PLANT VOLATILE SECONDARY METABOLITES: CITRAL CONTAINING ESSENTIAL OILS AS POTENTIAL TYROSINASE INHIBITORS

Tyrosinase is a copper containing enzyme characteristic of several bacteria fungi, animals, and plants. In human, tyrosinase is the key enzyme in the biosynthetic pathway of melanin, the biological pigment found in hair, skin and in the eye iris where it plays a crucial role in the absorption of free radicals and in the protection of the cell DNA from ionizing radiations. Genetic conditions, exogenous causes (i.e. exposure to UV light or to certain drugs and chemicals) and physiological processes (i.e. aging) can significantly increase melanin production leading to minor aesthetic problems, such as freckles and solar lentigo, as well as to serious dermatological conditions including cancer and post inflammatory melanoderma. The downregulation of tyrosinase is a very widespread approach to reduce the excessive melanin production and tyrosinase inhibitors as skin whitening agents are gaining significant prominence clinically and cosmetically. Plant extracts have often revealed to be valuable sources of tyrosinase inhibitors as three out five of the most medically/cosmetically employed tyrosinase inhibitors are plant secondary metabolites (i.e. Hydroquinone, -arbutin and aloesin). Up to date, phenolic compounds have mostly been investigated as potential tyrosinase inhibitor while fewer studies have evaluated the tyrosinase inhibition activity of plant volatile terpenoids. Citral (a mixture of two isomers, cis- and trans-3,7-dimethyl-2,6-octadienal in the typical 1/3 and 2/3 ratio) has proven to be a potential tyrosinase inhibitor as it blocks the enzymatic activity of mushroom tyrosinase, a fungal source of tyrosinase employed for preliminary and high throughput screenings as it is relatively cheap and readily available. Citral is an important fragrance ingredient present in considerable amount in the essential oils obtained from different botanical species including Cymbopogon schoenanthus (L.) Spreng., Litsea cubeba (Lour.) Pers., Melissa officinalis L. and Verbena officinalis L. To the best of author knowledge, only L. cubeba EO has been investigated for its tyrosinase inhibitory activity. This study aims at evaluating the in-vitro tyrosinase inhibitory activity of Cymbopogon schoenanthus, Litsea cubeba, Melissa officinalis and Verbena officinalis EOs to assess whether the different chemical composition may influence the EO overall inhibitory activity due to possible synergistic and/or competitive interactions among their components. Different concentrations of citral were in-vitro tested to confirm its biological activity and to identify the range in which tyrosinase inhibition percentage increases proportionally to citral concentration. The investigated EOs were first chemically characterized by gas chromatography-mass spectrometry to determine citral concentration in each EO by the external standard method. The tyrosinase inhibitory activity of the investigated EOs was then measured testing a suitable amount of EO to set citral final concentration within the afore mentioned linearity range. The inhibitory activity of M. officinalis EO was in-line with its citral composition while the other investigated EOs showed different trends. C. schoenanthus and L. cubeba EOs contained almost the same amount of citral although the tyrosinase inhibitory activity of L. cubeba EO was double that of C. schoenanthus and in both cases the inhibition percentage was not proportional to citral concentration; similar consideration were done for V. officinalis EO, whose activity was higher than expected from its content of citral. These preliminary results suggest that C. schoenanthus, L. cubeba and M. officinalis EO tyrosinase inhibitory activity is only partially ascribed to citral and further studies are under way to identify the other bioactive components contributing to the investigated activity through a bio-guided fractionation approach.