Structural damage to proteins caused by free radicals: asessment, protection by antioxidants, and influence of protein binding

Oxidative damage to proteins results in biological dysfunctions such as per turbed activity in enzymes, transport proteins, and receptors. Here, we inv estigated structural damage to proteins induced by free radicals. Structura l alterations to lysozyme, human serum albumin (HSA) and beta -lactoglobuli n A were monitored by capillary zone electrophoresis. Four well-known antio xidants (quercetin, melatonin, Trolox, and chlorogenic acid) were examined for their ability to inhibit protein damage and to bind to these proteins. Melatonin and chlorogenic acid, which did not bind to any of the three prot eins under study, showed scavenging and protective activities well correlat ed with the amount of free radicals generated. Trolox, which bound only to HSA, was a better protector of HSA than of the two other proteins, indicati ng that its antioxidant capacity is increased by a shielding effect. Finall y, quercetin was a good antioxidant in protecting lysozyme and beta -lactog lobulin A. but its binding to HSA resulted in a pro-oxidant effect that acc elerated HSA fragmentation. These results demonstrate that binding of an an tioxidant to a protein may potentiate protection or damage depending on the properties of the antioxidant. (C) 2001 Elsevier Science Inc. All rights r eserved.