Antioxidant and semicarbazide‐sensitive amine oxidase inhibitory activities of alginic acid hydroxamates

The commercial polysaccharides of alginic acid (medium (3500 cps, 2% solution) and low (250 cps, 2% solution) viscosities) were esterified with acidic methanol (1 mmol L^?1 HCl) at 4 °C with gentle stirring for 5 days to obtain methyl esters of medium‐viscosity alginic acid (ME‐MVA) and low‐viscosity alginic acid (ME‐LVA). These ME‐MVA and ME‐LVA were reacted with alkaline hydroxylamine to obtain medium‐viscosity alginic acid hydroxamates (MVA‐NHOH) and LVA‐NHOH. The percentages of hydroxamic acid content in MVA‐NHOH and LVA‐NHOH were calculated as 25% and 20%, respectively. The hydroxamate derivatives of alginic acid were used to test the antioxidant and semicarbazide‐sensitive amine oxidase (SSAO) inhibitory activities in comparison with original materials (MVA and LVA). The half‐inhibition concentrations, IC₅₀, of scavenging activity against 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) were 24.5 and 29.8 µg mL^?1 for MVA‐NHOH and LVA‐NHOH, respectively. However, few scavenging activities of the MVA and LVA were found at the same concentrations. The IC₅₀ of the positive control of butylated hydroxytoluene was 5 µg mL^?1. The scavenging activity of DPPH radical was pH‐dependent, and the optimal pH for both of MVA‐NHOH and LVA‐NHOH was the Tris‐HCl buffer (pH 7.9). Using electron spin resonance (ESR) to detect the activity of scavenging hydroxyl radicals, both alginic acid hydroxamates showed dose‐dependent scavenging activities, and the IC₅₀ was 90 and 92 µg mL^?1, respectively, for MVA‐NHOH and LVA‐NHOH. Both alginic acid hydroxamates also exhibited protection against hydroxyl radical‐mediated DNA damage. Both MVA‐NHOH and LVA‐NHOH showed dose‐dependent inhibitory activities against bovine SSAO (2.53 units); the IC₅₀ was 0.16 and 0.09 µg mL^?1, respectively, for MVA‐NHOH and LVA‐NHOH, compared with 3.81 µg mL^?1 of semicarbazide (positive controls). Amine oxidase activity staining also revealed that both MVA‐NHOH and LVA‐NHOH exhibited SSAO inhibitory activities. Both MVA‐NHOH and LVA‐NHOH showed mixed non‐competitive inhibition against bovine SSAO. It was found that the V′_max value was reduced and the K′_m value was either increased (added MVA‐NHOH, 0.05 µg mL^?1) or reduced (added LVA‐NHOH, 0.11 µg mL^?1) in the presence of alginic acid hydroxamate. Copyright © 2006 Society of Chemical Industry