Inhibition of human placenta aldose reductase by tannic acid

Tannic acid was found to be a highly potent inhibitor of human placenta aldose reductase. The most potent inhibitory component of the tannic acid was isolated and identified as penta-O-galloyl-beta-D-glucose, which showed an IC50 value of 70 nM. The inhibition by the gallotannin was reversible and of mixed type with respect to DL-glyceraldehyde as the varied substrate.