Aminophospholipid glycation and its inhibitor screening system: A new role of pyridoxal 5′-phosphate as the inhibitor

Peroxidized phospholipid-mediated cytotoxity is involved in the pathophysiology of a number of diseases [i.e., the abnormal increase of phosphatidylcholine hydroperoxide (PCOOH) found in the plasma of type 2 diabetic patients]. The PCOOH accumulation may relate to Amadori-glycated phosphatidylethanolamine (deoxy-D-fructosyl PE, or Amadori-PE), because Amadori-PE causes oxidative stress. However, lipid glycation inhibitor has not been discovered yet because of the lack of a lipid glycation model useful for inhibitor screening. We optimized and developed a lipid glycation model considering various reaction conditions (glucose concentration, temperature, buffer type, and pH) between PE and glucose. Using the developed model, various protein glycation inhibitors (aminoguanidine, pyridoxamine, and carnosine), antioxidants (ascorbic acid, α-tocopherol, quercetin, and rutin), and other food compounds (L-lysine, L-cysteine, pyridoxine, pyridoxal, and pyridoxal 5′-phosphate) were evaluated for their antiglycative properties. Pyridoxal 5′-phosphate and pyridoxal (vitamin B₆ derivatives) were the most effective antiglycative compounds. These pyridoxals could easily be condensed with PE before the glucose/PE reaction occurred. Because PE-pyridoxal 5′-phosphate adduct was detectable in human red blood cells and the increased plasma Amadori-PE concentration in streptozotocininduced diabetic rats was decreased by dietary supplementation of pyridoxal 5′-phosphate, it is likely that pyridoxal 5′-phosphate acts as a lipid glycation inhibitor in vivo, which possibly contributes to diabetes prevention.