Plant Metabolism

Methoxyfenozide is metabolized in plants by the formation of a free phenol and products that result from the oxidation of the alkyl substituents. In addition, two polar carbohydrate conjugates have been identified in rotational crops. The parent compound is the residue of primary concern for dietary exposure assessments and tolerance setting. Studies carried out to elucidate the metabolic pathway of methoxyfenozide in plants are described in further detail in this section.

The metabolism, distribution and elimination of 14C-methoxyfenozide (labeled in each ring or in the t-butyl position in Figure 6.3) were studied in apples, grapes, cotton and rice.38 In apples and grapes, the half-life of methoxyfenozide was estimated to be <30 days with the decline in residues primarily as a result of fruit growth. Methoxyfenozide was not significantly metabolized in apples, with the majority of the residue remaining as methoxyfenozide. Two metabolites were present at <2% of the residue: RH-131364 and RH-131157 (see Figure 6.4). In grapes, methoxyfenozide remained the major component, with four minor components (each <4%) identified as the glucose conjugate of RH-117236 and RH-131364 and two tentatively identified components (see Figure 6.4). Analysis of a leaf sample also showed the majority of the residue

Figure 6.3 Structure of methoxyfenozide indicating position of radiolabel.

methoxyphenyl ring (A-ring)

Chapter 6 * Denotes Position of 14C label

Dimethylphenyl ring (B-ring)

Figure 6.3 Structure of methoxyfenozide indicating position of radiolabel.

Figure 6.4 Proposed metabolic pathway for methoxyfenozide in plants (a = apples, g = grapes, c = cotton, r = rice, CRC = confined rotational crop study and [ ] indicates tentative identification).

remained as unchanged methoxyfenozide. Similar results were noted in cotton plants where methoxyfenozide was not significantly metabolized, with the majority of the residue in mature bolls and plant samples remaining as methoxyfenozide. In rice treated with foliar applications of methoxyfenozide at the pre-flag leaf and post-flowering stages, the major component of the residue was methoxyfenozide, with 52-59% and 65-69% of the residue in grain and straw, respectively. RH-117236 (see Figure 6.4) was detected at 3.2-7.5% of the residue in grain and e3% in the straw. Other metabolites comprised <5% of the radioactivity in grain or straw.38

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