Pyrrolizidine alkaloids are naturally occurring genotoxic chemicals produced by a large number of plants. The high toxicity of many
pyrrolizidine alkaloids has caused considerable loss of free-ranging livestock due to liver and pulmonary lesions. Chronic exposure of toxic
pyrrolizidine alkaloids to laboratory animals induces
cancer. This investigation studies the metabolic activation of
retrorsine, a representative naturally occurring tumorigenic
pyrrolizidine alkaloid, and shows that a genotoxic mechanism is correlated to the tumorigenicity of
retrorsine. Metabolism of
retrorsine by liver microsomes of F344 female rats produced two metabolites, 6, 7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP), at a rate of 4.8 +/- 0.1 nmol/mg/min, and
retrorsine-N-oxide, at a rate of 17.6 +/- 0.5 nmol/mg/min. Metabolism was enhanced 1.7-fold by using liver microsomes prepared from
dexamethasone-treated rats. DHP formation was inhibited 77% and
retrorsine N-oxide formation was inhibited 29% by
troleandomycin, a P450 3A
enzyme inhibitor. Metabolism of
retrorsine with lung, kidney, and spleen microsomes from
dexamethasone-treated rats also generated DHP and the N-
oxide derivative. When rat liver microsomal metabolism of
retrorsine occurred in the presence of
calf thymus DNA, a set of DHP-derived
DNA adducts was formed; these adducts were detected and quantified by using a previously developed 32P-postlabeling/HPLC method. These same
DNA adducts were also found in liver
DNA of rats gavaged with
retrorsine. Since DHP-derived
DNA adducts are suggested to be potential
biomarkers of
riddelliine-induced tumorigenicity, our results indicate that (i) similar to the metabolic activation of
riddelliine, the mechanism of
retrorsine-induced carcinogenicity in rats is also through a genotoxic mechanism involving DHP; and (ii) the set of DHP-derived
DNA adducts found in liver
DNA of rats gavaged with
retrorsine or
riddelliine can serve as
biomarkers for the tumorigenicity induced by
retronecine-type
pyrrolizidine alkaloids.