Interaction of polyphenolic metabolites with human serum albumin: a circular dichroism study.

Binding sites of polyphenolic compounds on human serum albumin (HSA) were investigated using induced Cotton effects on the circular dichroism (CD) spectra. Polyphenolic compounds used in this study are known to be metabolites from tannins and their related polyphenols in food and medicinal plants. The present investigation revealed that the structural differences markedly affected the binding of the compounds to HSA. Protocatechuic acid, together with its methylated compounds vanillic and isovanillic acids, were assigned to be bound to sites I and II of HSA, based on the competitive relationships with site-I-binding phenylbutazone (PB) and site-II-binding diazepam (DP). 4-O-Methylgallic acid, which is the metabolite from gallic acid, was bound to site I on HSA, while gallic acid did not affect the binding of PB and DP at the concentration examined. Neither ellagic acid nor its metabolite urolithin A was competitive with PB and DP on HSA. The addition of digitoxin did not affect the induced CD of the polyphenolic acids examined.