Sterigmatocystin-DNA interactions: identification of a major adduct formed after metabolic activation in vitro.

Abstract

Sterigmatocystin (ST), a potent hepatocarcinogen, was covalently bound to calf thymus DNA by incubation in the presence of phenobarbital-induced rat liver microsomes. Acid hydrolysis of ST-modified DNA liberated a major guanine-containing adduct, present in DNA at an estimated level of 1 ST residue per 100-150 nucleotides. The adduct was isolated by high-pressure liquid chromatography and subjected to structural analysis. Spectral and chemical data identified the adduct as 1dihydro-2(NI-guanyl)--hydroxysterigmatocystin, the guanine and hydroxyl moieties being in a trans configuration. The structure and stereochemistry of this adduct indicated that the exo-ST-1,2-oxide was the metabolite that reacted with DNA, and the quantitative yield of adduct indicated that this metabolite was a major product of the in vitro metabolism of ST.Sterigmatocystin 'ST) is a carcinogenic mycotoxin produced as a secondary metabolite by Aspergillus, Penicillium, and Bipolaris species (1, 2). ST is acutely toxic to the liver of most animals tested (3, 4), and its carcinogenicity has been demonstrated with organ specificity varying with species and route and frequency of administration (5-9). In rats, ST induces hepatocellular carcinomas after oral administration (6) or intraperitoneal injection (5) and squamous cell carcinomas after repeated application to the skin (7). Despite its potent toxic and carcinogenic properties in animals, the importance of ST as a human health hazard is unknown because surveillance programs have detected its presence in foods only infrequently and at low concentrations even though ST-producing fungi are widely distributed (10).Nonetheless, ST is of interest as a model compound for cancer induction because of its structural similarity to aflatoxin B1 (AFB1). In rats and monkeys, the lethal potency of ST is about 1/10th that of AFB1 (3), and ST is between 1 and 2 orders of magnitude less potent as a hepatocarcinogen for the rat (5, 6). A comparable quantitative difference exists in the toxicities and mutagenicities of ST and AFB1 in Salmonella typhimurium (11,12).In contrast to the large literature on AFB1, little information has been published on the metabolism and biochemical effects of ST. It has been shown that a large portion of the dose administered to monkeys is converted to a ST-glucuronide (13) and that metabolic activation is required for the toxicity and mutagenicity of ST in bacteria and some cultured cells (11,12,(14)(15)(16). Mammalian cells in culture exposed to ST display nucleolar aberrations, inhibited mitosis, inhibited uptake of thymidine and uridine, and stimulated DNA repair synthesis (17)(18)(19)(20). ST also has been demonstrated to inhibit RNA synthesis in rat liver (21) (N7-guanyl)-l-hydroxysterigmatocystin (ST-N7-Gua). § MATERIALS AND METHODSST was metabolically activated in the presence of calf thymus DNA by using phenobarbital-induced rat liver microsomes. The incubation mixture (400 ml; divided into eight 50-ml portions) included approximately 1 mg of microsomal protein...

Scientific information on mycotoxins and natural plant toxicants

“…A paper by Essigmann et al (1979) indicates that the stereochemistry of the STC epoxide is identical to that of AFB 1 and that a major target for both epoxides is the N7 atom of guanine, which is located in the major groove of DNA in a position apparently readily accessible to attack by bulky chemicals. Moreover, it is conceivable that the toxicological similarities and differences between STC and AFB 1 may be attributed to their mechanisms of epoxidation and interaction with DNA.…”

“…It has been shown (Beasley, 1999;Essigmann et al, 1979) that a large portion of the dose administered orally to monkeys is converted to a STC-glucoronide and that metabolic activation is required for the toxicity and mutagenicity of STC in bacteria and some cultured cells.…”

“…The present document has been produced and adopted by the bodies identified above as author(s (Essigmann et al, 1979).…”

“…Like AFB 1 , STC has been found to be metabolized by the cytochrome P4503A4 (Yamazaki et al, 1995) system to an active epoxide, which can then interact with nucleic acids to form a DNA-adduct at the N 7 -− guanyl position in the target cells (Essigmann et al, 1979(Essigmann et al, , 1980(Essigmann et al, , 1982Terao, 1983).…”

Scientific Opinion on the risk for public and animal health related to the presence of sterigmatocystin in food and feed

“…The first observation of covalent binding of STC to DNA was made following incubation of calf thymus DNA with STC in the presence of phenobarbital-induced rat liver microsomes (Essigmann et al, 1979). Spectral and chemical data identified the adduct as 1,2-dihydro-2-(N 7 -guanyl)-1-hydroxysterigmatocystin, the guanine and hydroxyl moieties being in a trans configuration.…”