Lens proteins block the copper-mediated formation of reactive oxygen species during glycation reactions in vitro

The formation of advanced glycation endproducts (AGEs) from glucose in vitro requires both oxygen and a transition metal ion, usually copper. These elements combine to produce reactive oxygen species (ROS) which degrade glucose to AGE-forming compounds. We measured the ability of Cu(2+) to accelerate ROS formation, and the effect of added lens proteins on these reactions. Increasing levels of Cu(2+) accelerated the formation of superoxide anion with glucose and fructosyllysine, but the addition of 2.0 mg/ml calf lens proteins completely blocked superoxide formation up to 100 mu M of added Cu(2+). Lens proteins, however, had no effect on superoxide generated by the hypoxanthine/xanthine oxidase system. The oxidation of ascorbic acid was increased 170-fold by the addition of 10 mu M Cu(2+), but was also completely prevented by added lens proteins. Hydroxyl radical formation, as measured by the conversion of benzoate to salicylate, was increased to 30 nmoles/ml after 18 h by the addition of 100 mu M Cu(2+) and 2.5 mM H2O2. This increase was also blocked by the addition of lens proteins. However, hydroxyl radical formation, as estimated by the crosslinking and fragmentation of lens proteins, was observed in the presence of 100 mu M Cu(2+), Likely at the sites of Cu(2+) binding. Since the ratio of lens proteins to Cu(2+) in human lens is at least 1000-fold higher than those used here, the data argue that Cu(2+) in the lens would be tightly bound to protein, preventing ROS-mediated AGE formation hom glucose in vivo. (C) 1999 Academic Press.