Review

Tyrosinase: a comprehensive review of its mechanism

Plant tissue culture plays a central role in the agricultural, horticultural, research, and conservation sectors. It facilitates precise control over plant propagation and manipulation, resulting in enhanced crop yields, effective disease management, and the preservation of endangered plant species. Browning, a well-acknowledged limitation in plant tissue culture, poses potential challenges to successful in vitro plant multiplication. Browning primarily occurs in response to enzymatic reactions due to explant damage. Left untreated, it can lead to a reduced in the regeneration capacity, hindered callus proliferation, impeded development of adventitious shoots, and, in extreme cases, tissue necrosis. To mitigate the issue of browning, several in vitro strategies have been implemented i.e., submerging the explants in specialized solutions designed to inhibit browning, incorporating anti-browning agents into the growth medium, and adhering to certain cultural techniques. This article aims to comprehensively examine the factors contributing to browning and the multitude of strategies employed to effectively manage browning problems in plant tissue cultures. Furthermore, it explores the potential of encapsulating natural products as a cutting-edge method for addressing browning in plant tissue culture. These innovative approaches offer promising avenues for controlling browning in plant tissue culture, thereby contributing to the advancement of sustainable agricultural practices and conservation efforts.

The defatted seeds of evening primrose (DE), a by-product of evening primrose oil extraction, are currently underutilized. This study aimed to valorize DE by examining its effects on melanogenesis and tyrosinase activity in zebrafish embryos and in vitro, and an innovative affinity-labeled molecular networking workflow was proposed for the rapid identification of tyrosinase inhibitors in DE. Our results indicated DE significantly reduced melanin content (53.3 % at 100 μg/mL) and tyrosinse activity (80.05 % for monophenolase and 70.40 % for diphenolase at 100 μg/mL). Furthermore, through the affinity-labeled molecular networking approach, 20 compounds were identified as potential tyrosinase inhibitors within DE, predominantly flavonoids and tannins characterized by catechin and galloyl substructures. Seven of these compounds were isolated and their inhibitory effects on tyrosinase were validated using functional assays. This study not only underscores the potential of DE as a rich source of natural tyrosinase inhibitors but also establishes the effectiveness of affinity-labeled molecular networking in pinpointing bioactive compounds in complex biological matrices.

As the β-phenyl-α,β-unsaturated carbonyl (PUSC) structure was previously identified to play a key role in tyrosinase inhibition, 14 analogs with a PUSC structure built on a thiazol-4(5H)-one scaffold were synthesized using Knoevenagel condensation to serve as potential tyrosinase inhibitors. Through mushroom tyrosinase inhibition experiments, two analogs 9 and 11 were identified as potent tyrosinase inhibitors, with 11 exhibiting an IC₅₀ value of 0.4 ± 0.01 μM, which indicates its 26-fold greater potency than kojic acid. Kinetic studies using Lineweaver-Burk plots revealed that 9 and 11 are competitive and mixed-type inhibitors, respectively; these kinetic results were supported by docking simulations. According to the B16F10 cell-based experiments, 9 and 11 inhibited melanogenesis more effectively than kojic acid due to their potent cellular tyrosinase inhibitory activity. In addition, analogs 9 and 11 exhibited moderate-to-strong antioxidant capacity, scavenging ABTS⁺, DPPH, and ROS radicals. In particular, analog 12 with a catechol moiety exhibited very strong ROS-scavenging activity, similar to Trolox. These results suggest that analogs 9 and 11, which exhibit potent tyrosinase inhibitory activity in mushroom and mammalian cells and anti-melanogenic effects in B16F10 cells, are promising antibrowning agents for crops and skin lightening agents for hyperpigmentation-related diseases.

Tyrosinases (TYRs; EC 1.14.18.1) catalyze two sequential oxidative reactions of the melanin biosynthesis pathway and play an important role in mammalian pigmentation and enzymatic browning of fruit and vegetables. Inhibition of TYR activity is therefore an attractive target for new drugs and/or food ingredients. In addition, increasing evidence suggests that TYR regulation could be a novel target for treatments of cancer and Parkinson's disease. Biomasses, notably industrial byproducts and biowaste, are good sustainable sources of phytochemicals that may be valorized into bioactive compounds including TYR inhibitors. This review presents potential applications of biomass-derived polyphenols targeting TYR inhibition. Insights into structure–activity relationships of several polyphenols and their glycosides are highlighted. Finally, some remarks and perspectives on research into new TYR inhibitors from biomass waste are provided.

The tyrosinase enzyme, which is widely found in microorganisms, animals and plants, has a significant position in melanogenesis, plays an important role in undesirable browning of fruits and vegetables, antibiotic resistance, skin pigment formation, sclerotization of cuticle, neurodegeneration, etc. Therefore, with the wide potential application fields of tyrosinase in food, agriculture, cosmetics and pharmaceutical industries, which has become the target enzyme for the development of therapeutic agents such as antibrowning, anticancer, antibacterial, skin whitening, insecticides, etc., a large number of synthetic tyrosinase inhibitors have been widely reported in recent years. The triazole ring, which has a broad spectrum of biological action, is of increasing interest in the synthesis of new tyrosinase inhibitors. In this review, tyrosinase inhibition effects, structure-activity relationships, enzyme inhibition kinetics and mechanisms of action of 1,2,3- or 1,2,4-triazole derivatives were investigated. The data gathered is anticipated to supply rational guidance and an influential strategy for the development of novel, potent and safe tyrosinase inhibitors for better practical application in the future.

Polyphenol oxidase (PPO) enzymes induce undesirable browning in fruits, vegetables, and juices like sugarcane and apple. Inactivation of PPO is crucial to mitigate this issue, achieved through methods like heat, radiation, ultrasound, pH adjustment, adsorbent addition, and inhibitors. Inhibitor selection is traditionally a time-consuming trial-and-error process. This study aimed to develop a homology model of sugarcane PPO and to streamline inhibitor selection using a rational approach, relying on knowledge of PPO's structure and active site. The research utilized homology modeling through SWISS-MODELER to predict the 3D structures of two types of sugarcane copper-based PPOs: met-PPO (Cu-Cu active site) and oxy-PPO (Cu-O-Cu active site), both composed of 235 amino acids. Model quality was assessed using Global Model Quality Estimation (GMQE) and Qualitative Model Energy Analysis (QMEAN), yielding favorable scores of 0.17 (GMQE), 0.44 ± 0.05 (QMEAN Co-Global) and z-score of –4.03. Ramachandran analysis indicated 78.8 % and 17.5 % of amino acid residues fell within the most favored and additional allowed regions, respectively. Docking analysis revealed strong affinities between m-hydroxybenzoic acid and p-hydroxybenzoic acid with binding energies of –4.40 Kcal/mol and –4.38 Kcal/mol, respectively, for met-PPO and oxy-PPO. Identified active site residues are ARG369, HIS325, GLU321, ALA361, and TRP313. This study enhances comprehension of sugarcane PPO and its active site, aiding rational inhibitor design to counteract undesired browning reactions.