Metabolic Activation of Pyrrolizidine Alkaloids – Generation of Multiple Reactive Metabolites Leading to DNA Adduct Formation and Potentially Liver Tumor Initiation

Abstract

Pyrrolizidine alkaloids (PAs) are toxic and carcinogenic phytochemicals widespread in the world. Metabolic formation of dehydro‐PAs and 6,7‐dihydro‐7‐hydroxy‐1‐hydroxymethyl‐5H‐pyrrolizine (DHP) has long been considered the principle metabolic activation pathway. We determined that a set of DHP‐DNA adducts are generally formed from the metabolism of a series of carcinogenic PAs in vitro, in cells and in vivo. The level of DHP‐DNA adducts well correlated with liver tumor potency for a set of PAs. We have proposed that this set of DHP‐DNA adducts is a potential biomarker of PA exposure and PA‐induced liver tumor initiation. Our present studies have found that, in addition to dehydro‐PAs and DHP, there are another seven reactive pyrrolic metabolites that are capable of binding to cellular DNA to produce the same set of DHP‐DNA adducts. These DNA reactive metabolites are: 7‐glutathione‐DHP, 7‐cysteine‐DHP, 7‐N‐acetylcysteine‐ DHP, 1‐CHO‐DHP, cysteinyl‐[2′‐S‐7]‐1‐CHO‐DHP, cysteinyl‐[3′‐N‐7]‐1‐CHO‐DHP, and 1‐cysteinylimino‐DHP. These pyrrolic metabolites are enzymatically and/or chemically formed from the reaction of the corresponding amino acids (and glutathione) with either dehydro‐PAs or DHP. Also, these pyrrolic metabolites are interconvertible, apparently ascribed to their unique reactive bifunctional alkylating character. These overall results represent the first time that concerning the metabolic activation of PAs: (i) there involve the formation of multiple DNA reactive pyrrolic metabolites; and (ii) metabolism of carcinogenic and potential carcinogenic PAs all generate the identical DHP‐DNA adducts. [This abstract is not an official U.S. Food and Drug Administration (FDA) guidance or policy statement. No official support or endorsement by the U.S. FDA is intended or should be inferred. The authors declare no competing financial interest.]