Real-time and simultaneous assay of monophenolase and diphenolase activity in tyrosinase catalyzed cascade reactions by combination of three-way calibration and excitation-emission matrix fluorescence

Inhibitory mechanism of scutellarein on tyrosinase by kinetics, spectroscopy and molecular simulation

Tyrosinase plays an important role in melanin synthesis. Inhibition against tyrosinase activity has been extensively focused on for pharmaceutical, food, cosmetic, and agricultural purpose. The inhibitory mechanism of scutellarein on tyrosinase was elaborated by coupling enzyme kinetics, multi-spectroscopy and computational simulation. Scutellarein remarkably inhibited tyrosinase activity with an IC₅₀ value of 91 μM. Scutellarein reversibly inhibited tyrosinase in a competitive manner. Fluorescence quenching validated that interaction of scutellarein with tyrosinase occurred to form a complex with a binding constant of 6.11 × 10⁴ M^-1. Thermodynamic parameters suggested that scutellarein spontaneously bound with tyrosinase via hydrogen bond and van der Waals force. Three-dimensional fluorescence spectra and circular dichroism spectra revealed that scutellarein induced an obvious conformational change in tyrosinase. Molecular docking result predicted that scutellarein mainly bound with tyrosinase via Arg268 residue. Scutellarein effectively controlled the enzymatic browning of apple slices during storage. This research could give theoretical guiding significance in various application for tyrosinase inhibitors.

Determination of monophenolase activity based on backpropagation neural network analysis of three-dimensional fluorescence spectroscopy

Tyrosinase is pivotal for melanin formation. Measuring monophenolase activity is of great importance for both fundamental research and industrial applications. For the first time, a backpropagation (BP) artificial neural network with three-dimensional fluorescence spectroscopy was applied for the real-time determination of tyrosinase monophenolase activity. Principal component analysis (PCA) was utilized for the dimension reduction of three-dimensional fluorescence data. The four principal components served as inputs for the neural network. Network parameters were optimized using a genetic algorithm (GA). BP learning algorithm was applied to train the network model to determine tyrosine levels in a binary mixture containing tyrosine and L-DOPA without any chemical separation. The time course of tyrosine consumption by monophenolase was determined to calculate the initial velocity of the enzymatic reaction. The limit of detection of the monophenolase assay was 0.0615 U·mL^-1. This combined strategy of PCA, GAs, and BP artificial neural networks for three-dimensional fluorescence spectroscopy was efficient for the real-time and in-situ determination of monophenolase activity in a cascade reaction.

Monophenolase assay using excitation-emission matrix fluorescence and ELMAN neural network assisted by whale optimization algorithm

Tyrosinase plays a vital role for melanogenesis and inherently involves both monophenolase activity and diphenolase activity. Monophenolase catalyzes hydroxylation of tyrosine to l-DOPA (L-3,4-dihydroxyphenylalanine). Real-time monophenolase assay method is of outstanding interest for both scientific research and industrial application. A combined strategy of three-dimensional excitation-emission matrix (EEM) fluorescence spectra and artificial neural network was developed to determine monophenolase activity. A quantitation system for tyrosine in presence of l-DOPA was designed based on ELMAN neural network. Principal component analysis (PCA) was conducted to reduce the dimensionality of fluorescence spectra. Four principal components was used as input variables. Whale optimization algorithm (WOA) was implemented to optimize the initial weights and threshold network. Real-time concentration of tyrosine in monophenolase reaction was monitored to calculate the initial velocity for tyrosine consumption. The exclusive monophenolase activity without interference from diphenolase reaction was determined. Limit of detection (LOD) for monophenolase assay is 0.0113 U mL^-1. Using the proposed method, enzyme kinetics for monophenolase was investigate. K_m was calculated as 14.16 μM. Inhibitor for monophenolase was screened by using model molecule kojic acid with IC₅₀ of 3.49 μM. The assay method exhibited a promising prospect to characterize the kinetics and inhibitor of monophenolase.