Glucose oxidase as a model enzyme for antidiabetic activity evaluation of medicinal plants: In vitro and in silico evidence

Diabetes mellitus is a major public health problem in the world. In Africa, more than 80% of patients use plants for their treatment. However, the methods of validation of endogenous knowledge usually used are costly. The alternative method developed in this study aims at creating hyperglycemia in vitro and exploiting the metabolic pathway involving glucose oxidase for UV-visible spectrophotometric screening of medicinal plants’ antidiabetic activity. The evolution of glucose oxidation as a function of drug concentration is followed by UV-visible spectrophotometry. The formation of the stable complex between the enzyme and the inhibitor is studied using molecular docking. Drugs used (Gliben) and plant extracts exhibited an in vitro hypoglycemic effect by reducing exponentially, in vitro, the level of free glucose. The results also showed that L. multiflora is more active than V. amygdalina (IC₅₀: 1.36 ± 0.09 mg/mL Vs IC₅₀: 3.00 ± 0.54 mg/mL). Gliben (0.5 mg/mL) and L. multiflora (2 mg/mL) reduced both the rate of oxidation of glucose by glucose oxidase (catalytic power V_max: 0.84 ± 0.11 mg*mL^-1*min^-1 for Gliben and 1.72 ± 0.13 mg*mL^-1*min^-1 for ^{L. multiflora}); and the affinity of this enzyme for its substrate-glucose (K_M: 15.11 ± 2.72 mg*mL^-1 for Gliben and 9.17 ± 1.56 mg*mL^-1 for L. multiflora) when these results are compared to enzyme catalysis in the absence of inhibitor (V_max: 2.86 ± 0.44 mg*mL^-1*min-1; K_M: 8.07 ± 1.96 mg*mL^-1). The binding of GOX (1GAL) to selected phytocompounds derived from L. multiflora was confirmed by molecular docking. The most stable complexes were obtained for four compounds; 8 (-10.1±0.0 Kcal/mol), 6 (-9.5±0.1 Kcal/mol), 3 (-8.3±0.0 Kcal/mol) and 9 (-8.2±0.1 Kcal/mol). Among these, compounds 8 and 6 formed complexes with the enzyme stabilized by hydrogen bonds, the compound 8 forms 5 hydrogen bonds (ASN514, ASP424, ARG95, TYP68, LEU65) while compound 6 forms 2 hydrogen bonds (ASN514 and SER422). However, no H-bonding interaction occurs in the complex that involves ligands 9 and 3 despite their high binding energy (-8.2±0.1 Kcal/mol and -8.3±0.0 Kcal/mol respectively). Glucose oxidase can serve as a marker enzyme for in vitro antidiabetic activity evaluation of medicinal plants.