Glycation of Liver Cystatin: Implication on its Structure and Function.

The increased level of reducing sugars and their derivatives in a diabetic condition has been the main cause of protein related complications. The changes in native state of proteins upon glycation induce loss in the function and structure of proteins. This further leads to cell damage and accumulation of immune system inducing AGE formation. Here in the present study cystatin was purified from liver (BLC) through affinity chromatography and was incubated with glucose, fructose and ribose. Changes were observed in the intensity of Trp absorption at 280 nm as well as AGE's specific fluorescence at 435 nm upon excitation at 370 nm to monitor the formation of BLC-sugar adducts. Protein intrinsic fluorescence showed marked conformational changes when BLC was incubated with D-ribose, glucose and fructose. Glycation with D-ribose induces BLC to misfold rapidly into an intermediate state retaining a low percentage of α-helical content compared to fructose and glucose as revealed by far-UV CD data. Furthermore, a caseinolytic assay of papain in presence of glycated liver cystatin showed decreased activity in the protein induced by these reducing sugars. Ribose had more effect on the structure as well as the function of liver cystatin followed by fructose and least for glucose. Absorption spectroscopy shows change in BLC and formation of AGE's. These results shows that liver cystatin-cathepsin imbalance is compromised in diabetic state which may lead to improper balance of proteinases leading to cirrhosis or liver damage.