Persistent protein damage despite reduced oxygen radical formation in the aging rat brain

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Abstract
The relation between cerebral oxygen radicals and the aging process was investigated in crude synaptosomal (P2) fractions from rats. The rate of formation of oxygen radicals was measured using the probe 2',7'-dichlorofluorescein diacetate (DCFH-DA), which is de-esterified and subsequently oxidized by oxygen radicals to a fluorescent product 2',7'-dichlorofluorescein (DCF). There was a significant age-dependent decrease in the formation rate of oxygen radicals, observed by decreased formation of DCF. No difference in oxygen radical formation was apparent between age groups following an in vitro challenge with an ascorbate/FeSO4 mixture. This age-dependent decrease in cerebral oxygen radical generation coincided with age-dependent increases in superoxide dismutase. No age-related alterations in lipid order in either the hydrophilic or lipophilic membrane regions were observed using fluorescence polarization analysis. Age-dependent losses in cerebral P2 tryptophan fluorescence (a measure of protein degradation), and increased liberation of [14C]protein fragments into the acid-soluble fraction (a measure of overall proteolytic activity) were observed. Results suggest that aging does not proceed as a result of elevated rates of generation of oxygen radicals, a finding that does not support the proposed free radical theory of aging. The observed age-dependent decrease in the formation of oxygen radicals does not effect membrane lipid order. These findings implicate modifications in proteins and activated protein catabolic pathways as major contributing factors in the normal physiological process of senescence.