Metabolism of furans in vitro: ipomeanine and 4-ipomeanol

Ipomeanine (IPN), 4-ipomeanol (4-IPO), 1-ipomeanol (1-IPO), and 1,4-ipomeadiol (DIOL) are toxic 3-substituted furans found in mold-damaged sweet potatoes. IPN and 4-IPO are the most toxic, but all produce pulmonary toxicity in cattle and rodents, and 4-IPO induces hepatotoxicity in humans. These furans require metabolic activation to elicit toxicity, but the limited information obtained from previous metabolism studies prompted us to initiate the investigation reported here. Our initial studies of 4-IPO metabolism by rat liver microsomes demonstrated that the oxidation of 4-IPO to IPN and reduction to DIOL occurred and that more IPN was metabolized to a reactive species than 4-IPO or DIOL. Incubation of IPN and Gly produced a 2'-pyrrolin-5'-one adduct establishing that IPN was metabolized to an enedial. N-Acetylcysteine reacted with the 5'-aldehyde of the enedial to give two 2',5'-dihydro-2'-hydroxyfurans stabilized by H bonding between the 2'-OH and 3'-keto group. Reaction of the enedial metabolite of IPN with one GSH gave several adducts including a pyrrole derived from the 1,2-addition of GSH to the 5'-aldehyde as well as two tricyclic 2'-pyrrolines derived from the 1,4-addition of GSH at the 4'-position. The identities of the pyrrole and 2'-pyrroline GSH adducts were confirmed by observation of structurally similar adducts from Cys conjugation with the enedial metabolite of IPN. Several minor adducts from the conjugation of the enedial metabolite of IPN with two GSH were also detected. Mono-GSH and bis-GSH adducts were derived from both the 1,2-and 1,4-addition of GSH to the enedial metabolite of 4-IPO in rat liver microsomal incubations of 4-IPO and GSH. Sequential oxidation of 4-IPO to IPN and then to the enedial metabolite followed by GSH conjugation also occurred in the 4-IPO incubations. The complex structures of the reaction products of the enedial with biological nucleophiles may explain why the many attempts to identify 4-IPO adducts to protein have not been successful.