Rice Albumin N-Terminal (Asp-His-His-Gln) Prevents against Copper Ion-Catalyzed Oxidations

Abstract

Chromatographic separation of soluble proteins from rice (Oryza sativa L.) yielded a major albumin protein (16 kDa), with the DHHQVYSPGEQ sequence in the N terminus, showing antioxidant action. The rice albumin was more potent than other rice proteins in preventing Cu2+-induced low-density lipoprotein (LDL) oxidation. Additionally, it also exhibited a remarkable suppression of HOCl oxidation. In a further study, albumin inhibited Cu2+-induced oxidation of LDL in a stoichiometric manner with an EC50 value of 4.3 microM, close to that of serum albumins. Moreover, after digestion with trypsin or chymotrypsin, it maintained its antioxidant action. In an experiment to see the involvement of the N terminus in antioxidant action, a synthetic tetrapeptide, equivalent to the N terminus DHHQ, was found to inhibit Cu2+-induced LDL oxidation or degradation of apolipoprotein B, similar to that of rice albumin. In mechanistic analyses, the action of rice albumin or tetrapeptide is primarily due to the removal of Cu2+, as suggested from its inhibitory effect on Cu2+/diphenylcarbohydrazide (DPCH) complex formation. However, despite its similar inhibitory effect on Cu2+-induced oxidation of LDL, rice albumin was less effective than serum albumin in inhibiting Cu2+/DPCH complex formation, suggesting that the number of Cu2+-binding sites in rice albumin may be less than that in serum albumins. Taken together, rice albumin exerts a potent preventive action against Cu2+-induced oxidations, which is due to the Cu2+ binding by DHHQ in the N-terminal sequence. Such a role as a Cu2+ chelator would add up to the application of rice albumin protein.