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Mechanisms of anti-carcinogenesis by indole-3-carbinol. Studies of enzyme induction, electrophile-scavenging, and inhibition of aflatoxin B1 activation.

ABSTRACT: The induction of oxidation and conjugation enzymes, the scavenging of carcinogen electrophiles, and the inhibition of aflatoxin B1 (AFB1) activation were examined as possible mechanisms of anti-carcinogenesis by indole-3-carbinol (I3C). Liver microsomal 7-ethoxycoumarin O-deethylase and 7-ethoxyresorufin O-deethylase activities were not induced significantly in rainbow trout fed diets containing 500-2000 ppm I3C for 8 days compared to trout fed the control diet. Furthermore, no detectable changes in the specific contents of cytochrome P-450 isozymes LM2 and LM4b, as measured by Western-blotting and immunoquantitation, were found in liver microsomes following dietary I3C administration. Dietary I3C had no significant effect on liver microsomal uridine diphosphate-glucuronyl-transferase activity, measured using the substrates 1-naphthol and testosterone, or on cytosolic glutathione S-transferase activity, measured using the substrate styrene oxide. The ability of I3C or its acid reaction products (RXM; generated by the reaction of I3C with HCl) to act as scavengers for the direct alkylating agent AFB1-8,9-Cl2 was examined. Addition of I3C or RXM to in vitro incubations did not inhibit the covalent binding of AFB1-8,9-Cl2 to calf thymus DNA. Kinetic analyses of microsome-mediated binding of AFB1 to DNA in vitro indicated that RXM inhibited the metabolic activation of AFB1. RXM increased the apparent Km for the AFB1-DNA binding reaction without changing the associated Vmax; the apparent Km values at 0, 3.5, 35, and 350 microM RXM were 35, 38, 66, and 86 microM for trout liver microsomes. RXM also inhibited the activation of AFB1 by rat liver microsomes, but I3C was not an effective inhibitor against AFB1-DNA binding mediated by either rat or trout liver microsomes. The results of the present study indicate that inhibition of microsome-activated AFB1 binding to DNA by I3C products may be of significant importance in I3C inhibition of hepatocarcinogenesis in trout and other species. The inhibition of carcinogen activation by I3C is contrasted with the mechanism of anti-carcinogenesis by beta-naphthoflavone, which involves induction of xenobiotic metabolizing enzymes.

SUBMITTER: Fong AT

PROVIDER: S-EPMC2998067 | biostudies-literature | 1990 Jan

REPOSITORIES: biostudies-literature

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Mechanisms of anti-carcinogenesis by indole-3-carbinol. Studies of enzyme induction, electrophile-scavenging, and inhibition of aflatoxin B1 activation.

Fong A T AT Swanson H I HI Dashwood R H RH Williams D E DE Hendricks J D JD Bailey G S GS

Biochemical pharmacology 19900101 1

The induction of oxidation and conjugation enzymes, the scavenging of carcinogen electrophiles, and the inhibition of aflatoxin B1 (AFB1) activation were examined as possible mechanisms of anti-carcinogenesis by indole-3-carbinol (I3C). Liver microsomal 7-ethoxycoumarin O-deethylase and 7-ethoxyresorufin O-deethylase activities were not induced significantly in rainbow trout fed diets containing 500-2000 ppm I3C for 8 days compared to trout fed the control diet. Furthermore, no detectable change ...[more]

PMID: 2105094

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Project description:Aflatoxin B1 is a carcinogenic and mutagenic mycotoxin mainly produced by Aspergillus flavus and A. parasiticus, and prevalent in food and feed. Microbial degradation is a promising strategy which can be performed in mild and environmental friendly condition. This work is a step towards identifying the enzyme responsible for biodegradation of AFB1 by P. putida. Experiments were performed with P. putida lysate and compared with commercial lipase to see the degradation efficiency and the temperature stability. The cell free lysate of P. putida efficiently degraded AFB1 in a range of temperature from 20 to 90 °C. The lysate is thermostable and could retain its activity on pre-incubation up to 90 °C. Highest rate of degradation was observed at 70 °C. These observations show that the P. putida lysate is not only stable at higher temperatures but its enzymatic activity increases after incubation. Similarly, the commercial lipase degraded AFB1 efficiently. However, both, the P. putida lysate and lipase ceased degradation in presence of a lipase inhibitor, HgCl2. The Hill function accurately predicted enzyme activity at various times and temperatures. Like lipase, the lysate also hydrolyses the p-nitrophenyl palmitate to p-nitrophenol. Kinetic parameters such as Vmax, Km and n values are good measures to characterize the lysate response with respect to changing paranitro phenyl palmitate levels. The substrate specificity test of lipase showed linear correlation between the absorbance at 410 nm vs amount of product paranitro phenol. The value of Km, Vmax and n are 0.62 mM, 355.7 μmol min-1 and 1.29, respectively. The lipase gene presence in P. putida was confirmed using PCR technique. These observations indicate that the main enzyme responsible for AFB1 degradation by P. putida is lipase. Thus, lipase as a multifunctional biocatalyst provides a promising future for a variety of industries and may also help to ensure the food safety by degrading the mycotoxins.

Dataset Information

Mechanisms of anti-carcinogenesis by indole-3-carbinol. Studies of enzyme induction, electrophile-scavenging, and inhibition of aflatoxin B1 activation.

Publications

Mechanisms of anti-carcinogenesis by indole-3-carbinol. Studies of enzyme induction, electrophile-scavenging, and inhibition of aflatoxin B1 activation.

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