Competitive binding of newly synthesized hydroxylated and carbonylated metabolites of chlorinated paraffins to transthyretin.

While biotransformation of chlorinated paraffins (CPs) has been demonstrated in various compartments, their hydroxylated and oxidized metabolites have only recently been identified using non-target screening approaches. The lack of authentic standards for these tentative metabolites impedes definitive structural characterization and toxicity assessment. To address this gap, reference standards for hydroxylated and carbonylated CP metabolites were synthesized, analyzed using liquid chromatography (LC) with high-resolution mass spectrometry (HRMS), and evaluated for their capacity to compete with thyroid hormone for binding to transthyretin (TTR). Matching LC-HRMS data demonstrated that the newly synthesized 3,4,7,8,10,11-hexachloro-1-undecanol and 1,2,5,8,9-pentachloro-4-undecanol (C11H17Cl6-OH and C11H18Cl5-OH) were in vitro metabolites of 1,2,4,5,8,9-hexachloroundecane (C11H18Cl6) following incubation with rat liver S9 fractions. CP single congeners exhibited limited aqueous solubility and weak TTR binding capacity. Hydroxylation increased aqueous solubility, enabling hydroxylated CPs to bind TTR at higher concentrations. In contrast, carbonylated CPs were less potent competitors than hydroxylated CPs for fluorescent thyroxine (FITC-T4) binding to TTR. This study confirms the biotransformation of CPs into hydroxylated metabolites, which may cause thyroid hormone system disruption. It is important to further investigate whether the binding of hydroxylated CPs to TTR facilitates their transport across the placenta or the blood-cerebrospinal fluid barrier, thereby posing risks to fetal development.