Site-specific modification of albumin by free radicals. Reaction with copper(II) and ascorbate

Abstract

Exposure of albumin to Cu(II) (10-100 microM) and ascorbate (0.1-2 mM) results in extensive molecular modifications, indicated by decreased fluorescence and chain breaks. The rate of utilization of molecular oxygen and ascorbate as a function of Cu(II) concentration is non-linear at copper/albumin ratios of greater than 1. It appears that Cu(II) bound to the tightest albumin-binding site is less available to the ascorbate than the more loosely bound cation. SDS/polyacrylamide-gel electrophoresis reveals new protein bands corresponding to 50, 47, 22, 18 and 3 kDa. For such a cleavage pattern, relatively few (approximately 3) and rather specific chain breaks occurred. Repeated addition of portions of ascorbate to the albumin/Cu(II) mixture results in increased intensity of the new bands. The absence of Cu(II) or the presence of metal chelating agents is inhibitory. There was no evidence of intermolecular cross-linking or of the formation of insoluble, albumin-derived, material. A mechanism is proposed wherein the loosely bound Cu(II) participates in a Fenton-type reaction. This generates OH. radicals, which rapidly inter-react with the protein and modify it in a 'site-specific' manner.