Antioxidant activity of a Schiff base of pyridoxal and aminoguanidine

We recently reported that PL-AG, a Schiff base of pyridoxal and aminoguanidine, was more effective than aminoguanidine (AG), a well-known anti-diabetic-complication compound, in preventing nephropathy in diabetic mice and presented brief data indicating the antioxidant activity of the adduct. In the present study, we additionally investigated the inhibitory activity of PL-AG in comparison with that of AG against in vitro and in vivo oxidation. PL-AG was more potent than AG and reference compounds such as pyridoxal and pyridoxamine in any of the five antioxidant activities examined in vitro, i.e., hydrogen peroxide-scavenging, hydroxyl radical-scavenging, superoxide radical-scavenging, ascorbic acid-autoxidation inhibitory, and low-density lipoprotein (LDL)-oxidation inhibitory activities, the last two of which were assessed in the presence of Cu2+. Unlike AG, PL-AG did not show the pro-oxidant activity. The inhibitory activity of PL-AG against lipid peroxidation in diabetic rats was higher than that of AG, for example, the amounts of malondialdehyde in erythrocytes (nmol/g hemoglobin; mean ± SD) in normal, untreated diabetic, AG-treated diabetic, and PL-AG-treated diabetic rats were 3.53 ± 0.35, 4.99 ± 0.23, 4.65 ± 0.45, and 4.06 ± 0.35, respectively. A fluorescent substance different from PL-AG was found in the plasma and urine of rats treated with PL-AG. The chemical structure of this substance, i.e., oxidized PL-AG, was determined by a combination of nuclear magnetic resonance, mass, and infrared spectrometry. AG dramatically decreased the pyridoxal phosphate level in the diabetic rat liver, whereas PL-AG only moderately affected it. Our results indicate that the antioxidant activity of PL-AG is due to its chelation with transition metal ions and to scavenging of reactive oxygen species. They also suggest that PL-AG is more promising for the treatment of diabetic complications than AG.