Research in Medicine

Download citation:
BibTeX | RIS | EndNote | Medlars | ProCite | Reference Manager | RefWorks
Send citation to:

---

Loss of transparency of the lens is termed cataract which is one of the commonest causes of blindness throughout the world. This major health problem appears to be much commoner in many developing countries such as India and Pakistan, where it causes visual disability at a substantially earlier age and several hypotheses have been put forward to explain this higher prevalence in these areas. Recently it has been suggested that the higher incidence of diarrhoea in :hese countries could be a major primary cause of the excess cataract there.It was postulated that four features of diarrhoea-malnutrition, acidemia, dehydration(Osmotic stress) and uraemia­could contribute to cataract formation. In this paper only the uraemia and its possible role in cataract formation is considered. Of several effects of urea on the lens its equilibrium with cyanate is important. At equilibrium 250mM urea kept at 38° C, pH7.4 contains 2mM cyanate. Cyanate is able to carbamylate protein, glutathione and other components of the lens. The ,reaction of cyanate witha-crystallin leads to conformational changes and oxidation of protein thiols to disulphides. Cyanate can also carbamylate the epsilon-amino groups of lysine residues, carbamylation of lysine leads to the formation of hemocitrulline which would change the charge profile of the lens proteins: the charged epsilon-amino group being converted to the uncharged amide structure. Such a change could destabilize the protein and cause a conformational change such as that which could be seen in human unclear cataract. There are perhaps three ways proposed to deal with enormous burden of blindness caused by cataract: extraction, prevention and drug therapy. Prevention has not been discussed widely and as yet use of drugs to treat cataract is less well developed. Since rheumatoid arthritis patients receiving aspirin seem less lilely to have cataract than their fellows trated in other ways, the benefits of aspirin therapy in patients with early cataract has recently been considered. It has been shown that lens soluble proteins incubated with cyanate steadily incorporate cyanate, this incorporation is inhibited by aspirin. Salicylate has much less effect at higher dose, and lower concentration has no effect. Since aspirin can acetylate proteins, thus it has been suggested that the acetyl group of aspirin(acetyl salicylic acid) is important in protecting the proteins against cyanate. To investigate the possible relationship between diarrhoea and cataract in Khuzestan province, 100 lenses extracted from cataractous patients were sent to the Nuffield laboratory of ophthal­mology of Oxford University for analysis. The preliminary results have not shown hemocitrulline in any lens but two other components, peaks A and B were found in many of them. These components are rarely seen in analyses of Oxford lenses but they have been found in many lenses from India. The nature of these components has not yet been elucidated; but it is thought that hemocitrulline ma y go on to form the components called peaks A and B that are found in many of the Iranian and Indian lenses. The amounts of peaks A and B found in the Iranian lenses are higher than in the average batch of lenses from India. The purpose of this paper is twofold: 1) to review the articles with regard to the possible causes of the cataract and a new hypothesis which explains the high prevalence of cataract in some tropical countries, and 2) to present some preliminary data obtained from the analyses of lenses of cataractous patients in Khuzestan province. 

Keywords: Diarrhoea, Cataract, Aspirin, Prevent

Full-Text [PDF 6909 kb]   (527 Downloads)