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Biotransformation of polycyclic aromatic hydrocarbons by trout liver S9 fractions: Evaluation of competitive inhibition using a substrate depletion approach
Citation:
Nichols, J., M. Ladd, A. Hoffman, AND P. Fitzsimmons. Biotransformation of polycyclic aromatic hydrocarbons by trout liver S9 fractions: Evaluation of competitive inhibition using a substrate depletion approach. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. Society of Environmental Toxicology and Chemistry, Pensacola, FL, 38(12):2729-2739, (2019). https://doi.org/10.1002/etc.4595
Impact/Purpose:
Environmental contaminants frequently occur as chemical mixtures. If one chemical inhibits the biotransformation of a second substance with which it coexists, failure to understand this interaction could lead to underprediction of the second compound’s potential to accumulate in exposed animals and exert toxic effects. The purpose of this study was to use an in vitro approach to investigate biotransformation of polycyclic aromatic hydrocarbons (PAH), tested as binary mixtures using trout liver fractions. PAHs are well metabolized by fish and generally occur in the environment as part of a complex mixture. Mixture effects on biotransformation of PAHs is therefore highly relevant. The results show that PAHs competitively inhibit one another’s metabolism, consistent with the suggestion that they are metabolized by one or a small number of enzymes. These findings may be used to determine the likelihood of competitive inhibition occuring when multiple PAH are tested in vitro and in vivo experimental settings. This research substantially advances current knowledge of mixture effects on chemical bioaccumulation and effects in fish, and have special relevance to chemical hazard assessments.
Description:
Environmental contaminants frequently occur as part of a chemical mixture, potentially resulting in competitive inhibition among multiple substrates metabolized by the same enzyme. Trout liver S9 fractions were used to evaluate the biotransformation of three polycyclic aromatic hydrocarbons (PAHs): phenanthrene (PHEN), pyrene (PYR) and benzo[a]pyrene (BAP), tested as binary mixtures. Initial rates of biotransformation were determined using a substrate depletion approach. The resulting datasets were then fitted by simultaneous non-linear regression to a competitive inhibition model. In each case, the PAH possessing the lower Michaelis-Menten affinity constant (KM) competitively inhibited biotransformation of the other compound. Inhibition constants (Ki) determined for the lower-KM compound were generally close to previously determined KM values, consistent with the suggestion that phase I biotransformation of PAHs is largely catalyzed by one, or a small number of CYPs. The use of a substrate depletion approach to perform enzyme inhibition studies imposes practical limitations on experimental design and complicates the interpretation of derived kinetic constants. Nevertheless, the resulting information may have utility for chemical hazard assessments as well as the design and interpretation of controlled laboratory studies. Depletion experiments informed by measured chemical concentrations in tissues may also provide a means of determining whether enzyme inhibition occurs under relevant environmental conditions.
URLs/Downloads:
DOI: Biotransformation of polycyclic aromatic hydrocarbons by trout liver S9 fractions: Evaluation of competitive inhibition using a substrate depletion approach