Relationship between lens protein glycation and membrane structure in human cataract

Glycation, which begins with the spontaneous reaction between sugar and proteins by the formation of Schiff bases, is known to especially damage long-lived proteins such as lens crystallins, and has been implicated in the ageing process and particularly in cataract formation. In this study of human senile cataract, a specific method is used to measure the formation of Amadori compounds in control postmortem and cataractous lenses, but no difference was found. However, the fluorescence of proteins at 430 nm (exc 350 nm), which has been attributed to advanced glycation, increased with normal ageing of the lens (cortex versus nucleus, and as a function of subject's age for each type of fibres) and was further enhanced in cataractous lenses. The precise molecular origin of this fluorescence remains to be elucidated. In parallel to the accumulation of non-tryptophan fluorophores, a decrease in the membrane fluidity was observed with lens ageing and more acutely with cataract. Both parameters are positively correlated (P < 1%). The modification of the membrane structure with glycation could explain the strong permeability changes occurring during cataract, measured here in terms of cation concentration and inositol leakage, as shown by the negative relationship between the fluorescence signal and the sodium to potassium ratio or the inositol level.