Identification of 2'-deoxyguanosine for an α-amylase inhibitor in the extracts of the earthworm Eisenia fetida and characterization of its inhibitory activity against porcine pancreatic α-amylase

Inhibitory activity (IA) against porcine pancreatic alpha-amylase (PPA) has been found in the water extract of Eisenia fetida. IA is recovered mostly in a fraction (U3EE) containing compounds of low molecular mass under 3 kDa. U3EE is treated with 85 % ethanol (EtOH) and the obtained EtOH extract is applied to the solid-phase extraction (SPE) system. An IA fraction named AI is separated from SPE by elution with water, and then a fraction AI* has been obtained by elution with 5 % EtOH. In the previous study, PPA inhibitors of guanine (Gua), guanosine (Guo), and inosine (Ino) have been isolated from AI. In the present study, IA of AI* was examined and a new inhibitor 2'-deoxyguanosine (2'-dGuo) was isolated by RP-HPLC. It was a mixed-type inhibitor showing IC50 of 0.7 +/- 0.1 mM and inhibitor constants Ki and Ki' of 2.0 +/- 0.5 and 0.9 +/- 0.2 mM, respectively, at pH 6 and at 37 degrees C. PPA inhibitors of U3EE were arranged in the order according to the inhibitory efficiency as Gua> 2'dGuo> Guo> Ino. This study reports that compounds related to nucleic acids could be PPA inhibitors. These inhibitors as well as U3EE might be suitable for functional foods to prevent obesity and type 2 diabetes.