Non-enzymically glycated proteins in human blood and lenticular tissue

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Non-enzymic glycosylation of protein can modify its structure and function. A role for the glycosylation of lens crystallins in cataract formation, particularly in the diabetic state, has been suggested (Stevens et al., 1978; Monnier et ul., 1979; Monnier L Ansari et al., 1980; Chiou et a/ . , 1981; Hiraoka et al., 1983; Pande et al., 1979; Gopalakrishna el a/., 1983) Glycosylation of crystallin may bring about a conformational change, such that the buried sulphydryl groups become exposed. This may lead to increased susceptibility of sulphydryl groups to oxidation, to the formation of interand intra-molecular disulphide bridges leading to formation of high molecular mass proteins, which cause opacity in the lens, especially in diabetic patients (Stevens et al., 1978; Monnier et al., 1979). It was further established that non-enzymic glycosylation of lens protein increased in diabetic patients (Mandel et ul., 1983; Gopalakrishna et al., 1983; Hiraoka et al., 1983). Garadi et al. (1982) suggested that the major polypeptide of lens membrane and lens fibre cells contained glycoprotein. Kamei & lwata (1983) suggested a role for glycosaminoglycan variation in protein aggregation and opacification of lens. An increase in mucoprotein content and a decrease in sialic acid and hexosamine content were observed by T. Sarwat & M.A. Rahman (unpublished work) in diabetic cataractous lenses. The purpose of this study was to estimate non-enzymic glycosylation of protein and the amount of bound carbohydrate levels in the blood and lenses of the same group of patients at various stages of cataractogenesis. Fasting blood and cataractous lenses were collected from patients admitted for operation in the Eye Ward of the Jinnah Postgraduate Medical Centre and other hospitals in Karachi. Patients having a history of ocular trauma, uveitis or glaucoma were not included in the study. Selected patients were examined by slit lamp before operation to determine the type of cataract. The senile cataractous lenses were classified as immature when the opacity did not totally obscure all normal anatomical regions of the lens; and as mature when a totally opaque lens in which no recognizable normal anatomical zone remained but in which there was no appreciable anteroposterior swelling. Totally opaque lenses that had undergone marked swelling in the anteroposterior dimension were classified as hypermature. The cataractous lenses of diabetic patients were classified as diabetic cataract. The cataractous lenses were transferred to preweighed plastic containers filled with 50% (v/v) buffered glycerol (Morton, 1955) which were transferred to the laboratory in ice and stored at 20°C. A 10% homogenate of each lens was prepared in 20 mhl-cold Tris( hydroxymethy1)amino-