The purpose of our study was to determine the amount of glycated proteins and advanced glycation end products (AGE) in cataractous lens homogenates of patients who underwent phacoemulsification, and to define a simple in vitro protein model of glycoxidation. Analysis of 30 cataractous lenses (15 diabetic and 15 non-diabetic) revealed a significant increase in both glycated lens proteins of diabetics compared with the controls (0.15 vs 0.08 nmol/mg protein, P < 0.01) and AGE-linked fluorescence at 440 nm (4.8 vs 2.8 AU/mg protein, P < 0.01). The presence of AGE fluorescence in lenses indicates the role of oxidative stress in cataractogenesis. Fifty-six days incubation of alanine and aspartate aminotransferases, used as model proteins, with 500 mM D-fructose at 25 and 37 degrees C led to a complete inhibition of ALT and AST activities. The fluorescence of both aminotransferases rose according to the chosen incubation temperature: 37 degrees C > 25 degrees C > 4 degrees C. ALT and AST incubated in a medium containing D-fructose are subject to nonenzymatic glycation followed by a consequent formation of AGE products. Our data: i) support the concept of glycation-glycoxidation pathway appearing in diabetic patients; ii) form a base for determination of the efficiency of various antioxidative compounds in vitro.