Green tea polyphenols: novel irreversible inhibitors of dopa decarboxylase.

The green tea gallocatechins, (-)-epigallocatechin-3-O-gallate (EGCG), and (-)-epigallocatechin (EGC) were found to be inhibitors of Dopa decarboxylase (DDC). EGCG and EGC inactivate the enzyme in both a time- and concentration-dependent manner and exhibit saturation of the rate of inactivation at high concentrations, with efficiency of inactivation values (k(inact)/K(i)) of 868 and 1511 M(-1) min(-1), respectively. In contrast, gallic acid behaves as a weak inhibitor of DDC. Protection against inactivation by EGCG and EGC was observed in the presence of the active site-directed inhibitor D-Dopa. Either EGCG or EGC induce changes in the absorbance and CD bands of the visible spectrum of enzyme-bound PLP. Taken together, these findings indicate the active site nature of the interaction of DDC with both polyphenols. On the basis of the properties of the EGCG-inactivated enzyme, it can be suggested that inactivation could be ascribed to a covalent modification of not yet identified residue(s) of the active site of DDC.