3.1 Metabolic Activation 3. Metabolism of PAHs and Health Effects (ERs) or inhibit estrogens from binding to ERs. A chemical structure-biological activity relationship is considered the mechanism (red route) (Hayakawa et al., 2007). As a result, the number of papers on the relationship between PAHs and endocrine disruption has been increasing (Cathey et al., 2020). The third metabolic pathway involves the formation of QPAHs. Several QPAHs, such as 9,10-phenanthrenequinone (9,10-PQ), overproduce reactive oxygen species (ROS) through a redox cycle (blue route). On the other hand, PAHs are nitrated to NPAHs during combustion processes and via secondary atmospheric reactions. In microorganisms, NPAHs are metabolized to nitroso-, hydroxyamino-, and finally, amino-derivatives of PAHs. In this pathway, hydroxyamino PAHs cause DNA adduct formation. This is known to be an indirect-acting mutagenic pathway of NPAHs (brown route) (Hayakawa, 2016). In accordance with the most accepted metabolic activation of PAHs (Straif et al., 2013), Fig. 3 gives an overview of the metabolic activation of PAHs in animals and microorganisms, using benzo[a]pyrene (BaP) as a representative PAH (Hayakawa, 2016). BaP can bind to the aryl hydrocarbon receptor (AhR) that induces the cytochrome P450 (CYP) family (e.g., CYP1A1, CYP1A2, CYP1B1), which forms epoxides, quinones and phenols of BaP. The first metabolic activation pathway involves the formation of BaP-7,8-diol-,9,10-epoxide, which leads to DNA adduct formation, exhibiting carcinogenicity (green route). The second metabolic pathway involves the formation of OHPAHs in the presence of CYP. Several OHPAHs have been found to bind to estrogen receptors Polycyclic Aromatic Hydrocarbons from Vegetation Burning and Health Effects Fig. 2 Atmospheric PM2.5-bound PAHs, NPAHs, OHPAHs and QPAHs. PM2.5 samples were collected for one week in the winter of 2018. Fig. 3 Metabolic activation of BaP. 15

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