In vitro metabolism of naphthalene and its alkylated congeners by human and rat liver microsomes via alkyl side chain or aromatic oxidation

Mineral oils are widely applied in food production and processing and may contain polycyclic aromatic hydrocarbons (PAHs). The PAHs that may be present in mineral oils are typically alkylated, and have been barely studied. Metabolic oxidation of the aromatic ring is a key step to form DNA-reactive PAH metabolites, but may be less prominent for alkylated PAHs since alkyl substituents would facilitate side chain oxidation as an alternative. The current study investigates this hypothesis of preferential side chain oxidation at the cost of aromatic oxidation using naphthalene and a series of its alkyl substituted analogues as model compounds. The metabolism was assessed by measuring metabolite formation in rat and human liver microsomal incubations using UPLC and GC-MS/MS. The presence of an alkyl side chain markedly reduced aromatic oxidation for all alkyl-substituted naphthalenes that were converted. 1-n-Dodecyl-naphthalene was not metabolized under the experimental conditions applied. With rat liver microsomes for 1-methyl-, 2-methyl-, 1-ethyl-, and 2-ethyl- naphthalene, alkyl side chain oxidation was preferred over aromatic oxidation. With human liver microsomes this was the case for 2-methyl-, and 2-ethyl-naphthalene. It is concluded that addition of an alkyl substituent in naphthalene shifts metabolism in favor of alkyl side chain oxidation at the cost of aromatic ring oxidation. Furthermore, alkyl side chains of 6 or more carbon atoms appeared to seriously hamper and reduce overall metabolism, metabolic conversion being no longer observed with the C12 alkyl side chain. In summary, alkylation of PAHs likely reduces their chances of aromatic oxidation and bioactivation.