Tyrosinase inhibition and antioxidant properties of Asphodelus microcarpus extracts

Melanin is a pigment which plays an important role in the protection against UV damage and represents an important defense system of the skin against harmful factors. Despite its advantages, melanin is also involved in abnormal pigmentation and melanoma; therefore, different approaches to the study of skin disorders has been developed [1–3].

Tyrosinase (EC 1.14.18.1) is the key enzyme in the first two steps of melanin biosynthesis, catalyzing the hydroxylation L-tyrosine to the 3,4-dihydroxyphenylalanine (DOPA) and the oxidation of DOPA to dopaquinone.

Overproduction and accumulation of melanin occur in several skin disorders including solar melanosis, ephelides, melasma, senile lentigos and postinflammatory hyperpigmentation [4]. Since tyrosinase is the limiting step enzyme in melanogenesis, its inhibitors have become increasingly important as depigmenting agents in hyperpigmentation disorders.

However, currently available tyrosinase inhibitors suffer from toxicity and/or a lack of efficacy and there is a constant quest for better inhibitors from natural sources as they are expected to be free of harmful side effects [5, 6].

Several investigations have been done for identification of many naturally occurring substances in higher plants with antioxidant and other protective biochemical functions. The raw extracts or chemical constituents might be used for treatment of various human diseases [7]. Reactive oxygen species are considered as a major contributor to age-related symptoms and pathogenesis of many diseases. The skin is also vulnerable to oxidative stress, and exposure to repeated oxidative stress contributes to its aging [8]. Many synthetic antioxidants have potential hazards to health [9], therefore, there is a tendency to develop and utilize effective natural antioxidants to reduce the health risks.

In the traditional medicine, plants of genus Asphodelus are used to treat skin disorders, as well as ectodermal parasites, psoriasis, microbial infection and for lightening freckles [10, 11]. Various biological activities have been reported for A. microcarpus, [12, 13]. Phytochemical studies on A. microcarpus revealed the presence of lipids, carbohydrates, sterols, anthraquinones and arylcoumarins [13, 14]. It is well known that the last two compounds have tyrosinase inhibitory activity [15–17] and plant extracts with antimelanogenic activity typically possess polyphenols such as flavonoids, which are usually the factors responsible for the activities in plant extracts [18]. The aim of this study was to investigate the inhibitory activity of three different extracts of A. microcarpus on tyrosinase activity and on melanogenesis in B16F10 melanoma cells. In addition, total phenols, flavonoids contents and antioxidant capacities of the extracts have also been analyzed.