potassium-cyanate and Cataract

Incubation of lens crystallins with aspirin inhibits the development of opacities caused by cyanate. Cyanate-induced opacities are thought to be due to carbamylation of the lysyl residues which causes a decrease in the protein charge and subsequent conformational changes that permit disulfide bonding. Because aspirin can also react with lysyl residues, it has been proposed that the aspirin inhibition of cataractogenesis is due to acetylation of the lysyl residues which would block their reaction with cyanate. However, acetylation of lysyl residues also lowers the protein charge and would be expected to effect changes in protein conformation similar to those caused by carbamylation. Therefore, acetylation of the lysyl residues is not a satisfactory explanation for the inhibitory effect of aspirin on lens opacification. Our investigations of the reactions of cyanate and aspirin with bovine gamma II-crystallins show that the cysteinyl residues are also carbamylated and acetylated at pH 7.4. At this pH, the carbamylation at the cysteinyl residues is reversible, leading to regeneration of the thiol group and disulfide bonding. In contrast, the acetylation at cysteinyl residues is stable at pH 7.4 and can prevent disulfide bonding. This difference in stability explains how cyanate promotes, and aspirin inhibits, cataractogenesis.

Topics: Acetylation; Amino Acid Sequence; Animals; Aspirin; Cataract; Cattle; Crystallins; Cyanates; Cysteine; Lens, Crystalline; Molecular Sequence Data; Pepsin A; Peptide Fragments

Cataract, the major cause of blindness world-wide, may be caused partly by modification of lens proteins by carbamylation and non-enzymic glycosylation (glycation) in some patients. Aspirin has been found to protect against these modifications and to prevent cyanate-induced opacification occurring in whole rate lenses. Ibuprofen is an aspirin-like anti-inflammatory drug which appeared as a protective factor against cataract in an Oxford case-control study. The binding of cyanate, galactose and glucose 6-phosphate to lens proteins, and the effect of ibuprofen on this reaction was investigated, as was cyanate-induced opacification in whole rat lenses. Labelled metabolite was incubated with bovine lens homogenate in the presence and absence of ibuprofen, and the incorporation of label into the lens homogenate was followed. Simultaneous and preincubation experiments were performed. Intact rat lenses were incubated in culture medium with and without cyanate and ibuprofen. The phase separation temperature was noted as the temperature at which opacity first appeared on cooling. Cyanate, galactose and glucose 6-phosphate bind progressively to lens proteins. Simultaneous incubation with ibuprofen reduces cyanate and galactose binding but not glucose 6-phosphate. Ibuprofen protects against opacities due to cyanate-induced phase separation. Ibuprofen has protected against cataract in the models of cataractogenesis in this study. It appears to have a different mechanism of action from that of aspirin. These studies provide some support for the idea, based on epidemiological findings, that ibuprofen might be a useful anti-cataract drug.

Topics: Animals; Cataract; Cattle; Crystallins; Cyanates; Galactose; Ibuprofen; Lens, Crystalline; Male; Rats; Temperature

Incubation of mixed bovine lens crystallins with 100 mM potassium cyanate causes almost all the protein to form large aggregates. These aggregates are not dispersed by powerful chaotropic agents and are held together by disulphide bonds. Experiments with beta L-crystallin show that carbamylation of this one protein class can bring about the aggregation of other unmodified crystallins. The carbamylated crystallin served as a nucleus for aggregation of other crystallins. These changes are related to the chemical modification of crystallins and the ensuing conformational changes in cataractogenesis.

Topics: Animals; Binding Sites; Cataract; Cattle; Chromatography, Gel; Crystallins; Cyanates; Disulfides; Molecular Weight; Protein Conformation; Protein Denaturation

The carbamylation of lens proteins by cyanate causes conformational changes, and cyanate causes cataract. There is some evidence that aspirin is beneficial to cataract patients, so its effect on the carbamylation of lens proteins and on opacification produced by cyanate in vitro was studied. Aspirin decreased the phase separation temperature in lenses exposed to cyanate, and was found to reduce the rate of carbamylation of most, if not all, soluble lens proteins. Studies with radiolabelled aspirin lead to the conclusion that the drug achieves this protection by chemically modifying the proteins. The nature of this modification and the relevance of these results to human cataract is discussed.

Topics: Animals; Aspirin; Cataract; Cattle; Crystallins; Cyanates; In Vitro Techniques; Lens, Crystalline; Protein Conformation; Rats; Rats, Inbred Strains; Temperature; Time Factors