Dityrosine and tyrosine oxidation products are endogenous markers for the selective proteolysis of oxidatively modified red blood cell hemoglobin by (the 19 S) proteasome.

Abstract

Cells exposed to oxidative stress have been shown previously to exhibit both protein oxidation and increased proteolysis. Experiments conducted with purified proteins in vitro have indicated that oxidatively modified proteins may be selectively degraded by intracellular proteases, but a definitive cause-and-effect relationship has not been demonstrated previously in intact cells. Several investigators have proposed that oxidatively modified proteins are selectively degraded within cells, but the possibility that oxidants may activate intracellular proteases (directly or indirectly) to catalyze the indiscriminate degradation of undamaged proteins has not been discounted. Armed with the knowledge that dityrosine is a specific product of protein oxidation, we undertook a series of experiments to test the hypothesis that oxidized proteins undergo selective intracellular degradation. Our results demonstrate that dityrosine is produced in the hemoglobin molecule when red blood cells are exposed to a continuous flux of hydrogen peroxide (H2O2). The dityrosine so produced is only released from the hemoglobin by proteolysis and is stable to prolonged incubation with cell extracts. Inhibitors of proteolysis have no effect on dityrosine production but do effectively prevent dityrosine release. Proteasome (the 670-kDa multicatalytic proteinase complex, that we have previously called macroxyproteinase or MOP (Pacifici, R. E., Salo, D. C., and Davies, K. J. A. (1989) Free Radical Biol. & Med. 7, 521-526; Salo, D. C., Pacifici, R. E., Lin, S. W., Giulivi, C., and Davies, K. J. A. (1990) J. Biol. Chem. 265, 11919-11927; Pacifici, R. E., and Davies, K. J. A. (1991) Gerontology 37, 166-180) appears responsible for dityrosine release during the selective degradation of oxidatively modified proteins in red blood cells and red cell extracts. We conclude that the elevated rates of proteolysis observed in response to oxidative stress do, indeed, reflect selective degradation of oxidatively modified (damaged) proteins. Despite a relatively low production rate, dityrosine has a high fluorometric quantum yield and is, of course, a specific product of protein oxidation. As an apparently stable metabolic end product, dityrosine may prove to be an extremely valuable (cellular or urinary) marker or index of organismal oxidative stress.