The toxicity and chemistry of Thamnosma texana

Abstract

Oral route of exposure of sheep to the air-dried, aerial portions of Thamnosma texana (Dutchman's breeches), resulted in severe photosensitization. Animals fed the plant at 9 or 12 g/kg of body weight/day and held in direct sunlight exhibited signs of phototoxicity within 24-48 hours. The signs manifested were elevated body temperature, photophobia, edema of the muzzle, ears, and vulva, keratoconjunctivitis with edema of the cornea, and exudative dermatitis of the skin of the ears, muzzle, and vulva. Lesser dosages of the plant produced similar effects after several days, except that corneal edema and opacity did not develop. Hepatic lesions were not observed by histopathological studies, a finding consistent with primary photosensitization. The photosensitizing effects of Thamnosma texana can be attributed to the presence of phototoxic linear furocourmarins (psoralens) in the plant. Nine linear furocoumarins and three coumarins were isolated (thin plate chromatography, high pressure liquid chromatography) and identified (mass spectrometry, nuclear magnetic resonance, coinjection) from Thamnosma texana. The identified compounds were xanthotoxin, imperatorin, bergapten, alloimperatorin methyl ether oxide, heraclenin, isopimpinellin, psoralen, oxypeucedanin, byakangelicol, and the coumarins herniarin, osthol, and thamnosmin. These compounds appear to be the agents that account for the observed phototoxicity properties of Thamnosma texana, and consequently for one of its colloquial names, "blisterweed".