A modified method for determining protein binding capacity of plant polyphenolics using radiolabelled protein

A modified radiochemical protein binding method for determining the protein binding capacity of plant polyphenolics (tannins) is described. Purified tannin or unfractionated plant extracts were immobilised on filter paper discs and incubated with the 125I-labelled bovine serum albumin. Protein bound to the disc was proportional to the amount of tannin applied to the disc, although at high concentrations of polyphenolics the discs became saturated and the relationship was no longer applicable. The method was validated using purified procyanidin from Sorghum grain and has been applied to crude polyphenolic extracts from maple, white oak, black oak, walnut and tulip poplar leaves. Specific chemical assays for the determination of proanthocyanidins (acid butanol method) and hydrolysable tannins (modified potassium iodate method) were employed to validate the new protein binding method with the complex plant extracts.