Illek B, Lizarzaburu M E, Lee V, Nantz M H, Kurth M J, Fischer H
Children's Hospital Oakland Research Institute, Oakland 94609, California, USA.
Am J Physiol Cell Physiol. 2000 Dec;279(6):C1838-46. doi: 10.1152/ajpcell.2000.279.6.C1838.
Apigenin (4',5,7-trihydroxyflavone) is an activator of cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl(-) currents across epithelia at low concentrations and a blocker at high concentrations. We determined the roles of structural components of apigenin for both stimulation and block of Cl(-) currents across Calu-3 epithelia. The half-maximal binding affinity of apigenin for current stimulation (K(s)) was 9.1 +/- 1.3 microM, and the rank-order of molecular structures was 7-hydroxyl > pyrone = 4'-hydroxyl > 5-hydroxyl. Both the 7-hydroxyl and the 4'-hydroxyl served as H-bond acceptors, whereas the 5-hydroxyl was an H-bond donor. The half-maximal binding affinity of apigenin during current block was 74 +/- 11 microM. Blocked Cl(-) currents were structurally determined by 7-hydroxyl = 4'-hydroxyl > pyrone > 5-hydroxyl. Prestimulation of tissues with forskolin significantly affected activation kinetics and binding characteristics. After forskolin stimulation, K(s) was 4.1 +/- 0.9 microM, which was structurally determined by pyrone > all hydroxyls > single hydroxyls. In contrast, block of Cl(-) current by apigenin was not affected by forskolin stimulation. We conclude that apigenin binds to a stimulatory and an inhibitory binding site, which are distinguished by their affinities and the molecular interactions during binding.
