Phthalate-induced disruption of metabolic and redox homeostasis in rat liver via modulation of lactate dehydrogenase activity: Insights from toxicokinetic and computational analyses

Phthalates represent the emergent and pervasive environmental pollutants derived from phthalic acids and their constituents, primarily used as plasticizers in polyvinyl chloride products such as packaging materials and toys. Their extensive use and leaching potential raise concerns about exposure and human health. Phthalates are recognized as endocrine-disrupting agents that significantly modify the hepatic enzymes' activities. This study has envisaged assessing the toxicological impact of dicyclohexyl phthalate (DCHP) and diisononyl phthalate (DiNP) on the kinetics of lactate dehydrogenase (LDH) catalyzed reversible reaction in rat liver. The forward and reverse reactions catalyzed by the enzyme, demonstrating the optimal activity at pH 7.4, temperature 37 °C, and pH 6.6, temperature 47 °C, respectively. The enzyme showed a low K_m for substrate of the forward reaction while exhibiting a high K_m for the substrates of the reverse reaction in the absence of phthalates (DCHP and DiNP). However, the phthalates induced mixed enzyme inhibition and altered the toxicokinetic parameters. Additionally, the IC₅₀, t_1/2, and K_i values indicated that DCHP impaired the function of LDH more effectively in both the forward and reverse reactions. The in-silico analysis revealed that DCHP and DiNP induced conformational changes in the mobile loop, active site, and substrate-binding site (Arg169, Arg171), leading to steric hindrance within LDH. The binding affinity and K_i confirmed that DCHP more strongly inhibits the forward reaction of LDH than DiNP. These findings suggested that phthalates induced the structural rearrangements that compromise the catalytic function of LDH and significantly disrupt glycolytic metabolism and energy production by modulating LDH activity.