Vegetable soybean (Glycine max (L.) Merr.) leaf extracts: Functional components and antioxidant and anti-inflammatory activities

To investigate ways to extract greater dietary value from the leaves of the vegetable soybean (Glycine max (L.) Merr.) cultivar 'Kaohsiung No. 9'. Our results indicate that phenolic content and flavonoid content are highest in extracts prepared with 70% methanol and 70% ethanol. The 70% ethanol extracts also had the highest quercetin (135 ± 1.62 µg/g) and kaempferol (450 ± 1.35 µg/g) contents. These results show that flavonoids are a dominant class of compounds in these vegetable soybean leaf extracts and serve as their main source of antioxidants. At an extract concentration of 5 mg/ml, the 70% methanol extracts achieved good antioxidant effects, with a DPPH radical scavenging rate of 80%, and a reducing power of 88%. In assays of anti-inflammatory capacity using lipopolysaccharide-stimulated RAW 264.7 macrophages. The 70% methanol extracts displayed the most significant inhibition of nitric oxide (NO) synthesis, achieving up to 86% inhibition. As a similar trend was observed in expression levels of inducible nitric oxide synthase (iNOS), we deduced that vegetable soybean leaves may regulate NO synthesis through inhibiting iNOS. We also observed a significant decrease in cyclooxygenase (COX)-2 gene expression. Analysis of proinflammatory cytokine synthesis revealed that the 70% methanol and 70% ethanol extracts significantly reduced TNF-α, IL-6, and IL-1β synthesis, and increased the intracellular reduced glutathione/oxidized glutathione (GSH/GSSG) ratio from 8 to 12.8. These results indicate that vegetable soybean leaves possess antioxidant activities and exert inhibitory effects on inflammatory mediators, suggesting their potential for use as dietary supplements.