Interconversion of the peptide isoforms of aspartate: stability of isoaspartates.

A common modification of human long-lived proteins is spontaneous isomerisation of aspartate residues, and its biological importance can be inferred from the ubiquitous presence of protein isoaspartate methyl transferase (PIMT), that repairs this damage. Cyclisation of L-Asp residues yields four isomers: L-Asp, L-isoAsp, D-Asp and D-isoAsp, however little is known about their rate of formation or interconversion. This is important because PIMT is inactive towards D-isoAsp.

Accelerated aging of Asp 58 in αA crystallin and human cataract formation.

Racemisation of amino acids is one of the most abundant modifications in long-lived proteins. In this study racemisation of Asp 58 in the small heat shock protein, αA crystallin, was investigated. In normal human lenses, levels of l-isoAsp, d-isoAsp and d-Asp increased with age, such that by age 70 they accounted for approximately half of the total Asp at this site.

Age-dependent racemization of serine residues in a human chaperone protein.

Racemization is one of the most abundant modifications in long-lived proteins. It has been proposed that the accumulation of such modifications over time could lead to changes in tissues and ultimately human age-related diseases. Serine is one of the main amino acids involved in racemization; however, the site of D-Ser in any aged protein has yet to be reported.

Age-dependent deamidation of glutamine residues in human γS crystallin: deamidation and unstructured regions.

Human aging is associated with the deterioration of long-lived proteins. Gradual cumulative modifications to the life-long proteins of the lens may ultimately be responsible for the pronounced alterations to the optical and physical properties that characterize lenses from older people. γS crystallin, a major human lens protein, is known to undergo several age-dependent changes.

Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine?

Since crystallins in the human lens do not turnover, they are susceptible to modification by reactive molecules over time. Methylation is a major post-translational lens modification, however the source of the methyl group is not known and the extent of modification across all crystallins has yet to be determined. Sites of methylation in human lens proteins were determined using HPLC/mass spectrometry following digestion with trypsin.

Racemization of two proteins over our lifespan: deamidation of asparagine 76 in γS crystallin is greater in cataract than in normal lenses across the age range.

Purpose: Long-lived proteins are widespread in man, yet little is known about the processes that affect their function over time, or their role in age-related diseases.

Methods: Racemization of two proteins from normal and cataract human lenses were compared with age using tryptic digestion and LC/mass spectrometry. Asp 151 in αA crystallin and Asn 76 in γS crystallin were studied.

Racemisation and human cataract. D-Ser, D-Asp/Asn and D-Thr are higher in the lifelong proteins of cataract lenses than in age-matched normal lenses.

ASTRACT: Several amino acids were found to undergo progressive age-dependent racemisation in the lifelong proteins of normal human lenses. The two most highly racemised were Ser and Asx. By age 70, 4.