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

Figures & data

Figure 1.  Chemical structures of isolated compounds 1-5 in stem barks of M. lhou. Arrows indicate HMBC correlations.

Table I.  Inhibitory effects of isolated compounds 1-5 on mushroom tyrosinase activities.

	IC50 (μM) values^†
Compound	l–tyrosine	l–DOPA	Inhibition type	Kinetic parameters (μM)
1	87.8	214.8	Competitive	Ki = 49.2
2	91.6	160.8	Competitive	Ki = 46.4
3	>250.3	>300	NT^‡	NT
4	134.9	>300	Competitive	Ki = 80.5
5	1.2	>300	Competitive	Ki = 0.61
Kojic acid	12.5	NT	NT	NT

^† All compounds were examined in a set of experiments repeated three times; IC₅₀ values of compounds represent the concentration that caused 50% enzyme activity loss

^‡ Not tested.

Figure 2.  Effect of compounds (1, 2, 4, and 5) on the activity of tyrosinase for the catalysis of l–tyrosine at 30 °C.

Figure 3.  Relationship of the catalytic activity of mushroom tyrosinase with the enzyme concentrations at different concentrations of compound 4. Concentrations of compound 4 for curves 0-4 were, 0, 40, 80, 120, and 160 μM, respectively.

Figure 4.  Time course of oxidation of l–tyrosine catalyzed by mushroom tyrosinase in the presence of compounds (1-5) for curves. Conditions were as follows: 180 μM tyrosine, 0.25 M phosphate buffer, pH 6.8, not preincubated at 30 °C. (A) Concentrations of compounds 1, 2, and 4 were 80 μM, 50 μM, and 100 μM. (B) Concentrations of compounds 3 and 5 were 100 μM and 25 nM, respectively.

Figure 5.  Lineweaver-Burk plots for inhibition of compounds 1, 2, 4, and 5 on mushroom tyrosinase for the catalysis of l–tyrosine. Conditions were as follows: 180 μM l–tyrosine, 144 units tyrosinase, 0.25 M phosphate buffer (pH 6.8), at 30 °C. In the presence of different concentrations of compounds for curves from bottom to top were, (A) for compound 1: 0, 60, and 160 μM; (B) for compound 2: 0, 41.7, 83.3, and 166.7 μM; (C) for compound 4: 0, 83.3, 166.7, and 250 μM; (D) for compound 5: 0, 0.6, 1.2, and 2.4 μM.

Figure 6.  Time-dependent inhibition of tyrosinase in the presence of norartocarpetin (5). (A) Conditions were as follows: 180 μM l–tyrosine, 144 units tyrosinase, and concentrations of norartocarpetin for curves from top to bottom were 0, 9.76, 19.53, 39.06, and 78.12 nM. The k_obs values at each inhibition concentration were determined by fitting the data to Equation (2). (B) Dependence of the values for k_obs on the concentration of norartocarpetin. The k_obs values, determined in panel A, were fitted to Equation (3).