Synchronous fluorometric method for continuous assay of monophenolase activity

Enzymatic colorimetric method for turn-on determination of l-lactic acid through indicator displacement assay

l-Lactic acid is a natural α-hydroxy carboxylic acid and is commonly used as an addictive. Quantitation of l-lactic acid is indispensable in food and cosmetic industries. An enzymatic colorimetric method was developed for the determination of l-lactic acid by competitive indicator displacement assay. Boric acid inhibited the colorimetric reaction of l-3,4-dihydroxyphenylalanine (l-DOPA) catalyzed by tyrosinase. l-Lactic acid competitively displaced and released l-DOPA bound with boric acid to serve as substrate, and thus restored the tyrosinase activity. Recovery of color reaction could be spectrophotometrically determined at 475 nm and was proportional to the amount of l-lactic acid. A calibration curve between l-lactic acid concentration and recovery of absorbance were built. The concentration range of the l-lactic acid was 0.25-2.25 mM. The limit of detection (LOD) and the limit of quantification (LOQ) for l-lactic acid was estimated to be 0.05 mM and 0.16 mM, respectively. The method achieved turn-on and visual sensing with good precision, accuracy, specificity, and robustness. The assay method exhibited a promising prospect to determine the content of l-lactic acid in foods and cosmetics.

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.

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

A real-time assay for multiple enzyme activities in cascade reactions is required for research on metabolism and bioengineering. Tyrosinase has the bifunctional activity of monophenolase and diphenolase. A combined strategy of three-way calibration with excitation-emission matrix (EEM) fluorescence was developed for real-time and simultaneous determination of monophenolase and diphenolase activity with tyrosine as a substrate. Mathematical separation and second-order advantage were utilized to solve spectral overlapping and uncalibrated interferents during complex dynamic enzymatic processes. Kinetic evolution profiles of EEM were monitored to stack a fusion three-way data array together with static samples. Using a parallel factor analysis (PARAFAC) algorithm, pseudo-univariate calibration curves with limits of detection (LODs) of 3.00 μM and 0.85 μM were established to simultaneously and real-time measure tyrosine and DOPA. Progress curves for tyrosine consumption by monophenolase and DOPA consumption by diphenolase were obtained using the law of mass conservation to calculate the initial velocity. The LODs for monophenolase and diphenolase were 0.0232 U⋅mL^-1 and 0.0316 U⋅mL^-1. The method achieved real-time and simultaneous assays of multiple enzyme activities in cascade reactions. It showed potential application in the metabolic pathway and biochemical industry.

First derivative synchronous fluorometric method to continuously measure monophenolase activity

Tyrosinase plays an essential role in melanin biosynthesis and inherently exhibits both monophenolase and diphenolase activity. A first derivative synchronous fluorometric assay was established for directly monitoring monophenolase activity. The zero-crossing point at 322 nm for the first-derivative under synchronous fluorescence with Δλ = 67 nm was utilized to selectively quantify tyrosine in the presence of the reaction product dihydroxyphenylalanine (DOPA). The limit of detection (LOD) for tyrosine was 0.54 μM. The fluorescence intensity of tyrosine was monitored at intervals of 30 s to establish the time course of tyrosine consumption. The LOD for the monophenolase activity was 0.0706 U⋅ mL^-1. The Michaelis-Menten e constant and maximum speed were 21.83 μM and 1.12 μM min^-1, respectively. Zinc ions competitively inhibited the monophenolase activity, with an IC₅₀ value of 14.36 μM. This assay is easily and rapidly executed and is of great significance for analyzing the kinetics of enzymatic reactions and in fundamental research on monophenolase. This approach has potential applications in the discovery of tyrosinase inhibitors for medicine and cosmetics, as well as in the industrial synthesis of substituted o-diphenol intermediates.

Considerations about the Continuous Assay Methods, Spectrophotometric and Spectrofluorometric, of the Monophenolase Activity of Tyrosinase

With the purpose to obtain the more useful tyrosinase assay for the monophenolase activity of tyrosinase between the spectrofluorometric and spectrophotometric continuous assays, simulated assays were made by means of numerical integration of the equations that characterize the mechanism of monophenolase activity. These assays showed that the rate of disappearance of monophenol (VssM,M) is equal to the rate of accumulation of dopachrome (VssM,DC) or to the rate of accumulation of its oxidized adduct, originated by the nucleophilic attack on o-quinone by a nucleophile such as 3-methyl-2-benzothiazolinone (MBTH), (VssM, A−ox), despite the existence of coupled reactions. It is shown that the spectrophotometric methods that use MBTH are more useful, as they do not have the restrictions of the L-tyrosine disappearance measurement method, of working at pH = 8 and not having a linear response from 100 μM of L-tyrosine. It is possible to obtain low LOD^M (limit of detection of the monophenolase activity) values with spectrophotometric methods. The spectrofluorimetric methods had a lower LOD^M than spectrophotometric methods. In the case of 4-hydroxyphenil-propionic acid, the LOD^M obtained by us was 0.25 U/mL. Considering the relative sensitivities of 4-hydroxyanisole, compared with 4-hydroxyphenil-propionic acid, LOD^M values like those obtained by fluorescent methods would be expected.