Protein damage and degradation by oxygen radicals. III. Modification of secondary and tertiary structure.

Proteins which have been exposed to the hydroxyl radical (.OH) or to the combination of .OH plus the superoxide anion radical and oxygen (.OH + O2- + O2) exhibit altered primary structure and increased proteolytic susceptibility. The present work reveals that alterations to primary structure result in gross distortions of secondary and tertiary structure. Denaturation/increased hydrophobicity of bovine serum albumin (BSA) by .OH, or by .OH + O2- + O2 was maximal at a radical/BSA molar ratio of 24 (all .OH or 50% .OH + 50% O2-). BSA exposed to .OH also underwent progressive covalent cross-linking to form dimers, trimers, and tetramers, partially due to the formation of intermolecular bityrosine. In contrast, .OH + O2- + O2 caused spontaneous BSA fragmentation. Fragmentation of BSA produced new carbonyl groups with no apparent increase in free amino groups. Fragmentation may involve reaction of (.OH-induced) alpha-carbon radicals with O2 to form peroxyl radicals which decompose to fragment the polypeptide chain at the alpha-carbon, rather than at peptide bonds. BSA fragments induced by .OH + O2- + O2 exhibited molecular weights of 7,000-60,000 following electrophoresis under denaturing conditions, but could be visualized as hydrophobic aggregates in nondenaturing gels (confirmed with [3H]BSA following treatment with urea or acid). Combinations of various chemical radical scavengers (mannitol, urate, t-butyl alcohol, isopropyl alcohol) and gases (N2O, O2, N2) revealed that .OH is the primary species responsible for alteration of BSA secondary and tertiary structure. Oxygen, and O2- serve only to modify the outcome of .OH reaction. Furthermore, direct studies of O2- + O2 (in the absence of .OH) revealed no measurable changes in BSA structure. The process of denaturation/increased hydrophobicity was found to precede either covalent cross-linking (by .OH) or fragmentation (by .OH + O2- + O2). Denaturation was half-maximal at a radical/BSA molar ratio of 9.6, whereas half-maximal aggregation or fragmentation occurred at a ratio of 19.4. Denaturation/hydrophobicity may hold important clues for the mechanism(s) by which oxygen radicals can increase proteolytic susceptibility.