Coumarin-induced changes in delta-aminolaevulinic acid synthase and cytochrome P-450 in chick embryo liver

Co-mutagenicity of coumarin (1,2-benzopyrone) with aflatoxin B1 and human liver S9 in mammalian cells

Coumarin (1,2-benzopyrone), a natural dietary constituent and drug currently under evaluation for treatment of certain cancers and lymphedema, reduces polycyclic aromatic hydrocarbon-induced neoplasms in rodents. Because most rodents metabolize coumarin through 3,4-epoxidation, whereas 7-hydroxylation predominates in humans, their suitability as a model for coumarin effects in humans has been questioned. We examined coumarin chemoprotection against the promutagen and dietary contaminant aflatoxin B1 with human liver S9 bioactivation in the Chinese hamster ovary cell/hypoxanthine-guanine phosphoribosyltransferase mutation assay. Coumarin in the absence of aflatoxin B1 was not mutagenic or cytotoxic up to 500 microM. When included with either 1 or 10 microM aflatoxin B1, coumarin produced a dose-dependent increase in mutant frequency and cytotoxicity. At concentrations greater than 50 microM, coumarin stimulated human liver S9 bioactivation of aflatoxin B1 to the mutagenic 8,9-epoxide. This increase was 12- and fivefold at 500 microM coumarin with 1 and 10 microM aflatoxin B1, respectively, compared with incubations with aflatoxin B1 alone. These findings differ from previous results with liver S9 from other species, and indicate that coumarin co-mutagenicity with aflatoxin B1 and human liver S9 is through increased aflatoxin B1 bioactivation.

Coumarin chemoprotection against aflatoxin B1-induced gene mutation in a mammalian cell system: a species difference in mutagen activation and protection with chick embryo and rat liver S9

Coumarin (1,2-benzopyrone), a natural food constituent, prevents polycyclic aromatic hydrocarbon-induced neoplasms in rats and mice, but has not been studied with other chemical carcinogens. We examined coumarin chemoprotection against aflatoxin B1 using the 6-thioguanine resistance mutation assay in two different Chinese hamster ovary cell lines (K1BH4 and AS52) with liver S9 from rats and 19-day-old chick embryos for aflatoxin B1 bioactivation. Laboratory rodents metabolize coumarin through 3-hydroxylation, whereas 7-hydroxylation predominates in chick embryos and humans. Chick embryo liver S9 was approximately 25-fold more effective in activating aflatoxin B1 to the mutagenic and cytotoxic metabolite(s) than rat liver S9. Coumarin added at 50 and 500 microM with chick embryo liver S9 reduced the mutant frequency of 1 microM aflatoxin B1 by 40 and 85%, respectively. Coumarin up to 500 microM had no effect on aflatoxin B1 mutagenicity with rat liver S9. When liver S9 from chick embryos pretreated with coumarin was used for aflatoxin B1 bioactivation, mutant frequency and cytotoxicity were decreased compared to liver S9 from vehicle-treated controls. Liver S9 from coumarin-treated rats did not significantly affect mutant frequency or cytotoxicity. HPLC analysis of chick embryo liver S9 incubated with 1 microM aflatoxin B1 showed a dose-dependent decrease by coumarin of aflatoxin B1 activation to the 8,9-epoxide ranging from 70% of controls at 5 microM coumarin to 4% of controls at 500 microM coumarin. In contrast, coumarin produced a dose-dependent increase in 20 microM aflatoxin B1 activation by rat liver S9, reaching twice the control levels at 500 microM coumarin. These findings, using a mammalian cell system as a mutagenic endpoint, demonstrate marked species differences in chemoprotection by coumarin.

Effects of chloroquine in hematoporphyrin-treated animals

Porphyrins and related compounds are useful in photodynamic therapy but can cause cutaneous photosensitivity. We determined whether chloroquine, which is effective in treating porphyria cutanea tarda, would mobilize an administered porphyrin from tissues and enhance its excretion. Hematoporphyrin with and without chloroquine was administered to chick embryos, mice, and rats. Tissue and plasma porphyrin levels were markedly increased after hematoporphyrin dosing. Porphyrin concentrations in liver, spleen, and kidney were not significantly affected by chloroquine. Total urinary and fecal porphyrin excretion in rats treated with hematoporphyrin (50 mg/kg, i.p.) was not influenced by chloroquine treatment (100 mg/kg, s.c.). Excretion of heptacarboxylporphyrin, normally a minor fraction of urinary porphyrins, was significantly increased in chloroquine-treated rats. These results suggest that chloroquine is unlikely to be useful after photodynamic therapy for mobilizing exogenous porphyrins from tissues such as liver, spleen, and kidney. Increased urinary excretion of heptacarboxylporphyrin may contribute to the beneficial effect of chloroquine in porphyria cutanea tarda.

Species differences in the metabolism and hepatotoxicity of coumarin

1. Investigations of coumarin metabolism and hepatotoxicity have been reviewed. 2. Species differences in coumarin hepatotoxicity appear to be metabolism-mediated. 3. The rat, in which it is markedly hepatotoxic, primarily metabolises coumarin via 3-hydroxylation and cleavage of the heterocyclic ring. 4. Coumarin is less toxic in the baboon, gerbil and certain strains of mice, which resemble man in their extensive formation of the 7-hydroxy metabolite. 5. Liver toxicity in patients receiving relatively high daily doses of coumarin is very rare. 6. Recent studies indicate that coumarin 3,4-epoxide is the metabolic intermediate responsible for hepatotoxicity in the rat.

Biochemical characterization of coumarin 7-hydroxylase activity in chick embryo liver microsomes

Coumarin occurs naturally in the diet and can induce and inhibit cytochrome P450 enzymes. Hepatic coumarin 7-hydroxylase activity is the major pathway for coumarin metabolism in humans but not in rats, most strains of mice, or other laboratory animals. Coumarin 7-hydroxylase activity and the effects of chemical inhibitors and inducers on this activity were studied in 19-day-old chick embryo liver microsomes. Activity was between 35 and 75 nmol/mg protein/hr which is approximately 2-fold higher than reported for human liver microsomes. The pH optimum was 7.8 and the Km determined by both an ether extraction and a high performance liquid chromatography method was 7.3 +/- 0.9 (+/- SD) microM. Substrate inhibition was evident at coumarin concentrations above 250 microM (activities at 1000 and 4000 microM coumarin were 84 and 40% of Vmax, respectively). The Ki values (+/- SD) for inhibitors of microsomal coumarin 7-hydroxylase activity in vitro were: alpha-naphthoflavone, 46.9 +/- 19.8 nM; metyrapone, 0.8 +/- 0.9 microM; aniline, 12.3 +/- 8.2 microM; cimetidine, 70.9 +/- 27.9 microM; N-nitrosodimethylamine, 0.7 +/- 0.9 mM; and dimethyl sulfoxide, 7.9 +/- 1.9 mM. Treatment of chick embryos with pyrazole (40 mumol) increased coumarin 7-hydroxylase by 50% at 24 hr, but this activity was unaffected by treatment of embryos with 3-methylcholanthrene (2 mumol) or glutethimide (8 mumol). Thus, hepatic coumarin 7-hydroxylase activity in 19-day-old chick embryos is higher than in most laboratory animals and has similar biochemical properties as the enzyme in humans and mice. The chick embryo liver may be a useful system for studies on the biochemical effects of coumarin and the regulation of cytochrome P450-dependent coumarin 7-hydroxylase.