Pyrrolizidine alkaloids: Chemical basis of toxicity

Abstract

In humans, livestock and experimental animals, pyrrolizidine alkaloids (PAs) are toxic as a consequence of their hepatic metabolism to reactive pyrrolic esters, or dehydroalkaloids (DHAs). Despite their similarity in structures, PAs often vary markedly in their lethality (LD₅₀s) and in the organs in which toxicity is expressed. We have examined whether there are differences in the physicochemical properties of certain DHAs which are associated with differences in patterns of metabolism and toxicity produced by the parent PA. Using a potentiometric method to measure hydrolysis, it was determined that the half-lives of the corresponding DHAs of retrorsine, seneciphylline, monocrotaline and trichodesmine were 1.06, 1.60, 3.39 and 5.36 sec, respectively. These values were supported by similar results from experiments measuring reactivity of DHAs toward 4-(p-nitrobenzyl)pyridine. Studies from the isolated rat liver perfused with PAs show that DHA stability is related to patterns of metabolism and toxicity. Perfusion of the primarily hepatotoxic retrorsine and seneciphylline is associated with a greater proportion of metabolite released as non-toxic 7-glutathionyl-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine (7-GSDHP), a greater proportion alkylating liver macromolecules, and a lower proportion released as DHA into the circulation. Perfusion with monocrotaline and trichodesmine, PAs producing extrahepatic toxicity, produced lower proportions of 7-GSDHP release and liver alkylation, and higher proportions of DHA released into the circulation. Other studies characterizing DHAs included the use of an in vitro enzyme assay in which DHAs were shown to inhibit the phosphotransferase activity of yeast and rat brain hexokinase. Parent PAs, and the hydrolysis product of DHAs, (±)-6,7dihydro-1-hydroxymethyl-5H-pyrrolizine (DHP) did not affect enzyme activity. In vivo studies in rats have established that glutathione and cysteine-conjugated pyrrolic metabolites of PAs likely represent detoxication pathways, providing further support for DHAs as the primary toxic metabolite. We have examined the chemical form of sulfur-bound pyrroles to establish the importance of the 7-ester position in PA toxicity. Additionally, we have developed an efficient technique for the rapid separation and purification of large quantities of PAs using high-speed counter-current chromatography.