The fate of [(14)C]xanthotoxin (8-methoxypsoralen) was studied in larvae of insect species that are tolerant (Papilio polyxenes Stoll) or sensitive (Spodoptera frugiperda J.E. Smith) to the phototoxic effects of photosensitizing psoralens. Both insects metabolize xanthotoxin by oxidative cleavage of the furan ring, but the detoxification occurs at a much more rapid rate inP. polyxenes in which >95% of an oral 5 μg/g xanthotoxin dose is metabolized within 1.5 hr after treatment. The detoxification of psoralens byP. polyxenes appears to occur primarily in the midgut tissue prior to absorption, with the result that the intact phototoxin does not reach appreciable levels in body tissues. Studies with an angular furanocoumarin indicated that isopsoralens are metabolized byP. polyxenes at a somewhat slower rate than observed for psoralens; however, a reduced rate of metabolic detoxification of isopsoralens probably does not explain the fact that psoralen tolerance inP. polyxenes does not extend to the isopsoralen series.

vie uC-labeled potassium 3,4-dichloro-5-isothiazolecarboxylate (PDIC) was applied to individual leaves (100 Aig/leaf) of field-grown cotton plants for studies of absorption, photodecomposition, and metabolism and to whole plants in a small plot (1121 g of /ha) for studies of radioactive residues in different parts of the cotton plant. Also, 14C-labeled PDIC was administered orally to white rats to determine metabolism, accumulation in tissues, and excretion. There was some photodecomposition (<1% of dose) of PDIC on leaf surfaces. The chemical was rapidly absorbed (55% after 24 h) from leaf surfaces and then readily translocated throughout the plant. Appreciable residues of radiocarbon (>200 ppm) accumulated in cottonseed; most of this was the parent compound. PDIC was rapidly excreted (ca. 95% in 24 h) in the urine of white rats; only minimum concentrations (0.01-0.17 ppm) remained in any tissues after 24 h.