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[Aging at the molecular level exemplified by proteins].

During their life span, human proteins may undergo post-translational modifications that can be interpreted as manifestations of protein aging. The most frequently discussed modifications in that context are oxidation, glycation, deamidation, isomerization and racemization. Their pathophysiological relevance depends on the availability of repair mechanisms and on the protein turnover. Proteins with high turnover will be exchanged before molecular modifications become relevant. The most affected proteins are long-living and permanent proteins. Such proteins can be identified by the determination of their D-aspartic content (as a measure of an in vivo racemization and isomerization of aspartyl and asparaginyl residues). Using this method it could be demonstrated that numerous proteins in diverse human tissues are long-living or permanent and cannot escape post-translational modification by turnover. The human organism is confronted with an accumulation of "abnormal", post-translationally modified proteins during aging, especially in the extracellular space, but also at a cellular level. The exact pathophysiological dimension of post-translational modifications as manifestations of protein aging has to be further elucidated. However, they have been already discussed as relevant factors in the pathogenesis of diseases of old age.