Differential metabolism of the pyrrolizidine alkaloid, senecionine, in fischer 344 and sprague-dawley Rats

The attenuation effect of licorice on the hepatotoxicity of Euodiae Fructus by inhibiting the formation of protein conjugates and GSH depletion

ETHNOPHARMACOLOGICAL RELEVANCE

As a traditional Chinese medicine and food, Euodiae Fructus (EF) is widely used in clinics to relieve pain and prevent vomiting and for making tea for more than a thousand years. In recent years, hepatotoxic reactions to EF have been reported. The intermediates produced by evodiamine and rutaecarpine metabolism in vitro were captured by glutathione (GSH), suggesting that the toxicity of EF may be related to metabolic activation. Whether licorice can inhibit the metabolic activation of EF has not been reported, which needed an effective strategy to clarify the correlation between protein conjugates and hepatotoxicity and the attenuation mechanism of licorice processing.

AIM OF THE STUDY

This study aimed to explore the toxic components and mechanisms of EF based on metabolic activation and the detoxification of licorice.

MATERIALS AND METHODS

The content and toxicity index of protein conjugates in the liver were determined by orally administering mice and rats with EF. The attenuation mechanism of licorice was examined in cell and enzymology experiments.

RESULTS

The change in evodiamine-cysteinylglycine (EVO-Cys-Gly) and evodiamine-cysteine (EVO-Cys) levels was consistent with the change in hepatotoxicity. Licorice inhibited the formation of the protein conjugates of EF and increased the content of GSH in L02 cells.

CONCLUSION

EF mediated by P450 enzymes produced toxic intermediates, which combined with cysteine residues in animal liver and inactivate them, leading to hepatotoxicity. Interestingly, licorice can alleviate the GSH depletion caused by EF and inhibit the production of protein conjugates by inhibiting P450 enzymes.

Quantification of Usaramine and Its N-oxide Metabolite in Rat Plasma Using Liquid Chromatography-Tandem Mass Spectrometry

A sensitive, fast and robust liquid chromatography--tandem mass spectrometry (LC–MS-MS) method was developed and validated for the determination of usaramine (URM) and usaramine N-oxide (UNO) in rat plasma. The separation was conducted on an ACQUITY UPLC BEH C₁₈ Column (50 × 2.1 mm, 1.7 μm) and gradient eluted with mobile phase A (0.1% formic acid with 5 mM ammonium acetate in water) and B (0.1% formic acid in acetonitrile/methanol, 9/1, v/v). The method was linear over the range of 1–2,000 ng/mL for both analytes. The validated method was applied to investigate the pharmacokinetic behaviors and sex differences of URM and its N-oxide metabolite in rats. After intravenous administration of URM at 1 mg/kg, the AUC_0-t values for URM and UNO were 363 ± 65 and 172 ± 32 ng/mL*h in male rats, while 744 ± 122 and 30.7 ± 7.4 ng/mL*h in females, respectively. The clearance of URM was significantly higher in male rats than in females (2.77 ± 0.50 vs 1.35 ± 0.19 L/h/kg, P < 0.05). After oral administration of URM at 10 mg/kg, the AUC_0-t values of URM and UNO were 1,960 ± 208 and 1,637 ± 246 ng/mL*h in male rats, while 6,073 ± 488 and 300 ± 62 ng/mL*h in females, respectively. The oral bioavailability of URM in female rats (81.7%) was much higher than in males (54.0%). In conclusion, sex-based differences were observed in the pharmacokinetics, N-oxide metabolism and oral bioavailability of URM.

Structure-dependent hepato-cytotoxic potencies of selected pyrrolizidine alkaloids in primary rat hepatocyte culture

Contamination of food, feed and herbal medicines with plants containing pyrrolizidine alkaloids (PA) leads to measurable amounts of PA in many products. Since a number of PA are hepatotoxic in humans and animals and hepato-carcinogenic in animal experiments, the assessment of the relative toxic potencies of widely occurring PA contaminants warrants detailed investigation. Here, we studied the hepato-cytotoxic potencies of a number of relevant PA congeners in rat hepatocytes in primary culture. It was found that cyclic and open di-esters were much more toxic than mono-esters. Furthermore, the hepatocellular levels of cytochrome P450-catalyzed 7-benzoxyresorufin O-dealkylase (BROD) activity decreasing over time in culture, played an important role for activation of PA into cytotoxic metabolites. With a highly toxic PA (lasiocarpine), inhibition of BROD activity with ketoconazole markedly reduced toxicity while this was not obvious with the less toxic congener lycopsamine. Depletion of cellular glutathione with buthionine sulfoximine had no significant influence on the effects of highly toxic PA whereas it slightly increased toxicity of less potent congeners. Overall, our data partially confirm previously published structure-dependent interim Relative Potency (iREP) factors although for echimidine and monocrotaline in particular, substantial deviations were found, possibly due to specific toxicokinetic properties of these congeners.

In vitro biotransformation of pyrrolizidine alkaloids in different species. Part I: Microsomal degradation

Pyrrolizidine alkaloids (PA) are secondary metabolites of certain flowering plants. The ingestion of PAs may result in acute and chronic effects in man and livestock with hepatotoxicity, mutagenicity, and carcinogenicity being identified as predominant effects. Several hundred PAs sharing the diol pyrrolizidine as a core structure are formed by plants. Although many congeners may cause adverse effects, differences in the toxic potency have been detected in animal tests. It is generally accepted that PAs themselves are biologically and toxicologically inactive and require metabolic activation. Consequently, a strong relationship between activating metabolism and toxicity can be expected. Concerning PA susceptibility, marked differences between species were reported with a comparatively high susceptibility in horses, while goat and sheep seem to be almost resistant. Therefore, we investigated the in vitro degradation rate of four frequently occurring PAs by liver enzymes present in S9 fractions from human, pig, cow, horse, rat, rabbit, goat, and sheep liver. Unexpectedly, almost no metabolic degradation of any PA was observed for susceptible species such as human, pig, horse, or cow. If the formation of toxic metabolites represents a crucial bioactivation step, the found inverse conversion rates of PAs compared to the known susceptibility require further investigation.