Biotransformation of Polycyclic Aromatic Hydrocarbons by Trout Liver S9 Fractions: Evaluation of Competitive Inhibition Using a Substrate Depletion Approach

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 3 polycyclic aromatic hydrocarbons (PAHs): phenanthrene, pyrene, and benzo[a]pyrene, tested as binary mixtures. Initial rates of biotransformation were determined using a substrate‐depletion approach. The resulting data were then fitted by simultaneous nonlinear 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 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 cytochrome P450 enzymes. 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. Environ Toxicol Chem 2019;38:2729–2739. Published 2019 Wiley Periodicals, Inc. on behalf of SETAC. This article is a US government work, and as such, is in the public domain in the United States of America