α-Glucosidase Inhibition-Guided Fractionation of Kalincung (Uncaria gambir Roxb. By-Product) using Diaion HP-20 Resin

Kalincung is a semi-solid by-product generated during the traditional processing of Uncaria gambir Roxb. and may undergo spontaneous fermentation during post-processing storage. However, it remains underexplored despite its potential as a polyphenol-rich resource. This study compared methanolic extracts of kalincung and dried gambir leaves and evaluated α-glucosidase inhibition-guided fractionation using Diaion HP-20 resin. An assessment of phytochemical constituents was carried out, including the determination of total phenolic (TPC), total flavonoid content (TFC), and vanillin–HCl–reactive flavan-3-ol equivalent profile (VAE; expressed as catechin equivalents, CAE), followed by antioxidant assessment using the DPPH assay (IC₅₀) and α-glucosidase inhibition (IC₅₀). Kalincung exhibited higher TPC (40.10 ± 0.07% GAE) and TFC (35.92 ± 1.12% RAE) than leaves (30.94 ± 0.71% GAE and 21.30 ± 0.71% RAE; p < 0.05), while leaves showed higher VAE values (27.46 ± 0.06% CAE) than kalincung (9.84 ± 0.36% CAE; p < 0.05), reflecting differences in vanillin-reactive flavan-3-ol units rather than absolute proanthocyanidin content. Kalincung demonstrated strong DPPH scavenging activity (IC₅₀ = 9.13 ± 0.05 µg/mL) and α-glucosidase inhibition (IC₅₀ = 32.32 ± 1.20 µg/mL), comparable to leaves. Solvent fractionation enriched bioactivity, with the butanol fraction showing improved α-glucosidase inhibition. Diaion HP-20 fractionation yielded a selected active fraction with increased VAE compared with the crude kalincung extract (48.33 ± 0.39% CAE), accompanied by enhanced functional potency (DPPH IC₅₀ = 8.03 ± 0.07 µg/mL; α-glucosidase IC₅₀ = 17.07 ± 0.42 µg/mL), showing activity within the same order of magnitude as (+)-catechin and showing lower IC₅₀ than acarbose under the present in vitro assay conditions. Taken together, these findings position kalincung as a largely overlooked yet functionally relevant source of polar polyphenolic constituents. Its α-glucosidase inhibitory effect, supported by antioxidant performance, suggests meaningful prospects for early-stage functional evaluation.