Aging and the alteration of enzymes: a review.

The present status of research on age-related alterations of enzymes is reviewed. Evidence which makes it clear that "altered" enzymes do, in fact, exist in old animals is presented. The alteration may result from either a sequence change or a post-synthetic modification. The "error hypothesis" would support the former mechanism; a proposal based upon increasing half-lives of proteins with age could explain the latter. Evidence supporting or contradicting these alternatives is discussed.