Modification of protein surface hydrophobicity and methionine oxidation by oxidative systems

Aging and some pathological conditions are associated with the accumulation of altered (inactive or less active) forms of enzymes. It was suggested that these age-related alterations reflect spontaneous changes in protein conformation and/or posttranslational modifications (e.g., oxidation). Because changes in protein conformations are often associated with changes in surface hydrophobicity, we have examined the effects of aging and oxygen radical-dependent oxidation on the hydrophobicity of rat liver proteins. As a measure of hydrophobicity, the increase in fluorescence associated with the binding of 8-anilino-1-naphthalene-sulfonic acid to hydrophobic regions on the proteins was used. By this criterion, the hydrophobicity of liver proteins of 24-month-old rats was 15% greater than that of 2-month-old animals. Exposure of liver proteins to a metal-catalyzed oxidation system (ascorbate/Fe(II)/H2O2) or a peroxyl radical generating system, 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) led to increases of 2% or 30% in surface hydrophobicity, respectively. Treatment of liver proteins with the metal-catalyzed oxidation system led to a significant increase in reactive carbonyl content and to conversion of methionine residues to methionine sulfoxide residues. Treatment with AAPH led also to oxidation of methionine, tyrosine, and tryptophan residues and to the precipitation of some proteins. Dityrosine was detected in AAPH-treated protein, both the precipitate and supernatant fraction. The oxidation-dependent increase of hydrophobicity was correlated with an increase in the levels of methionine sulfoxide and dityrosine. These results suggest that oxidative modification of proteins may be responsible for the age-related increase of protein surface hydrophobicity in vivo, and that the oxidation of methionine by an oxidative system may be an important event for the change of protein conformation.