Extraction optimisation, purification, characterisation, antibacterial, and antioxidant activity of polysaccharide from Platycodon grandiflorus leaves

Platycodon platycodon is a traditional Chinese medicine homologous food, which is rich in stems and leaves, but mostly discarded. In the present work, the extraction process of polysaccharide from P. grandiflorus leaves (PGLP) was optimised. After isolation and purification, the antibacterial and antioxidant activities of the polysaccharide were studied. The extraction rate was highest under the conditions of ultrasonic power 280 W, solid-to- solvent ratio 1:22 (w/v), extraction temperature 79 degrees C, and extraction time 3.5 h, reaching 10.11%, which was consistent with the model predicted value. The crude polysaccharide was isolated and purified by DEAE-52 cellulose chromatography and Sephadex G-100 chromatography column, and two components PGLP-2-1 and PGLP-4-1 were obtained. UV-Vis, FT-IR, and XRD analyses showed that PGLP-2-1 and PGLP-4-1 had typical characteristics of polysaccharides. PGLP-2-1 had the highest antibacterial activity against Staphylococcus aureus, , and the inhibitory zone diameter reached 17.46 mm; in addition, it had weaker antibacterial activity against Pseudomonas aeruginosa, , Salmonella Typhimurium, and Bacillus subtilis. . PGLP-4-1 had a weak antibacterial activity against Pseudomonas aeruginosa, , Salmonella Typhimurium, , and Staphylococcus aureus, , and the inhibitory zone diameters were 12.24, 11.35, and 12.52 mm, respectively. PGLP-2-1 had strong scavenging ability on superoxide anion, hydroxyl, DPPH, and ABTS free radicals, and the highest scavenging rate reached 86.68, 80.96, 74.82, and 78.88%, respectively. PGLP-4-1 also had strong scavenging ability on superoxide anion, hydroxyl, DPPH, and ABTS free radicals; and the highest scavenging rate reached 75.04, 72.23, 60.56, and 50.25%, respectively. The results indicated that PGLP-2-1 and PGLP-4-1 had strong antioxidant activity, and the antioxidant activity of PGLP-2-1 was higher than PGLP-4-1. The present work provided a theoretical basis for the comprehensive development and utilisation of P. platycodon leaves.