General Properties

Ellagic acid is a cream-colored powder slightly soluble in water and ethanol, in alkaline solution and pyridine, and practically insoluble in ether [4]. EA has high antioxidant activity [5], and is listed as a food additive in Japan. The hydroxyl groups of EA can chelate with metal ions [6,7].

IN VITRO STUDIES I

Ellagic acid inhibits mushroom-derived tyrosinase competitively and in a dose-dependent manner; the inhibition constant (ki) is 81.6 |M [8]. The decrease in copper concentration s and the reduction in tyrosinase activity by EA follow almost parallel patterns. Tyrosinase do

Figure 1 Ellagic acid.

Figure 1 Ellagic acid.

activity, after inhibition by EA, partially recovers after addition of cuprous or cupric ion (Fig. 2).

Growth of B16 melanoma cells in culture medium was not suppressed by EA at concentrations of less than 4 |M. At 4 |M, the inhibition of tyrosinase activity was 38.3% and the decrease in melanin concentration 54.4%. Although the color of the cells (reflecting the melanin concentration) became whitened in the presence of EA, cell color reverted to the original shade when EA was removed from the culture medium (Fig. 3). The addition of other metals, in place of the copper compounds, did not lead to recovery of the enzymic activity.

These results show that the inhibitory effect of EA is reversible, effective only in its presence, and specific to copper compounds. It is proposed that EA chelates to copper ion(s) at the active center of tyrosinase, which is a metaloprotein containing copper. Further structural changes then make the tyrosinase inactive. Because the molecular structure of EA is planar, EA may be able to penetrate into the active center of tyrosinase easily. It is clear that EA inhibits tyrosinase because of its molecular structure as well as its ability to chelate with copper.

Figure 2 Effects of addition of copper ion on recovery of tyrosinase activity. Cu+ or Cu+ (5 mM) were added to tyrosinase during inhibition by EA.

Figure 3 Effect of EA on melanoma cells. Cells were incubated with EA (4 uM) for 48 hours. Culture medium was changed to fresh medium in the presence or absence of EA (4 uM) and incubated for an additional 48 hours.

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