The formation of altered enzymes in aging animals.

Evidence is accumulating which supports the idea that the alteration of enzymes is due to post-synthetic modifications rather than to sequence changes. Although unequivocal proof is still lacking, the idea that the changes in old enzymes are due to a subtle denaturation rather than to covalent changes is strengthened by recent work. That altered enzymes may result from a slowed protein turnover in old organisms appears to be a viable hypothesis.