Chemical synthesis and tyrosinase inhibitory activity of rhododendrol glycosides

Tyrosinase Inhibition and Antimelanogenic Effects of Resorcinol-Containing Compounds

Tyrosinases (TYRs) are copper-containing metalloenzymes present in a large diversity of species. In human, hTYR is responsible for pivotal steps in melanogenesis, catalysing the oxidation of l-tyrosine to l-DOPA and further to dopaquinone. While numerous TYR inhibitors have been reported, polyphenolic compounds tend to dominate the literature. However, many of these compounds, particularly monophenols and catechols, have been identified as alternative substrates rather than true inhibitors, given their structural similarity to natural substrates. Resorcinol-containing compounds have emerged as promising candidates to address this challenge, as the meta-dihydroxy moiety in resorcinol demonstrates resistance to TYR-mediated oxidation, while retaining the favourable interactions with copper ions provided by the hydroxy groups. Although their precise mechanism of action remains debated, resorcinol derivatives have yielded some of the most active compounds against isolated mushroom and human TYRs, as well as clinically used dermocosmetic agents like rucinol and thiamidol, which exhibited very promising effects in patients with facial melasma. This review outlines the development of resorcinol-containing TYR inhibitors, categorized by scaffold type, ranging from simple alkyl analogues to intricate synthetic derivatives. Mechanistic insights about the resorcinol-TYR interaction are also presented and debated.

Incorporation of Nanocatalysts for the Production of Bio-Oil from Staphylea holocarpa Wood

Biomass has been recognized as the most common source of renewable energy. In recent years, researchers have paved the way for a search for suitable biomass resources to replace traditional fossil fuel energy and provide high energy output. Although there are plenty of studies of biomass as good biomaterials, there is little detailed information about Staphylea holocarpa wood (S. holocarpa) as a potential bio-oil material. The purpose of this study is to explore the potential of S. holocarpa wood as a bio-oil. Nanocatalyst cobalt (II) oxide (Co₃O₄) and Nickel (II) oxide (NiO) were used to improve the production of bio-oil from S. holocarpa wood. The preparation of biofuels and the extraction of bioactive drugs were performed by the rapid gasification of nanocatalysts. The result indicated that the abundant chemical components detected in the S. holocarpa wood extract could be used in biomedicine, cosmetics, and biofuels, and have a broad industrial application prospect. In addition, nanocatalyst cobalt tetraoxide (Co₃O₄) could improve the catalytic cracking of S. holocarpa wood and generate more bioactive molecules at high temperature, which is conducive to the utilization and development of S. holocarpa wood as biomass. This is the first time that S. holocarpa wood was used in combination with nanocatalysts. In the future, nanocatalysts can be used to solve the problem of sustainable development of biological resources.

Structure and inhibition mechanism of some synthetic compounds and phenolic derivatives as tyrosinase inhibitors: review and new insight

Safety concerns are the primary consideration to identify and detection of enzyme inhibitors. In this regard, safe and potent tyrosinase inhibitors play important role in enhancing nutritional quality, health promotion and also prevent further damages. The present review focuses on the recent and efficient tyrosinase inhibitors discovered from both synthetic sources and synthesized phenolic compounds, including flavonoid, carvacrol, thymol, cinnamic acid and resorcinol derivatives. The inhibitory activity of these compounds was analyzed according to chemical structure, IC₅₀, K_i and their binding energy. Further, inhibition mechanism and the biological effects of some these inhibitors with potential application in food, agricultural, cosmetic and pharmaceutical industries were briefly discussed. Molecular docking procedure was performed on some derivatives and demonstrated favorable binding affinity with amino acid residues of mushroom tyrosinase (PDB ID: 2Y9X). The information offered showed that the substitution pattern of hydroxyl groups at the phenyl ring is an important factor of tyrosinase inhibitory activity. The results confirmed that understanding structural modification of inhibitors is a key role in finding novel and efficacious tyrosinase inhibitors.Communicated by Ramaswamy H. Sarma.

Synthesis, structural revision, and tyrosinase inhibitory activity of proposed phloretin-4-O-β-D-glucopyranoside from Homalium stenophyllum

Phloretin-4-O-β-D-glucopyranoside (1), isolated from Homalium stenophyllum, was synthesized for the first time through aldol condensation and Schmidt glycosylation reactions aiming to develop a novel hydrophilic tyrosinase inhibitor. However, the specific rotation of synthetic 1 was found to be negative and different from that reported for natural product 1. Thus, L-glucoside 2 was chemically synthesized using the established synthetic route of 1, suggesting that the configuration of the natural product 1 was the same as that of 2, as their specific rotation and spectroscopic data were also the same. In addition, the evaluation of the inhibitory activity of 1 and 2 against tyrosinase indicated that 2 was 1.4 times more potent than 1, but they were both relatively weak. Therefore, the enantiomeric analogues 1 and 2 were proved to be unique tyrosinase inhibitors due to the chiral recognition from the tyrosinase active site.

