Relationship between antioxidant properties and α-amylase and α-glucosidase inhibitory capacity of Justicia spicigera leaf extracts

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This study evaluated the in vitro antioxidant and antidiabetic activities of seven extracts derived from Justicia spicigera leaf, using infusion, decoction, and ultrasound-assisted organic solvent extraction techniques. Total soluble phenols (TSP) and flavonoids (TF) were quantified in the aqueous (AF) and 50% aqueous methanol (AMF) fractions obtained through solid-phase extraction (SPE). High-resolution mass spectrometry (HRMS) analysis of the 14 SPE fractions identified three phenolic compounds: liquiritigenin, detected exclusively in AF, and kaempferitrin ( 1 ) and kaempferol ( 2 ), both consistently present across all AMF. These findings demonstrate a distinct fraction-specific distribution for these compounds. Correlation analysis revealed distinct bioactivity patterns between the two fractions. TF within the AF fractions showed a strong negative correlation with α-amylase inhibition (r = -0.774), while the TF in the AMF fractions showed a pronounced association with DPPH • radical-scavenging capacity (r = -0.755). The 70% aqueous ethanol ultrasound-assisted extract (EEAUS) exhibited the highest levels of phenolic compounds and enzyme-inhibitory effects. From this active extract, two flavonoids were isolated by chromatography and identified by one- and two-dimensional Nuclear Magnetic Resonance (NMR), kaempferitrin ( 1 ) and, for the first time in J. spicigera , kaempferol 3- O -α-L-arabinopyranoside-7- O -α-L-rhamnopyranoside ( 2 ). These results highlight the critical role of extraction methodology in fractionating compounds responsible for antioxidant and antidiabetic activities.