|
Oxidative damage due to copper ion and hydrogen peroxide induces GlcNAc-specific cleavage of an Asn-linked oligosaccharide.
Cleavage of an asparagine-linked sugar chain by hydrogen peroxide (H2O2) and a copper salt was investigated. Incubation of a 2-aminopyridine (PA)-labeled biantennary sugar chain, GlcNAcbeta1-2Manalpha1-6(GlcNAcbeta1-2Manalpha1-3)Manbeta1-4GlcNAcbeta1-4GlcNAc-PA, with H2O2 and Cu2+ led to formation of four major degradation products. Reversed phase high performance liquid chromatographic analysis coupled with glycosidase digestion indicated that the sugar chain is not randomly degraded but specifically degraded at a GlcNAc residue. Treatment with either of H2O2 or copper alone did not cleave nor degrade the sugar chain to any extent. Electron spin resonance (ESR) spectra obtained using a spin trap reagent were consistent with the generation of OH* or an OH*-like radical by the H2O2/copper salt mixture. The addition of ascorbic acid enhanced this radical generation as well as the degradation of the sugar chain. It was also found that H2O2/Cu2+ destroys the N-acetyl group of the monosaccharide GlcNAc, as judged by a decrease in the ultraviolet absorption spectrum of this group. On the other hand, replacement of copper by Fe2+ caused no cleavage of the sugar chain, although comparable levels of the same radical species were generated. Furthermore, spectrophotometric analysis showed that a GlcNAc-containing sugar chain coordinates to copper but not to iron, and, thus, the coordination appears to play an essential role in the degradation of the sugar chain. These findings suggest that coordination of copper ions to GlcNAc residues localizes the generation of a radical, which cleaves the glycosidic linkage, possibly involving alteration of the N-acetyl group, thereby allowing the GlcNAc-specific cleavage.
|
