New Inhibitors of Laccase and Tyrosinase by Examination of Cross-Inhibition between Copper-Containing Enzymes

Thin layer chromatography assay to detect laccase inhibitors

Thin-layer chromatography (TLC) hyphenated to bioassays is a modern tool used for discovery of biologically active compounds from complex mixtures. The first bioautographic assay for detecting laccase inhibitors on a TLC plate was developed in this study. The on-plate reaction of laccase with colourless ABTS that renders the blue ABTS∙+ radical was optimised. Combination of the enzymatic TLC-assay with a control TLC-assay, wherein ABTS∙+ radical is chemically generated and then applied on the TLC, allowed to differentiate between the pure laccase inhibitor sodium azide and radical scavengers such as gallic and kojic acids. The limit of detection and quantification for the method were 54.9 and 166 ng of sodium azide respectively. The methodology was applied successfully to a recently discovered laccase inhibitor chemotype: hydrazones. A model hydrazone was compared with several hydrazones synthesized for this study. For the first time, laccase inhibitors separated on a TLC plate can be detected individually.

Some Antioxidant Properties of Components from The Flower of Ochna integerrima and Their Beneficial Effects on HaCaT Keratinocytes and In Silico Analysis on Tyrosinase

Four compounds, luteolin ( 1 ), 6- γ , γ -Dimethylallylquercetin 7- O - β -ᴅ-glucopyranoside ( 2 ), 6- γ , γ -Dimethylallylkaempferol 7- O - β -ᴅ-gluco-pyranoside ( 3 ), and 6- γ , γ -Dimethylallyldihydrokaempferol 7- O - β -ᴅ-glucoside ( 4 ), were isolated for the first time from EtOAc extract of the O. integerrima   flower. We then evaluated the antioxidant effects of EtOAc, n -butanol, and MeOH extracts and their effects on H 2 O 2 against oxidative stress in HaCaT keratinocyte cell lines. Furthermore, 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH · ) radical scavenging activities of 1 - 4 were determined and their mechanisms of action on tyrosinase were predicted by in silico studies. The results revealed that the EtOAc extract and 1 - 3 exhibited good DPPH˙radical scavenging activity. Furthermore, this extract also had a significant protective effect against H 2 O 2 -induced oxidative stress in HaCaT cells. In silico studies indicated that the activity of 1 - 3 may be due to tyrosinase inhibition with MM-GBSA free binding energies of -78.9, -70.1, -71.1 kcal mol -1 , respectively, compared to 4 with an energy -56.9 kcal mol -1 .

Hydroxamic Acid as a Potent Metal-Binding Group for Inhibiting Tyrosinase

Tyrosinase, a metalloenzyme containing a dicopper cofactor, plays a central role in synthesizing melanin from tyrosine. Many studies have aimed to identify small-molecule inhibitors of tyrosinase for pharmaceutical, cosmetic, and agricultural purposes. In this study, we report that hydroxamic acid is a potent metal-binding group for interacting with dicopper atoms, thereby inhibiting tyrosinase. Hydroxamate-containing molecules, including anticancer drugs targeting histone deacetylase, vorinostat and panobinostat, significantly inhibited mushroom tyrosinase, with inhibitory constants in the submicromolar range. Of the tested molecules, benzohydroxamic acid was the most potent. Its inhibitory constant of 7 nM indicates that benzohydroxamic acid is one of the most potent tyrosinase inhibitors. Results from differential scanning fluorimetry revealed that direct binding mediates inhibition. The enzyme kinetics were studied to assess the inhibitory mechanism of the hydroxamate-containing molecules. Experiments with B16F10 cell lysates confirmed that the new inhibitors are inhibitory against mammalian tyrosinase. Docking simulation data revealed intermolecular contacts between hydroxamate-containing molecules and tyrosinase.