Inhibitory effects on mushroom tyrosinase by flavones from the stem barks of Morus lhou (S.) Koidz

Abstract

Five flavones displaying tyrosinase inhibitory activity were isolated from the stem barks of Morus lhou (S.) Koidz., a cultivated edible plant. The isolated compounds were identified as mormin (1), cyclomorusin (2), morusin (3), kuwanon C (4), and norartocarpetin (5). Mormin (1) was characterized as a new flavone possesing a 3-hydroxymethyl-2-butenyl at C-3. The inhibitory potencies of these flavonoids toward monophenolase activity of mushroom tyrosinase were investigated. The IC₅₀ values of compounds 1–5 for monophenolase activity were determined to be 0.088, 0.092, 0.250, 0.135 mM, and 1.2 μM, respectively. Mormin (1), cyclomorusin (2), kuwanon C (4) and norartocarpetin (5) exhibited competitive inhibition characteristics. Interestingly norartocarpetin (5) showed a time–dependent inhibition against oxidation of l–tyrosine: it also operated under the enzyme isomerization model (k₅ = 0.8424 min^{− 1}, k₆ = 0.0576 min^{− 1}, = 1.354 μM).

Keywords::

• Morus lhou (S.) Koidz. stem barks
• tyrosinase
• norartocarpetin
• time-dependent inhibition

Acknowledgements

This work was supported by Technology Development Program for Agricultural and Forestry, Ministry of Agriculture and Forestry, and the MOST/KOSEF to the Environmental Biotechnology National Core Research Center (grant R15-2003-012-02001-0), Republic of Korea. Ryu, Y.B. was supported by a grant from the BK 21 program.