Nuclear Cataract Formation Research Articles

Prevention of Direct and Photosensitized Ultraviolet Radiation Damage to the Ocular Lens

Exposure to ambient ultraviolet (UV) radiation (longer than 300 nm) over a lifetime appears to play a role in lens aging and nuclear cataract formation. The photochemical generation of fluorescent chromophores has been correlated with the increasing yellow to brown color of the aging lens nucleus and with polypeptide chain cross-linking progressively increasing the insoluble protein level. Acute exposure to 300 nm radiation has also been implicated in human, as well as in animal cortical cataracts. These cortical cataracts develop after rat lenses are exposed to 300 nm UV radiation at irradiance levels of 26 J or more. The Spectra-Shield® lens can be used as a UV filter and will completely prevent the generation of fluorescent compounds within the lens as well as the UV cortical cataracts. The Spectra-Shield lens also prevents the formation of 8-methoxypsoralen (8-MOP) protein photoreac-tion products in UV-exposed rat lenses derived from animals given a single dose of 8-MOP. Furthermore, the Spectra-Shield lens fully retains its ability to reflect all UV radiation after continuous exposure to over 2,500 J of 300 nm UV radiation.

Senile Cataracts—A Biochemical Review of Senile Cortical and Nuclear Cataracts

Summary Senile cortical and nuclear cataracts are formed through different mechanisms. During cortical cataract formation, Na+ in the lens increases, the protein content decreases and localized hydration changes are found in the cortex. These and other changes are consistent with alterations in the mechanisms responsible for maintaining the osmotic equilibrium of the lens. It is likely that the capsule and the fibre cell membranes have become leaky. Nuclear cataract formation is accompanied by a large and progressive insolubilization of proteins in the lens nucleus and by the formation of coloured protein aggregates which are held together by disulphide bonds and other covalent crosslinks. All of the changes observed appear to be the result of extensive oxidation due to failure of the normal protective systems.

Oxidative changes in human lens proteins during senile nuclear cataract formation

1.1.|Proteins from the cortex and nucleus of the human lens were studied to determine if any changes could be detected in their amino acids during senile cataract formation.2.2.|Senile nuclear cataract formation was found to be accompanied by a progressive oxidation of cysteine and methionine. The oxidation of methionine and changes in the distribution of the nuclear proteins did not appear to start until about 60% of the cysteine had been oxidized.3.3.|In the advanced nuclear cataractous lens, about 90% of the cysteine has been oxidized and 45% of the methionine is present as the sulphoxide in the nuclear proteins. The levels of other amino acids appeared to remain constant.4.4.|Similar, but smaller, changes were found in the cortical proteins in advanced nuclear cataractous lenses, suggesting that the oxidation spreads from the nucleus to the cortex.5.5.|These changes were discussed with regard to current views on cataract formation and it was concluded that they are probably the result of simple oxidation of the proteins with O2 or H2O2.

Changes in human lens proteins during nuclear cataract formation

The increase in water-insoluble proteins associated with senile nuclear cataract formation is due primarily to the increase in a urea-insoluble yellow protein fraction which can be solubilized with urea containing a reducing agent. It is not an artefact produced by aerobic extraction since prevention of disulphide bond formation during extraction does not result in a decreased yield of this fraction. The yellow protein fraction is localized in the nucleus of the lens and appears to contain trimers and larger aggregates of normal lens polypeptides which are resistant to deaggregation by 8 m-urea, 6 m-guanidine hydrochloride and 1% SDS in the presence of reducing agents. The soluble and urea-soluble proteins of the lens do not contain significant amounts of these aggregates. The proportion of these aggregates increases until they represent over 50% of the yellow protein fraction in advanced cataractous lenses.

The Identification of Anthranilic Acid in Proteolytic Digests of Cataractous Lens Proteins

The yellow protein fractions and brown insoluble proteins from nuclear cataractous lenses and the total proteins from normal lenses were digested with pronase, prolidase and leucine aminopeptidase and the digests were fractioned on Sephadex G-25. Anthranilic acid was identified as a component of the cataractous protein digests but was absent from the normal protein digest. It is suggested that a possible source of the anthranilic acid is tryptophan which was oxidized during nuclear cataract formation.