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

Abstract

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, 4.5% of all Ser residues had been racemised, along with >9% of Asx residues. Such a high level of inversion, equivalent to between 2 and 3 D- amino acids per polypeptide chain, is likely to induce significant denaturation of the crystallins in aged lenses. Thr, Glx and Phe underwent age-dependent racemisation to a smaller degree. In model experiments, D- amino acid content could be increased simply by exposing intact lenses to elevated temperature. In cataract lenses, the extent of racemisation of Ser, Asx and Thr residues was significantly greater than for age-matched normal lenses. This was true, even for cataract lenses removed from patients at the earliest ages where age-related cataract is observed clinically. Racemisation of amino acids in crystallins may arise due to prolonged exposure of these proteins to ocular temperatures and increased levels of racemisation may play a significant role in the opacification of human lenses.

Fig. 1

HPLC profile of amino acids in the acid digest of proteins from a 65-year-old human cataract lens

Fig. 2

Racemisation of Asx as a function of age, in normal and cataract lens proteins. Racemisation expressed as % . (normal; , R² = 0.916, two-sided P < 0.0001: cataract;, R² = 0.491, two-sided P = 0.0111). Mann–Whitney U test, two-sided P = 0.0067

Fig. 3

Racemisation of Ser as a function of age in normal and cataract lens proteins. Racemisation expressed as % . (normal; , R² = 0.901, two-sided P < 0.0001: cataract; , R² = 0.194, two-sided P = 0.152). Mann–Whitney U test, two-sided P = 0.0044

Fig. 4

Racemisation of Thr as a function of age in normal and cataract lens proteins. Racemisation expressed as % . (normal;, R² = 0.482, two-sided P = 0.0007: cataract; , R² = 0.0341, two-sided P = 0.566). Mann–Whitney U test, two-sided P = 0.016

Fig. 5

Racemisation of Glx as a function of age, in normal lens proteins. Racemisation expressed as % . (normal; , R² = 0.636, two-sided P < 0.0001)

Fig. 6

Racemisation of Phe as a function of age, in normal lens proteins. Racemisation expressed as % . (normal; , R² = 0.581, two-sided P < 0.0001)

Fig. 7

Racemisation of Leu as a function of age, in normal lens proteins. Racemisation expressed as % . (normal; , R² = 0.000763, two-sided P = 0.908)

Fig. 8

Racemisation of Ala as a function of age, in normal lens proteins. Racemisation expressed as % . (normal; , R² = 0.0675, two-sided P = 0.269)

Fig. 9

Thermal stress and racemisation of amino acid residues in intact lenses. Pig lenses were incubated at 60°C. ad-Asx as a function of time of incubation (, R² = 0.929, two-sided P = 0.0082). bd-Ser as a function of time of incubation. (, R² = 0.924, two-sided P = 0.0091)