Study of the inhibition of 3-/4-aminoacetophenones on tyrosinase

Evaluation of in vitro inhibitory effects of some natural compounds on tyrosinase activity and molecular docking study: Antimelanogenesis potential

Tyrosinase enzyme is a functional oxidase that is extensively divided in nature. It is the main enzyme in melanin synthesis and is also involved in designating the color of mammalian hair and skin. Additionally, it is accountable for the unfavorable enzymatic browning that happens in plant-derived foods, limiting the shelf-life of new-cut crops with the resultant economic harm. Recently, there has been a remarkable concern to study the inhibitory activity of the tyrosinase enzyme and some inhibitory molecules isolated from natural sources. For tyrosinase enzyme, afzelin, narcissoside, justiciresinol, thalassiolin B, carpachromene, neobavaisoflavone, and kojic acid (as standard) as natural phenols have IC₅₀ values in the range of 2.37-7.90 µM. Theoretical methods, such as gaussian software program and molecular modeling, were used to compare the biological and chemical activity values of molecules. To compare the biochemical and chemical activity values of molecules, chemical activities with quantum chemical parameters, and biological activities against tyrosinase with the ID of 5M8L molecules were investigated.

A comprehensive review on tyrosinase inhibitors

Tyrosinase is a multi-copper enzyme which is widely distributed in different organisms and plays an important role in the melanogenesis and enzymatic browning. Therefore, its inhibitors can be attractive in cosmetics and medicinal industries as depigmentation agents and also in food and agriculture industries as antibrowning compounds. For this purpose, many natural, semi-synthetic and synthetic inhibitors have been developed by different screening methods to date. This review has focused on the tyrosinase inhibitors discovered from all sources and biochemically characterised in the last four decades.

Structural and kinetic considerations on the catalysis of deoxyarbutin by tyrosinase

Deoxyarbutin, a potent inhibitor of tyrosinase, could act as substrate of the enzyme. Oxytyrosinase is able to hydroxylate deoxyarbutin and finishes the catalytic cycle by oxidizing the formed o-diphenol to quinone, while the enzyme becomes deoxytyrosinase, which evolves to oxytyrosinase in the presence of oxygen. This compound is the only one described that does not release o-diphenol after the hydroxylation step. Oxytyrosinase hydroxylates the deoxyarbutin in ortho position of the phenolic hydroxyl group by means of an aromatic electrophilic substitution. As the oxygen orbitals and the copper atoms are not coplanar, but in axial/equatorial position, the concerted oxidation/reduction cannot occur and the release of a copper atom to bind again in coplanar position, enabling the oxidation/reduction or release of the o-diphenol from the active site to the medium. In the case of deoxyarbutin, the o-diphenol formed is repulsed by the water due to its hydrophobicity, and so can bind correctly and be oxidized to a quinone before being released. Deoxyarbutin has been characterized with: [Formula: see text] = 1.95 ± 0.06 s-1 and [Formula: see text] = 33 ± 4 μM. Computational simulations of the interaction of β-arbutin, deoxyarbutin and their o-diphenol products with tyrosinase show how these ligands bind at the copper centre of tyrosinase. The presence of an energy barrier in the release of the o-diphenol product of deoxyarbutin, which is not present in the case of β-arbutin, together with the differences in polarity and, consequently differences in their interaction with water help understand the differences in the kinetic behaviour of both compounds. Therefore, it is proposed that the release of the o-diphenol product of deoxyarbutin from the active site might be slower than in the case of β-arbutin, contributing to its oxidation to a quinone before being released from the protein into the water phase.

Action of tyrosinase on alpha and beta-arbutin: A kinetic study

The known derivatives from hydroquinone, α and β-arbutin, are used as depigmenting agents. In this work, we demonstrate that the oxy form of tyrosinase (oxytyrosinase) hydroxylates α and β-arbutin in ortho position of the phenolic hydroxyl group, giving rise to a complex formed by met-tyrosinase with the hydroxylated α or β-arbutin. This complex could evolve in two ways: by oxidizing the originated o-diphenol to o-quinone and deoxy-tyrosinase, or by delivering the o-diphenol and met-tyrosinase to the medium, which would produce the self-activation of the system. Note that the quinones generated in both cases are unstable, so the catalysis cannot be studied quantitatively. However, if 3-methyl-2-benzothiazolinone hydrazone hydrochloride hydrate is used, the o-quinone is attacked, so that it becomes an adduct, which can be oxidized by another molecule of o-quinone, generating o-diphenol in the medium. In this way, the system reaches the steady state and originates a chromophore, which, in turn, has a high absorptivity in the visible spectrum. This reaction allowed us to characterize α and β-arbutin kinetically as substrates of tyrosinase for the first time, obtaining a Michaelis constant values of 6.5 ± 0.58 mM and 3 ± 0.19 mM, respectively. The data agree with those from docking studies that showed that the enzyme has a higher affinity for β-arbutin. Moreover, the catalytic constants obtained by the kinetic studies (catalytic constant = 4.43 ± 0.33 s-1 and 3.7 ± 0.29 s-1 for α and β-arbutin respectively) agree with our forecast based on 13 C NMR considerations. This kinetic characterization of α and β-arbutin as substrates of tyrosinase should be taken into account to explain possible adverse effects of these compounds.