Exposure of proteins to the hydroxyl radical (.OH) or to the combination of .OH plus the superoxide anion radical (.OH + O2-) causes gross structural modification. Such modified proteins can undergo spontaneous fragmentation or can exhibit substantial increases in proteolytic susceptibility. In the present study, with the representative protein bovine serum albumin (BSA), we report that alterations to primary structure underlie such gross structural modifications. All amino acids in BSA were susceptible to modification by both .OH and .OH + O2- +O2), although tryptophan, tyrosine, histidine, and cysteine were particularly sensitive. At a radical/BSA molar ratio (nmol of radicals/nmol of BSA) of 10, we observed an average 9-10% destruction of amino acids; whereas at a ratio of 100, the average loss was 45%. Decreasing tryptophan fluorescence provided a useful index of amino acid loss and exhibited a clear dose dependence with .OH or with .OH + O2- (+O2). Linear production of the biphenol bityrosine was observed with .OH treatment. In contrast, .OH + O2- (+O2) induced only a limited bityrosine production rate which reached an early plateau. Studies with various chemical scavengers (t-butyl alcohol, isopropyl alcohol, mannitol, urate) and gasses (N2O, N2, O2, air) revealed that .OH is the primary radical responsible for all amino acid modifications, but that O2- and O2 can further transform the products of .OH reactions. Thus, O2-/O2 can potentiate .OH-dependent destruction of many amino acids (e.g. tryptophan) while inhibiting production of bityrosine by reacting with tyrosyl (phenoxyl) radicals. No amino acid loss or bityrosine production occurred with exposure to O2- (+O2) alone. Amino acid modifications caused both by .OH alone and by .OH + O2- (+O2) progressively affected the overall electrical charge of BSA. In a pH range of 3.7-6.2, some 16 new isoelectric focusing bands were induced by .OH, and some eight new bands were induced by .OH + O2- (+O2). The alterations to primary structure observed provide the key to an understanding of the link between oxidative modification and increased proteolytic susceptibility.