Chemical synthesis and tyrosinase-inhibitory activity of isotachioside and its related glycosides

Isotachioside (1) and its related natural product 2 are isolated from Isotachis japonica and Protea neriifolia, respectively, and are categorized as analogs of arbutin (3), a tyrosinase inhibitor for practical use. Both of the natural products and several derivatives such as glucoside 4, xyloside 5, cellobioside 6, and maltoside 7 were synthesized via Schmidt glycosylation as a key step, and their tyrosinase inhibitory activity was evaluated. The half maximal inhibitory concentration (IC₅₀) of 1-3 could not be determined even when the concentration was increased to 1000 μM. Contrastingly, glycosides 4-7, missing methyl and benzoyl groups, acted as tyrosinase inhibitors with IC₅₀s of 417 μM, 852 μM, 623 μM, and 657 μM, respectively. Among these novel inhibitors, derivative 4 was the most potent, indicating that the structural combination of resorcinol and glucose was significant for inducing the inhibitory effect.

Substituent effect of benzaldehydes on tyrosinase inhibition

Benzaldehyde inhibited the oxidation of 4-t-butylcatechol catalyzed by mushroom tyrosinase with an IC₅₀ of 31.0 μM. The inhibition kinetics analyzed by Dixon plot indicated that it acts as a partial noncompetitive inhibitor. Further studies of several benzaldehydes, particularly those having a substitution at C-4, suggested that the partial inhibitory property diminished when using a bulk substituent. For example, 4-penthylbenzaldehyde showed a full and mixed type inhibition on diphenolase activity. Therefore, 4-substituted benzaldehyde on the aromatic ring primarily reflected the rate of product formation as it may act as a tight hydrophobic cover on the catalytic center of tyrosinase.

Dihydroresveratrol cellobioside and xylobioside as effective melanogenesis activators

Dihydroresveratrol cellobioside and xylobioside, whose structures were designed based on that of the naturally occurring melanogenesis-controlling agent dihydroresveratrol glucoside, were synthesized via Schmidt glycosylation as the key step. Both analogues stimulated melanogenesis with efficacies comparable to that of 8-methoxypsoralen, a well-known melanogenesis activator. This suggests that diglycosyl modification of the 4'-OH on the dihydroresveratrol skeleton leads to the activation of melanogenesis, both with and without hydroxymethyl groups in the sugar moieties.

Rhododenol and raspberry ketone impair the normal proliferation of melanocytes through reactive oxygen species-dependent activation of GADD45

Rhododenol or rhododendrol (RD, 4-(4-hydroxyphenyl)-2-butanol) occurs naturally in many plants along with raspberry ketone (RK, 4-(4-hydroxyphenyl)-2-butanone), a ketone derivative, which include Nikko maple tree (Acer nikoense) and white birch (Betula platyphylla). De-pigmenting activity of RD was discovered and it was used as a brightening ingredient for the skin whitening cosmetics. Recently, cosmetics containing RD were withdrawn from the market because a number of consumers developed leukoderma, inflammation and erythema on their face, neck and hands. Here, we explored the mechanism underlying the toxicity of RD and RK against melanocytes using B16F10 murine melanoma cells and human primary epidermal melanocytes. Treatment with RD or RK resulted in the decreased cell viability in a dose-dependent manner which appeared from cell growth arrest. Consistently, ROS generation was significantly increased by RD or RK as determined by DCF-enhanced fluorescence. An antioxidant enzyme, glutathione peroxidase was depleted as well. In line with ROS generation, oxidative damages and the arrest of normal cell proliferation, GADD genes (Growth Arrest and DNA Damage) that include GADD45 and GADD153, were significantly up-regulated. Prevention of ROS generation with an anti-oxidant, N-acetylcysteine (NAC) significantly rescued RD and RK-suppressed melanocyte proliferation. Consistently, up-regulation of GADD45 and GADD153 was significantly attenuated by NAC, suggesting that increased ROS and the resultant growth arrest of melanocytes may contribute to RD and RK-induced leukoderma.

Biochemical, cytological, and immunological mechanisms of rhododendrol-induced leukoderma

Recently, an unexpected outbreak of patients with leukoderma occurred in Japan with the use of brightening/lightening cosmetics containing rhododendrol (RD). Patients developed leukoderma mostly on the skin sites repeatedly applied with RD, but some patients also had vitiligo-like lesions on the non-applied sites. RD is a tyrosinase-competitive inhibiting substance, thereby serving as an inhibitor of melanin synthesis. Upon inhibition of tyrosinase, RD is converted to new products such as tyrosinase-catalyzed hydroxyl-metabolite, which damage melanocytes. The melanocyte cell lysates seem to induce T-cell response. The frequencies of CD8+ T cells in both lesional skin and peripheral blood are significantly higher in the RD leukoderma as well as non-segmental vitiligo patients than in normal controls. In HLA-A*02:01 positive cases, circulating Melan-A-specific cytotoxic T cells can be detected at a high frequency. It is thus suggested that RD-induced leukoderma is induced by not only cytolysis of melanocytes but also subsequent immune reactions toward melanocytes.