Isomerized aspartate residues in rodent eye lens protein act as a marker of biological aging

Abstract

Proteins in biological systems are known to accumulate spontaneous conformational alterations before breakdown and recycling. The racemization and isomerization of aspartic acid in proteins is one such modification that can result in loss of structure and function. Aspartic acid racemization specifically has been studied as a potential measure of chronological aging within low protein turnover mammalian tissues. However, very few of these studies have been optimized for shorter‐lived animals such as rodents, which are increasingly used as genetic and metabolic models of aging. In this study, we have developed a new sensitive approach for the measurement of biological age by taking advantage of the formation of isomerized aspartyl residues of the rodent eye lens. Using the human protein L‐isoaspartate/D‐aspartate methyltransferase (PCMT1) as an analytical reagent, we are able to rapidly and conveniently quantify the levels of L‐isoaspartate within lens protein extracts. We found the levels of isomerized aspartic acid to be about 35 times higher in the extracts of 18 month‐old rats versus two month‐old rats. Importantly, it was observed that the accumulation appeared to plateau in rats of 18 months and older, suggesting the presence of potentially novel mechanisms for the removal of altered proteins that may develop with age.Support or Funding InformationRuth L. Kirschstein National Research Service Award GM007185