Lack of effect of copper on advanced Maillard reaction and glucose autoxidation at physiological concentrations of albumin.

This study examines the possible action of copper on advanced glycation. Copper has been shown to induce fluorescence due to advanced-glycated-end-products (AGEs) on albumin incubated with glucose, and this was interpreted as activation of the glucose or Amadori product (AP) autoxidation. We glycated albumin (60 g/L) to several levels with increasing concentrations of glucose. The dialysed glucose-free glycated albumin was then incubated with 1.5 μmol/L copper or 1 mmol/L diethylenetriaminepentaacetic acid (DTPA), plus or minus glucose. The production of AP, measured as furosine, was similar whether DTPA or copper was present in the incubation medium. It linearly increased as a function of time and glucose concentration in both cases up to a maximum (furosine around 20 mmol/g protein), indicating saturation of the free NH2 residues on the protein. The fluorescence due to AGEs increased linearly over time for glycated albumin incubated without glucose, and exponentially when glucose was added to the incubation medium. This fluorescence was also unaffected by DTPA or copper for a glucose concentration below 125 mmol/L and initial furosine below 10 mmol/g. However copper caused a slight activation in samples with very high glucose (1.25 mol/L) and furosine (30-40 mmol/g) concentrations. We therefore find no effect of copper in this experiment, because the copper concentration is lower and the albumin higher than that used in previous studies. In these conditions, albumin chelates copper and inhibits its oxidative activity. The protein concentrations used in most in vitro studies showing a copper effect were below 10 g/L with copper often above 10 μmol/L, so that copper may act oxidatively. As the lens and arterial wall have high protein concentrations, copper should have no action on protein glycation in vivo, unless altered protein structure impedes the inactivation of copper by chelation.