Aloesin, a natural hydroxymethylchromone derivative isolated from aloe vera, acts by two different mechanisms of action on tyrosinase activity, e. g., aloesin inhibits the formation of DOPA quinone by competitive inhibition at the DOPA oxidation site, reduction of copper ions at the hydroxylase site, and consequently tyrosine hydroxylation by noncompetitive inhibition (24). In comparison with other depigmenting agents, aloesin shows no cytotoxicity in cell-based assays, no skin irritation in preliminary human studies and any genotoxicity or mutagenicity in the Ames assay. Cultured cells used in tyrosinase activity assays show no morphologic abnormalities when treated with aloesin, and human melanocytes appear normal with multiple dendrites (24).
Thus aloesin is a potent inhibitor of human tyrosinase. However, because of the hydrophilic nature of the compound and moderately high molecular weight, penetration of human skin was poor. Jones et al. (24) demonstrated aloesin dissolved in ethanol penetrates the skin slowly with approximately 1.59% of a finite dose penetrating the skin over a 32-hour period. At non-cytotoxic concentration aloesin probably acting as a competitive inhibitor on DOPA oxidation and as a non-competitive on tyrosine hydroxylase activity. Aloesin treatment showed pigmentation suppression in a dose – dependent manner; thus, aloesin might be used as an agent that inhibits melanin formation induced by UV radiation (25). In vivo, aloesin and arbutin co-treatment inhibits UV – induced melanogenesis in a synergistic manner.
The mixture of aloesin and arbutin showed a significant inhibition on tyrosinase activity of human melanocytes and reduced significantly melanin content, and had little influence on melanocytes viability (26).