Estimation of the Kinetic Parameters of the Inhibition of Tyrosinase by an Extract of S. Mombin (Root Bark) and the Investigation of Likely Interactions of Composite Phytochemicals Using Molecular Docking Calculations

The GCMS analysis of the ethyl acetate fraction of crude aqeous methanol extract of the root bark of Spondias mombin revealed the composition of 18 compounds of which two methyl esters of long chain carboxylic acids (methyl palmitate and (E)-9-octadecenoic acid methyl ester) account for 52% of the entire extract both having % peak area of 25.6% and 26.4% respectively. The ethyl acetate fraction of the S. mombin aqeous methanolic extract inhibited tyrosinase from Agaricus bisporus (mushroom) with an IC50 of 1.016 ± 0.003 mg/ml which was 25 fold higher than that of kojic acid which was used as a standard inhibitor of tyrosinase in a control experiment with an IC50 of 0.04 ± 0.006 mg/ml. The interaction between the EtOAc fraction of S. mombin and tyrosinase was investigated through fluorescence quenching studies. The fluorescence emission spectra of tyrosinase were recorded in the range of 300 – 500 nm with the excitation and emission wavelengths of tyrosinase at 290 nm and 345 nm respectively. The Intrinsic fluorescence quenching indicated that the test fraction interacted and quenched the fluorescence intensity of tyrosinase in a concentration dependent manner. Kinetic studies with the extract showed that the test fraction elicited a competitive mode of inhibition for the tyrosinase (from A. bisporus). The 3D structures of the 18 compounds detected as constituents of the fraction from GCMS analysis were generated and prepared for docking using a combination of software packages (ChemDraw Ultra 12.0 and MGL tools v1.5.4) and docked (using autodock vina v.1.1.2) with the 3D, X-ray crystallographic structure of the protein (obtained from the protein databank, rcsb.org, pdb code 2Y9X) in order to estimate their binding affinity and interactions with the protein. The docking calculations revealed that five compounds out of the eighteen had higher binding energy (-5.8 kcal/mol to -7.5 kcal/mol) relative to that of the standard, kojic acid (-5.6 kcal/mol). The compound identified to have the highest binding affinity for the tyrosinase is (E)-4-((4-(2-hydroxybenzamido)phenyl)amino)-4-oxobut-2-enoic acid with a binding energy of -7.5 kcal/mol.