Protein modification in aging.

The age-related accumulation of abnormal forms of enzymes is attributable to posttranslational modification of protein structure and to a progressive loss with age of proteases that preferentially degrade the modified forms. The protein modifications include, but are not limited to: the oxidation of amino acid side chains (especially, side chains of prolyl, arginyl, lysyl and histidinyl residues) by mixed-function oxidation systems; the deamidation of asparaginyl and glutaminyl residues; the racemization and isomerization of aspartyl and asparaginyl residues; the isomerization of prolyl residues; the oxidation of cysteine sulfhydryl groups; and spontaneous changes in protein conformation that are apparently unlinked to changes in amino acid composition. Evidence supporting the roles of these protein modifications and of the proteases that degrade abnormal enzymes during aging is discussed, as well as a consideration of some technical limitations of the methods used in their study.