An ultrasensitive bioassay for the detection of furocoumarins and other photosensitizing molecules

Detection of furocoumarins in plants and plant products with an ultrasensitive biological photoassay employing a DNA-repair-deficient bacterium

The application of an ultrasensitive photobiological assay which detects photosensitizing furocoumarins with sensitivities as high as 1 × 10(-11) g is discussed in relation to these molecules as phytoalexins. Examples of the utilization of this technique, verified by both HPLC and TLC, are the analyses of healthy and diseased celery and carrots, dry seeds, plant extracts and oils, and whole plants and leaves. The usefulness of this method in following the metabolic detoxification of furocoumarins is also illustrated. The extreme sensitivity of the test has permitted the detection, for the first time, of both 5-methoxypsoralen and 8-methoxypsoralen in fresh carrot roots.

Existing and potential applications of ultraviolet light in the food industry - a critical review

Short-wave ultraviolet light (UVC, 254 nm) can reduce dramatically the microbial load in air or on hard surfaces free from food residues, and can eliminate pathogens from potable water filtered to remove organic residues and 'clumps' of bacteria. More recently, approval of the Food and Drug Administration (USA) has been sought for a system for the destruction of pathogenic bacteria in fruit juices using UVC, and the same approach could perhaps be applied to remove spoilage organisms from cider or wines. In contrast, long-wave UV light (UVA, >320 nm) has limited microbiocidal properties, and for practical applications its effectiveness has to be enhanced by the presence of photosensitive compounds (eg furocoumarins) that will diffuse into a microbial cell prior to irradiation. The penetration of UVA into water is better than that of UVC, and its bacteriocidal action in the presence of photosensitisers can be rapid. However, pure furocoumarins are expensive and their addition to foodstuffs might be questioned on safety grounds. © 2000 Society of Chemical Industry.

Isopimpinellin is not phototoxic in a chick skin assay

Synthetic isopimpinellin (5,8-dimethoxypsoralen), confirmed to contain as impurities only trace quantities at most of psoralen, bergapten (5-methoxypsoralen) and xanthotoxin (8-methoxypsoralen), is not phototoxic when tested in a chick skin bioassay system. These findings are at variance with earlier studies showing isopimpinellin to be phototoxic against chick skin and support the conclusion that isopimpinellin is photobiologically inactive. As recently proposed by others, the several reports of isopimpinellin photoactivity are most likely attributable to contamination by small amounts of highly active psoralens such as bergapten or xanthotoxin.

Phototoxicity of plant secondary metabolites: insect and mammalian perspectives

Phototoxic chemicals produced by plants have been studied in a number of contexts, most notably as protective agents against mammalian and insect herbivores. Although there are commonalities in the responses of these two groups of herbivores to plant phototoxins, there are differences as well. Whereas a greater range of chemical classes has been demonstrated to display phototoxicity against insects, considerably more information is available on symptomology of phototoxicity and mechanisms of action in mammals. The commonalities include alterations in behavior following ingestion, notably photophobia, disruption of integumentary integrity following contact or ingestion, and metabolic detoxification following ingestion, in the case of furanocoumarins involving cytochrome P450 monooxygenases. Not yet known to exist in insects are phototoxin-mediated effects on sensory (particularly visual) systems and phototoxicity resulting from abnormal chlorophyll metabolism. In order to gain greater understanding of the ecological significance of phototoxin-mediated plant defense against both insects and mammals, there is a need for more studies centered on natural associations.

Phototoxic coumarins in limes

Coumarins in the rind and pulp of Persian and Key limes were quantified. In the rind of Persian limes, coumarin concentrations were in the order: limettin > bergapten > isopimpinellin > xanthotoxin > psoralen. In the rind of Key limes, psoralen and xanthotoxin were analytically absent; limettin was 10 times more concentrated than either bergapten or isopimpinellin, which were equal in concentration. Coumarin content in Persian lime pulp was in the order: isopimpinellin > limettin > bergapten > xanthotoxin > psoralen. For Key lime pulp, the concentrations of limettin, isopimpinellin and bergapten were equal; psoralen and xanthotoxin were not detected. Coumarins in lime pulp were 13 to 182 times less concentrated than those in the peel. Based on the amounts and types of coumarins, Persian limes appear to be potentially more phototoxic than Key limes. Although bergapten may be the main component of limes responsible for phytophotodermatitis, dermatological interaction assays with psoralen, bergapten, xanthotoxin and limettin should be conducted.

UV-A-mediated activity of p-methoxymethylcinnamate

Methyl esters of hydroxycinnamic acids are photobiologically active. Cis(Z) and trans(E) p-methoxymethyl-cinnamate photosensitize Escherichia coli and Chinese hamster ovary cells. They also produce sister chromatid exchanges. Photosensitization is oxygen independent, and the cinnamates are not genetically active in the absence of light in the Ames Salmonella typhimurium test.

