Antioxidant activity of differently sized and sulfated konjac glucomannan fragments prepared by the relay strategy

Konjac glucomannan (KGM) is a polysaccharide with potential medical and functional properties. Here, the antioxidant and cytoprotective effects of sulfated and differently sized KGM fractions were investigated using various in vitro assays. The sulfated KGMs (SKGMs) were prepared via a relay strategy. First, Vitamin C (Vc)-H₂O₂ degradation was employed to obtain three soluble KGM fractions with different molecular weights. Second, nine KGM derivatives with varying sulfate content were obtained by the sulfur trioxide-pyridine method. The scavenging of DPPH, superoxide, and hydroxyl radicals was measured in vitro. The antioxidant activity of SKGM correlated positively with sulfate content. SKGM-I-2 displayed the most potent radical scavenging activity. Its purification by cellulose DEAE-52 column chromatography yielded four homogeneous fractions (SKGM-I-2a, SKGM-I-2b, SKGM-I-2c, and SKGM-I-2d). Pretreatment with SKGM-I-2d increased the viability of RAW264.7 cells exposed to H₂O₂. Moreover, SKGM-I-2d significantly increased the activity of superoxide dismutase and catalase, as well as the levels of glutathione, while regulating the expression of Keap1, Nrf2, and HO-1 in RAW264.7 cells. The present study suggests that SKGM-I-2d protects RAW264.7 cells against H₂O₂-induced oxidative injury through the activation of the Nrf2/Keap1 signaling pathway. These results provide a scientific basis for future studies linking the structural and functional features of KGM.