Antioxidant activity of protein-bound quercetin

Bovine serum albumin (BSA) was derivatized by covalent attachment of different amounts of quercetin [ratios of BSA to quercetin were 20:1, 10:1, 7:1, 5:1, and 2:1 (w/w)]. The antioxidant activity of the protein-phenol derivatives was investigated using a modified TEAC assay. The results show that the covalent attachment of quercetin to BSA decreases the total antioxidant activity in comparison to an equivalent amount of free quercetin depending on the degree of derivatization. The derivative with the highest amount of covalently bound quercetin (the 2:1 derivative) showed an antioxidant activity of only 79% compared to an equivalent amount of free quercetin. After the enzymatic proteolysis of the BSA-quercetin derivatives with trypsin, the total antioxidant activity of the degradation products increases in comparison to the respective undigested derivatives but does not reach the activity of an equivalent amount of free quercetin. Even after 240 min of tryptic degradation, there is still a lack in antioxidant activity (for the 7:1 derivative nearly 33%) as compared to free quercetin.