Flavonoids from Dalea boliviana as tyrosinase inhibitors

PERALTA, MARIANA; SANTI, MD; AGNESE, MARIEL; ORTEGA, GABRIELA; CABRERA, JOSE LUIS

Cordoba

Otro; Primera reunión internacional de Ciencias Farmacéuticas (RICIFA); 2010

Docentes investigadores UNC y UNR

Introduction The melanin formation is one of the most important factors to determine the mammalian skin colour. Tyrosinase is a mixed function oxidase enzyme which catalyzes two different reactions in the synthesis of melanin: the formation of 3, 4-dihydroxyphenylalanine (L-DOPA) from tyrosine and further oxidizes L-DOPA to dopaquinone. Tyrosinase inhibitors may be of important to treat abnormal pigmentation disorders and to use as skin whitening agents in cosmetics.(1) Our research group began the chemical and pharmacological study of Dalea boliviana. We previously report the isolation and structural characterization of three new prenylated flavanones from the hexanes extract of D. boliviana roots: (2S) - 5, 7, 2 ´ - trihydroxy-5 ´-(1´´´,1´´´-dimethylallyl)-8-prenylflavanone (1), (2S) - 5, 7, 2´ - trihydroxy-8, 3 ´ - diprenylflavanone (2), and (2S) - 5, 2 ´ - dihydroxy-6 ", 6"- dimethylchromeno-(7,8:2´´,3´´) - 3 ´ - prenylflavanone (3), together with a known chromeno (dimethylpyrano) flavanone, obovatin (4).(2,3,4) Taking into account that similar compounds have been related to inhibition of tyrosinase activity, (5,6,7) we propose to evaluate compounds 1-4 as tyrosinase inhibitors. Materials and methods Plant material D. boliviana Britton was collected in February 2007, near Iturbe in Humahuaca Department, Jujuy province, Argentina. A voucher specimen is on deposit at the Botanical Museum - UNC as CORD 1066. The plant material was dried at room temperature and the roots (60 g) were separated from the aerial parts, powdered and extracted with hexanes (250 ml) at room temperature for 24 h. From 3g of hexane crude extract, by the application of chromatographic techniques (column and TLC), (2S) - 5, 7, 2 ´ - trihydroxy-5 ´-(1´´´,1´´´-dimethylallyl)-8-prenylflavanone (1, 25 mg), (2S) - 5, 7, 2 ´ - trihydroxy-8, 3 ´ - diprenylflavanone (2, 14 mg), (2S) - 5, 2 ´ - dihydroxy-6, 6 - dimethylchromeno-(7,8:2´´,3´´) - 3 ´ - prenylflavanone (3, 20 mg), and obovatin (4, 6 mg) were isolated.(4) Tyrosinase activity assay It was performed according to the method of Rahman et al. (8) adapted to work conditions. L - tyrosine (1.7mM) was used as a substrate of the enzyme and tyrosinase from mushroom (250U/mL) was the enzyme source. Activity was measured spectrophotometricaly at 475nm and 25 °C. Different concentrations of compounds 1, 2, 3 or 4 (25-100 μM) were evaluated. Results were expressed as percentages of inhibition using the following equation: % inhibition = [(Abscontrol − Abssample)/Abscontrol] × 100, where Abscontrol is the absorbance of the control and Abssample is the absorbance of the experimental sample. The IC50 values were estimated by using non-linear fitting of concentration-response data. Results As a result, 1 and 2 showed inhibitory activity with IC50 of 27.1 μM and 68.5 μM, respectively. On the other hand, 3 and 4 demonstrated a very low activity on tyrosinase enzyme even at 100 μM . Kojic acid(1) was used as positive control (IC50= 10.2 μM). Conclusions The present work reports the effect on tyrosinase activity of compounds 1-4, isolated from the hexanes extract of D. boliviana roots. They exhibited different capacities being 1 and 2 much better inhibitors than 3 and 4. It is important to highlight that the first two compounds present a 4-substituted resorcinol moiety at A ring, which has been demonstrated to be essential for inhibitory activity against tyrosinase activity(1). On contrary, 3 and 4 were weak inhibitors probably as a consequence of a cyclization of C-8 isoprenyl substitution, in agreement with Lee et al. (1) findings.