, was detected after a 4-day treating period with atrazine-14C (Shimabukuro et al., 1971b).This paper describes the isolation and identification of two chloroform-soluble metabolites of atrazine in sorghum. No product-precursor relationship existed between the two metabolites. The identification of the unknown metabolites indicated that one was an intermediate in the glutathione conjugation pathway, but the second was an intermediate in the N-dealkylation pathway. The structures of the metabolites, designated as XI and XIII, are shown in Figure 1. MATERIALS AND METHODSPlant Material. For small-scale herbicide treatments, sorghum seeds [Sorghum bicolor (L.) Moench var. North Dakota 104] were germinated in vermiculite for 11 days before transfer to continuously aerated Hoagland's solution. The plants in nutrient solution wer...

Atrazine metabolism in sorghum. Catabolism of the glutathione conjugate of atrazine

“…One triazine, 2-chloro-4-(ethylamino)-6-( 1-cyano-6-methylethylamino) -s-triazine, has been shown to be metabolized via the mercapturic acid pathway in the rat (Crayford and Hutson, 1972;Hutson et al, 1970). Metabolite VI was detected and isolated as the chloroform-soluble disulfide dimer in a concurrent study of the chloroform-soluble metabolites of atrazine in sorghum (Shimabukuro et al, 1973). T h e obligatory step in the mercapturic acid pathway, N-acetylation of the terminal amino residue of cysteine, was not detected in the metabolism of atrazine in sorghum.…”

Transgenic Rice Plants Expressing Human P450 Genes Involved in Xenobiotic Metabolism for Phytoremediation

“…N-dealkylation of atrazine occurs in many plant species, with preferential formation of deethylated atrazine over deisopropylated atrazine [Lamoureux et al, 1998]. N-dealkylation of a single side-chain results in a partial loss of phytotoxicity, and N-dealkylation of both side-chains causes complete loss of phytotoxicity [Shimabukuro et al, 1973]. Mammalian P450s metabolize atrazine by a similar dealkylation pathway.…”

The Metabolism of Atrazine and Related 2-Chloro-4,6-bis(alkylamino)-s-triazines in Plants

“…In pea and sorghum, N-dealkylation of atrazine occurs more readily with the ethyl side-chain than with the isopropyl sidechain and the concentration of desethyl atrazine (I) can be twice the concentration of desisopropyl atrazine (II) or atrazine (2) (Figure 2). N-Dealkylation of one sidechain results in partial loss of phytotoxicity while N-dealkylation of both side-chains results in nearly complete loss of phytotoxicity (2,8). N-Dealkylation proceeds primarily to mono-N-dealkylated metabolites in some moderately susceptible plant species while N-dealkylation of both side-chains becomes more important in some more tolerant species.…”

“…The rearrangement probably involves a 5-membered cyclic intermediate in which the sulfur of cysteine is displaced by the amine (Figure 11). The corresponding disulfide dimer of the Ncysteine conjugate was identified as a metabolite of atrazine in sorghum, accounting for 4% of the TRR (8). An N-cysteine conjugate of 2-methylthio-4-ethylamino-6-(l\2'-dimethylpropyl)amino-.y-triazine was also produced in rice from the GSH pathway and appears to be one of the few other cases where an ^/-cysteine conjugate has been observed from the GSH pathway (14).…”