Phototoxic phytochemicals

The number and structural diversity of phototoxic phytochemicals has grown tremendously over the last ten years. Our knowledge and understanding of the biological activity of these natural products will undoubtedly continue to expand as new structures are elucidated and new plant families are examined. In addition to exploratory studies, it is important that we begin to turn our attention toward the significance or function of photosensitizers. Do plants which... 

We recently surveyed a cross-section of plants from many tropical regions of the world in a search for photosensitizers to further test the above hypothesis. The methods used to test for phototoxic phytochemicals are described in detail elsewhere (22). Briefly, methanolic extracts were spotted onto sterile filter-paper discs and allowed to dry. The dried discs were placed onto replicate nutrient agar plates that had been spread with Ex B/r (a UV resistant bacterium). The plates were incubated in the dark at 3TC for 30 min. Half of the plates were irradiated for 60 min. with eight Sylvania F40BLB UVA lamps (18W m ), while the other half were kept in the dark. All plates were incubated overnight in the dark at 37°C, after which the zones of inhibition surrounding the filter paper discs were measured. 

It has also been reciystallised from MeOH, C6H6, pet ether or AcOH. [UV Zechmeister Sease J Am Chem Soc 69 273 1947, Steinkopf et al. Justus Liebigs Ann Chem 546 180 1941] It is a phototoxic nomatoridp. [Cooper Nitsche Bioorg Chem 13 36 1985, Chan et al. Phytochem 14 2295 1975]. [Beilstein 19 m/IV 4763, 19/9 V 226.]... 

Acetylenes, beta-carboline alkaloids, furocoumarins and lignans occur widely among the families listed in Table 1. Acetophenones (benzofurans and benzopyrans) and extended quinones have a much more limited distribution while furochromones, furoquinoline alkaloids and thiophenes are apparently restricted to single families. It should be mentioned that although acetylenic polylnes are phytochemical components of plants from numerous families (4), only those derivatives produced by members of the Asteraceae have been shown to be phototoxic Q). 

Figure 1. Structures of representative phototoxic plant products from various phytochemical classes. (I) 1-phenylhepta-1,2,3-triyne (acetylenic polyine) (II) 6-methoxyeuparin (benzofuran) (III) harmane (beta-carboline alkaloid) (IV) khellin (furo-chromone) (V) 8-methoxypsoralen (furocoumarin) (VI) dictamnine (furoqulnoline alkaloid) (VII) nordihydroguaiaretic acid (lignan) and (VIII) alpha-terthienyl (thiophene).

Over 400 tropical/subtropical species representing 76 families were collected at either Fairchild Tropical Gardens or Chapman Field, USDA Plant Induction Station in Miami, FL. Table I lists the number of genera and species of the families that were examined. Only five of the 76 families tested elicited phototoxic responses from E.coli. These families are listed in boldface caps in Table I. The Asteraceae has been studied extensively in regards to its phototoxic components (38. as have members of the Rutaceae (citrus family S.7)and the Moraceae (fig family 4Q). The compound(s) responsible for phototoxicity in the Sapotaceae (sapodilla family) is not known and is the only instance of light-enhanced toxicity from this family that has been reported to date (41). Elucidation of the structure of this phytochemical is in progress and we are also examining other members of the Sapotaceae for phototoxicity. 

Arnason, J.T., B.J.R. Philogene, C. Berg, A. Maceachern, J. Kaminski, L.C. Leitch, P. Morand, and J. Lam Phototoxicity of Naturally Occurring and Synthetic Thiophene and Acetylene Analogues to Mosquito Larvae. Phytochem. 25, 1609 (1986). 

Jarvenpaa, E.P., Jestoi, M.N. and Huopalahti, R. (1997) Quantitative determination of phototoxic furocoumarins in celeriac (Apium graveolens L. var. rapeceum) using supercritical fluid extraction and high performance liquid chromatography. Phytochemical Anal, 8, 250-256. 

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