Concentration dependent effects of commonly used pesticides on activation versus inhibition of the quince (Cydonia Oblonga) polyphenol oxidase

Polyphenol oxidase (PPO) catalyzes the oxidation of o-diphenols to their respective quinones which undergo autopolymerization and form dark pigments. The interaction of PPO with various substrates and effectors remains the focus of intensive investigations due to the enzyme's key role in pigments biosynthesis including animal melanogenesis and fruit/fungi enzymatic browning. In this study, the effect of a range of commonly used pesticides on the enzyme activity has been evaluated using the purified quince (Cydonia oblonga Miller) PPO. The biochemical analysis showed that, in the presence of high pesticide concentrations, the enzyme was competitively inhibited, particularly with benomyl, carbaryl, deltamethrine and parathion methyl for which inhibition constants (Ki) were 8.3, 5.7, 12 and 4 μM, respectively. At lower pesticide concentrations (2-10 μM), however, the catecholase activity was significantly activated (p < 0.01), suggesting a homotropic behavior of these chemical compounds. Furthermore, the use of in silico structure-based analyses, known as computational docking, highlighted the nature of the PPO-pesticides interactions and confirmed the in vitro observations. Catechol substrate and parathion methyl inhibitor showed lower total energy scores of −120.06 and −117.43 kcal mol−1, indicating that these ligands had higher PPO-binding affinities. The obtained data bring to light new pesticide functional features of great interest in the medicinal, agro-chemical and environmental circles.