The level of oxidatively modified protein and the enzymatic carboxyl methylation have been investigated in the isolated erythrocyte membrane of different ages separated by density gradient centrifugation. The level of 2, 4-dinitrophenylhydrazine (DNPH)-reactive membrane protein was increased according to the cell aging and the isolated membrane protein showed an age-related ability to act as a methyl accepting substrate.
When the isolated membrane protein had been incubated with 50 mM Tris-HC1, pH 74, containing 5 mM dithiothreitol and 50 uM FeCb, the amounts of DNPH-reactive protein and lipid peroxidation were shown as 255% and 364% of control, respectively. But the substrate efficiency of oxidized membrane protein for protein methylase II was decreased significantly in comparison with the nonoxidized membrane protein. The pH effect of the oxidizing buffer system on the decrease of substrate efficiency was shown to be maximum around pH 7.4 and substrate efficiency was sensitive to the concentration of ferric chloride. As the oxidation of membrane protein had been progressed for 20, 60 or 180 min the substrate efficiency of the oxidized protein were decreased to 80.4, 73.5 or 64.6% of control respectively.
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