Protein damage and degradation by oxygen radicals. I. general aspects.

Aggregation, fragmentation, amino acid modification, and proteolytic susceptibility have been studied following exposure of 17 proteins to oxygen radicals. The hydroxyl radical (.OH) produced covalently bound protein aggregates, but few or no fragmentation products. Extensive changes in net electrical charge (both + and -) were observed. Tryptophan was rapidly lost with .OH exposure, and significant production of bityrosine biphenol occurred. When incubated with cell-free extracts of human and rabbit erythrocytes, rabbit reticulocytes, or Escherichia coli, most .OH-modified proteins were proteolytically degraded up to 50 times faster than untreated proteins. The exceptions were alpha-casein and globin, which were rapidly degraded without .OH modification. ATP did not stimulate the degradation of .OH-modified proteins, but alpha-casein was more rapidly degraded. Leupeptin had little effect under any condition, and degradation was maximal at pH 7.8. The data indicate that proteins which have been denatured by .OH can be recognized and degraded rapidly and selectively by intracellular proteolytic systems. In both red blood cells and E. coli, the degradation appears to be conducted by soluble, ATP-independent (nonlysosomal) proteolytic enzymes. In contrast with the above results, superoxide (O2-) did not cause aggregation or fragmentation, tryptophan loss, or bityrosine production. The combination of .OH + O2- (+O2), which may mimic biological exposure to oxygen radicals, induced charge changes, tryptophan loss, and bityrosine production. The pattern of such changes was similar to that seen with .OH alone, although the extent was generally less severe. In contrast with .OH alone, however, .OH + O2- (+O2) caused extensive protein fragmentation and little or no aggregation. More than 98% of the protein fragments had molecular weights greater than 5000 and formed clusters of ionic and hydrophobic bonds which could be dispersed by denaturing agents. The results indicate a general sensitivity of proteins to oxygen radicals. Oxidative modification can involve direct fragmentation or may provide denatured substrates for intracellular proteolysis.