Protein damage and degradation by oxygen radicals. IV. Degradation of denatured protein.

Proteolytic degradation of oxidatively damaged [3H] bovine serum albumin [( 3H]BSA) was studied during incubation with cell-free erythrocyte extracts and a wide variety (14) of purified proteases. [3H]BSA was pretreated by exposure (60Co radiation) to the hydroxyl radical (.OH), the superoxide anion radical (O2-), or the combination of .OH + O2- + oxygen. Treated (and untreated) samples were dialyzed and then incubated with erythrocyte extract or proteases for measurements of proteolytic susceptibility (release of acid-soluble counts). Both .OH and .OH + O2- + caused severalfold increases in proteolytic susceptibility (with extract and proteases), but O2- alone had no effect. Proteolytic susceptibility reached a maximum at 15 nmol of .OH/nmol of BSA and declined thereafter. In contrast, proteolytic susceptibility was still increasing at an .OH + O2-/BSA molar ratio of 100 (50% .OH + 50% O2-). Degradation in erythrocyte extracts was conducted by a novel ATP- and Ca2+-independent pathway, with maximal activity at pH 7.8. Inhibitor profiles indicate that this pathway may involve metalloproteases and serine proteases. Comparisons of proteolytic susceptibility with multiple modifications to BSA primary, secondary, and tertiary structure revealed a high correlation (r = 0.98) with denaturation/increased hydrophobicity by low concentrations of .OH. Covalent aggregation reactions (BSA cross-linking) may explain the declining proteolytic susceptibility observed at .OH/BSA molar ratios greater than 20. Protein denaturation may also have caused the increased proteolytic susceptibility induced by .OH + O2- + O2, but no simple correlation could be obtained. Results with .OH + O2- + O2 appear to have been complicated by direct BSA fragmentation reactions involving (.OH-induced) protein radicals and oxygen. These data indicate a direct and quantitative relationship between protein damage by oxygen radicals and increased proteolytic susceptibility. Oxidative denaturation may exemplify a simple, yet effective inherent mechanism for intracellular proteolysis.