MODIFICATION OF PROTEIN SURFACE HYDROPHOBICITY AND METHIONINE OXIDATION BY OXIDATIVE SYSTEMS

Aging and some pathological conditions are associated with the accumul ation of altered (inactive or less active) forms of enzymes, It was su ggested 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 associ ated with changes in surface hydrophobicity, we have examined the effe cts of aging and oxygen radical-dependent oxidation on the hydrophobic ity of rat liver proteins, As a measure of hydrophobicity, the increas e in fluorescence associated with the binding of 8-anilino-1-naphthale ne-sulfonic acid to hydrophobic regions on the proteins was used, By t his criterion, the hydrophobicity of liver proteins of 24-month-old ra ts 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 hydr ophobicity, respectively, Treatment of liver proteins with the metal-c atalyzed oxidation system led to a significant increase in reactive ca rbonyl content and to conversion of methionine residues to methionine sulfoxide residues, Treatment with AAPH led also to oxidation of methi onine, tyrosine, and tryptophan residues and to the precipitation of s ome proteins, Dityrosine was detected in AAPH-treated protein, both th e precipitate and supernatant fraction, The oxidation-dependent increa se of hydrophobicity was correlated with an increase in the levels of methionine sulfoxide and dityrosine. These results suggest that oxidat ive modification of proteins may be responsible for the age-related in crease of protein surface hydrophobicity in vivo, and that the oxidati on of methionine by an oxidative system may be an important event for the change of protein conformation.