Application of the concept of relative photomutagenic potencies to selected furocoumarins in V79 cells

Furocoumarins are phototoxic and photomutagenic natural plant constituents found in many medicinal plants and food items. Since plants contain mixtures of several furocoumarins, there is a need for a comparative risk assessment of a large number of furocoumarins. Previously, we have introduced the concept of relative Photomutagenicity Equivalency Factors (PMEFs) derived from the slope of the concentration–response curve of photomutagenicity of individual furocoumarins in V79 cells using the HPRT mutation assay in the presence of UVA irradiation at 125 mJ/cm2. Here we have applied this method to the furocoumarins bergamottin, isopimpinellin and psoralen using 5-methoxypsoralen (5-MOP) as a reference compound with a PMEF of 1.0. We found that neither bergamottin nor isopimpinellin, two furocoumarins abundant in plants, food etc., exerted any significant photomutagenicity while psoralen was clearly photomutagenic with a PMEF of 0.36. Similarly, isopimpinellin was not phototoxic in V79 cells, while bergamottin showed some cytotoxicity which, however, was completely independent of UVA irradiation. Only psoralen was photocytotoxic showing a similar concentration–response relationship for photomutagenicity, and for photocytotoxicity (at 72 h after irradiation). Data from the micronucleus assay for DNA damage at 20 h after irradiation were in complete agreement with the HPRT mutation data. Our findings indicate that individual furocoumarins differ enormously in their photomutagenic potency, and that a specific toxicological risk assessment is required for each furocoumarin instead of an assessment based on the sum of furocoumarins in a given sample.