Current status of age altered enzymes: alternative mechanisms.

The occurrence of inactive enzyme molecules in a variety of tissues and animal species has been shown to be of a general nature. The levels of inactive enzyme molecules found in old animals were produced by amino acid analogs in young animals. These levels have been shown to be initially detrimental but subsequently the young system shows recovery by efficiently disposing of the analog-modified proteins. In old animals this disposal is considerably less efficient. Evidence is presented which suggests that post-translational modifications of proteins are the main cause of enzyme inactivation in old animals. Amino acid substitutions and modifications involving charge differences apparently do not contribute significantly to this phenomenon.