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

Abstract

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.Electronic supplementary materialThe online version of this article (10.1007/s00204-017-2114-7) contains supplementary material, which is available to authorized users.

In vitro biotransformation of pyrrolizidine alkaloids in different species: part II—identification and quantitative assessment of the metabolite profile of six structurally different pyrrolizidine alkaloids

“…In the previous studies, we demonstrated that liver microsomes from species considered to be sensitive to PAs showed a lower metabolic rate than liver microsomes from species considered to be more resistant (Kolrep et al 2018 ). This observation could be explained by the fact that the observed high overall rate of microsomal biotransformation in non-susceptible species mainly represents a detoxification.…”

“…Although much research has focused on the identification of PA metabolites (Buhler and Kedzierski 1986 ; Fashe et al 2014 , 2015 ; Fu et al 2004a ), their quantification revealed that a large portion is still unknown. Furthermore, GSH-DHP conjugates like 7,9-diglutathionyl-6,7-dihydro-1-hydroymethyl-5H-pyrrolizidine (diGSH-DHP), and 9-glutathionyl-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizidine (monoGSH-DHP) suggested as biomarkers for in vitro metabolic activation were not detectable when CYP content is low—for instance when using S9 as metabolic system instead of microsomes, or were only detectable after long-term incubation (Ebmeyer et al 2019 ; Geburek et al 2020 ; Kolrep et al 2018 ).…”

“…For an unambiguous confirmation of screened candidates as PA metabolites, highly resolved product ion scans were acquired and the fragments were compared with those of the reference substances and literature data (Fashe et al 2014 , 2015 ; Ma et al 2015 ). We expanded the recently published method for the detection and quantification of the metabolic transformation and GSH conjugate formation of PAs in microsomes from different species (Geburek et al 2020 ; Kolrep et al 2018 ) to elucidate the metabolite profile of six PAs (Fig. 1 ).…”

Pyrrolizidine alkaloids in commercial feedstuffs for horses

“…Lasiocarpine has the shortest in vitro half‐life (10 min) in most species. In horses, intermedine and senkirkine have the longest half‐lives (>1400 min) . Intermedine and senkirkine were found in the feed products tested in the current study, but only at low levels.…”

“…In vivo acute toxicity studies show significant differences in the toxicity of individual pyrrolizidine alkaloids in rats and mice . Similarly, significant differences in pyrrolizidine susceptibility between species have been identified under in vitro conditions . In vitro transformation of intermedine, lasiocarpine, senecionine and senkirkine by liver enzymes have different half‐lives in humans and animal species such as pigs, horses, cows, goats, sheep, rabbits and rats.…”