Abstract
The effect of epoxide hydratase upon the mutagenicity of benzo(a)pyrene was investigated using two Salmonella typhimurium strains (TA 1537 and TA 98). These two bacterial strains were found to differ characteristically in their susceptibility to different mutagens biologically produced from benzo(a)pyrene providing a diagnostic tool to investigate which types of mutagenic metabolites were produced in various metabolic situations. The results showed that the pattern of mutagenic metabolites produced by microsomes from methylcholanthrene-treated mice was very different from that produced by microsomes from phenobarbital-treated or untreated mice. However in all cases at least two mutagenic metabolites were produced. Epoxide hydratase was very efficient at reducing the mutagenic effect when benzo(a)pyrene was activated by microsomes from untreated or phenobarbital-treated mice. However, when microsomes from methylcholanthrene-treated mice were used the effect of hydratase depended upon the benzo(a)pyrene concentration. At low concentrations the mutagenicity was increased by addition of epoxide hydratase and decreased by inhibition of the hydratase. At high concentrations the reverse was true. These findings indicate that when microsomes from untreated and phenobarbital-treated mice were used the main contributors to the mutagenicity were simple epoxides (or compounds arising non-enzymically from them). The activation of dihydrodiols must, however, contribute to a significant extent when microsomes from methylcholanthrene-treated mice were used. Thus the role of epoxide hydratase was determined by the monooxygenase form present in the microsomes in the activating system.
Zusammenfassung
Die Rolle der Epoxidhydratase wurde untersucht in bezug auf die Mutagenität von Benzo(a)pyren. Benzo(a)pyren wurde mit Lebermikrosomen aktiviert. Mutagene wurden festgestellt anhand der Reversion der his − Salmonella typhimurium-Stämme TA 1537 und TA 98. Die beiden Stämme wurden sehr unterschiedlich durch verschiedene mutagene Benzo(a)pyren-Metabolite rückmutiert. Es zeigte sich, daß das Muster der mutagenen Metabolite, die durch Mikrosomen von Methylcholanthren-behandelten Mäusen aus Benzo(a)pyren gebildet wurden, sehr verschieden war vom Muster bei Aktivierung durch Mikrosomen von Kontroll-oder von Phenobarbital-behandelten Mäusen. Jedoch trugen in allen drei Fällen wenigstens zwei verschiedene mutagene Metabolite signifikant zur Mutagenität bei. Epoxidhydratase reduzierte sehr effektiv die Mutagenität, wenn Benzo(a)pyren durch Mikrosomen von Kontroll-oder von Phenobarbital-behandelten Mäusen aktiviert wurde. Wenn jedoch Mikrosomen von Methylcholanthren-behandelten Tieren verwendet wurden, war der Effekt der Epoxidhydratase stark von der Benzo(a)pyren-Konzentration abhängig. Bei niedriger Konzentration erhöhte Zugabe von Epoxidhydratase und erniedrigten Epoxidhydratasehemmstoffe die Mutagenität. Bei hohen Konzentrationen wurde das Umgekehrte festgestellt.
Diese Befunde wurden dahingehend interpretiert, daß bei der Aktivierung mit Mikrosomen von unbehandelten und von Phenobarbital-induzierten Mäusen einfache Epoxide (oder Substanzen, die nicht-enzymatisch daraus gebildet wurden) hauptsächlich für die Mutagenität verantwortlich waren, daß dagegen Mutagene, die über Dihydrodiole gebildet wurden, bedeutend zur Mutagenität beitrugen, wenn Mikrosomen von Methylcholanthren-behandelten Mäusen verwendet wurden.
Die Rolle der Epoxidhydratase, ob aktivierend oder inaktivierend, wird demnach bestimmt durch die Form der Monooxygenase, die an der Aktivierung beteiligt ist.
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Bentley, P., Oesch, F. & Glatt, H. Dual role of epoxide hydratase in both activation and inactivation of benzo(a)pyrene. Arch. Toxicol. 39, 65–75 (1977). https://doi.org/10.1007/BF00343276
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DOI: https://doi.org/10.1007/BF00343276