Light-activated plant defence

Sunlight provides the energy required for all biochemical, physiological and developmental processes necessary for plant growth, reproduction and survival. The role of light in photosynthesis and photomorphogenesis has been appreciated for some time; however, the role of light in plant defence is a comparatively recent realization. Between 75-100 photosensitizers or phototoxins, molecules that become toxic in the presence of light, have been extracted from higher plant tissues. These biologically-active compounds have diverse biosynthetic origins and belong to at least 15 different phytochemical classes (i.e. acetophenones, acetylenes, benzophenanthrenes, β-carbolines, coumarins, extended quinones, furanochromones, furanocoumarins, furanoquinolines, isoquinolines, lignans, pterocarpans, quinolines, sesquiterpenes and thiophenes). Of more than 100 angiosperm families assayed, phototoxins and/or phototoxic activity have been reported in c. 40 families representing 32 orders and 8 subclasses of the Magnoliophyta. Most of these allelochemicals are acute toxins with little organism-specificity. As such, they are effective biocides capable of killing a wide-range of potentially harmful organisms including: viruses, pathogenic bacteria and fungi, nematodes and herbivorous insects, as well as competing plant species. This review focuses on the phytochemistry, taxonomic occurrence and toxicological consequences of phototoxic metabolites in flowering plants. The available information suggests that phototoxic phytochemicals: (i) are broad-spectrum allelochemicals capable of protecting plants against a variety of detrimental organisms in nature, (ii) represent a successful defensive strategy in both primitive and advanced plant taxa, and (iii) may be an effective defence under a variety of photoenvironmental conditions. CONTENTS Summary 401 I. Introduction 401 II. Chemistry/biochemistry of plant photosensitizers 402 III. Botanical distribution of photosensitizers 403 IV. Toxicology 405 V. Biological activity/defensive role of photosensitizers 408 VI. Concluding remarks 413 References 415.

The photobiological activity of 5-geranoxypsoralen and its photoproducts

5-geranoxypsoralen (Bergamottin) does not photosensitize bacteria or a bacterial virus. It does, however, photosensitize mammalian cells in tissue culture. Irradiation with either black light (300-400 nm) or fluorescent ceiling lights produced at least four photobiologically active degradation products, the chemical nature of which still remains to be elucidated. Prolonged exposure to black light resulted in the formation of inactive molecule(s).

Phototoxicity testing in guinea-pigs

The photoirritant potential of topically applied chemicals was studied using guinea-pigs. Solutions of test chemicals were applied to the skin, and after 30 min the animals were irradiated with near-ultraviolet radiation. Skin reactions were assessed subjectively between 3 and 72 hr after the start of treatment. Acridine and anthracene caused immediate photoirritation, whereas reaction to 8-methoxy-psoralen (8-MOP) was delayed; acridine was weakly active compared with the strong photoirritancy of anthracene and 8-MOP. Ethanol and a mixture of dimethylacetamide-acetone-ethanol (DAE) were satisfactory solvents, and a time interval of 15 to 30 min between application and irradiation was optimal. It is concluded that animal tests should not be recommended if ingredients have negligible absorption of sunlight. The safety hazard of ingredients absorbing near-ultraviolet and visible radiation may be assessed by laboratory animal procedures to satisfy governmental regulations. The use of guinea-pigs allows the study to act as a preliminary test for the selection of non-photoirritant concentrations for photoallergy testing and reduction of animal numbers. The risk to consumers of photoactive products may be properly assessed by human studies.

Photobiological properties of a novel, naturally occurring furoisocoumarin, coriandrin

The photobiological properties of a novel, naturally occurring furoisocoumarin isolated from coriander and named coriandrin are described. Photosensitized lethal and mutagenic effects in bacteria indicate that it is more active than psoralen. It is a weak frameshift mutagen in the dark. Mammalian cells in tissue culture are photosensitized more actively with coriandrin than with psoralen even though preliminary evidence from interrupted radiation experiments and DNA analysis suggest that coriandrin does not form DNA interstrand crosslinks. Sister chromatid exchanges were induced with a unit dose of 1.1 x 10(-2) with coriandrin; the value for psoralen is 3 x 10(-3). Coriandrin appears to be metabolized more rapidly than furocoumarins by liver mixed function oxidases. Skin photosensitizing activity is very weak compared with psoralen, a surprising observation considering its potency in biological test systems.

Role of cloned carotenoid genes expressed in Escherichia coli in protecting against inactivation by near-UV light and specific phototoxic molecules

Genes controlling carotenoid synthesis were cloned from Erwinia herbicola and expressed in an Escherichia coli strain. Carotenoids protect against high fluences of near-UV (NUV; 320 to 400 nm) but not against far-UV (200-300 nm). Protection of E. coli cells was not observed following treatment with either psoralen or 8-methoxypsoralen plus NUV. However, significant protection of cells producing carotenoids was observed with three photosensitizing molecules activated by NUV (alpha-terthienyl, harmine, and phenylheptatriyne) which are thought to have the membrane as an important lethal target. Protection of carotenoid-producing cells against inactivation was not observed with acridine orange plus visible light but was seen with toluidine blue O plus visible light.

Alternariol, a dibenzopyrone mycotoxin of Alternaria spp., is a new photosensitizing and DNA cross-linking agent

The mycotoxin alternariol (3,4',5-trihydroxy-6'-methyldibenzo [a] pyrone) but not alternariol monomethyl ether (3,4'-dihydroxy-5-methoxy-6'-methyldibenzo [a] pyrone) is phototoxic to Escherichia coli in the presence of near UV light (320-400 nm). The phototoxicity bioassays with a DNA repair-deficient mutant of E. coli suggested that DNA may be the molecular target for photo-induced toxicity of alternariol. Interactions between alternariol and double-stranded, supercoiled DNA suggest that alternariol interacts with DNA by intercalation. No DNA breakage was detected in this system; however, alternariol forms a complex and cross-links double-stranded DNA in near UV light. These results suggest that alternariol is a new phototoxic, DNA-intercalating agent and is a DNA cross-linking mycotoxin in near UV light.

Identification of psoralen, 8-methoxypsoralen, isopimpinellin, and 5,7-dimethoxycoumarin in Pelea anisata H. Mann

5,7-dimethoxycoumarin and isopimpinellin, together with the well-known phototoxic, photo-irritant furanocoumarins psoralen and 8-methoxypsoralen, were isolated and identified from leaves and fruits of Pelea anisata H. Mann, a plant whose fruit are used in the construction of mohikana leis used in parts of the Hawaiian Islands. The presence of phototoxic furanocoumarins would explain the occurrence of photodermatitis in the wearers of the leis.