Abstract

A series of 45 flavonoids and related compounds were tested as inhibitors of liver fluke cyclic nucleotide phosphodiesterase. Many were found to be potent inhibitors; seven were as potent or more potent than any inhibitor previously tested. The kinetics of six compounds spanning a wide range of activities were investigated and found to be competitive. The most potent inhibitors, cyanidin chloride and quercetin, had K_i values of 10 ± 3 µM and 13 ± 6 µM, respectively, approaching the K_m for CAMP (8 µM). Structure/ activity studies showed that adding exocyclic substituents to the basic flavonoid skeleton affected activity only slightly, while changing the planarity of the heterocyclic ring greatly decreased activity. This observation, taken with the competitive kinetics, suggests that flavonoids compete with cAMP for a nucleotide binding site at which stacking occurs, perhaps similar to the binding sites of bovine pancreatic ribonuclease A and lobster glyceraldehyde-3-phosphate dehydrogenase. Quantum chemical calculations further suggest that the competition arises from the mimicking of the pyrimidine ring in cAMP by the pyranone ring of the flavonoids. If the flavonoids are comparably potent inhibitors of other phosphodiesterases, several reported pharmacological effects of the flavonoids might be explained.