Phytocatalytic Redox Cycling of Tire-Derived p -Phenylenediamines: A Hidden Driver for Quinone Metabolite Formation in Rice

In this study, the absorption, translocation, and transformation of p -phenylenediamines (PPDs) in rice were investigated by using a hydroponic cultivation method. The N -(1,3-dimethylbutyl)- N ′- phenyl- p -phenylenediamine (6PPD), N -isopropyl- N ′-phenyl- p -phenylenediamine (IPPD), and N -phenyl- N ′-cyclohexyl- p -phenylenediamine (CPPD) were more prone to be accumulated in the rice root compared with N, N ′-bis(methylphenyl)-1,4-benzenediamine (DTPD). The hydrolysis, methylated, hydroxylated, oxidized, reductive, C–N bond cleavage, glutathione-conjugated, and glucuronic acid-conjugated products were identified. A particularly concerning finding was that PPDs were found to generate PPD-Qs in plants for the first time. This transformation, alongside the interconversion between PPDs and their methylated or demethylated degradation products, could significantly prolong the environmental persistence of PPDs. The cytochrome P450 (CYP450), glycosyltransferases, glutathione S-transferases, and methyltransferases genes might be responsible for the PPD transformation. Additionally, PPDs strongly interacted with CYP450 enzymes, with DTPD demonstrating particularly significant binding affinity ranging from −8.17 to −9.23 kcal/mol. These results introduce uncertainties in understanding the ecological risks of PPDs and their potential human exposure.