Biocatalytic structural diversification of saponins in Zygophyllum decumbens: HR-LC-ESI-TOF-MS/MS profiling and bioactivity modulation

Abstract

Zygophyllum decumbens has been traditionally used in the treatment of gout, fungal infections, diabetes, and rheumatism. These therapeutic effects are primarily attributed to its triterpenoid saponins (TS). The present study explores the biocatalytic transformation of TS to examine structural modifications, identify rare saponins, and evaluate associated changes in biological activity. Triterpenoid saponins were isolated from shade-dried aerial parts by cold maceration in aqueous alcohol, followed by fractionation and precipitation with acetone, yielding (TS). Three microbial strains, Aspergillus niger, Lactobacillus acidophilus, and Bacillus subtilis, were evaluated for their biocatalytic potential. A. niger demonstrated the highest efficiency and was therefore selected for large-scale biocatalytic transformation, producing a modified saponin fraction (TSM). Both TS and TSM were characterized using high-resolution LC-ESI-TOF-MS/MS. Antifungal activity was assessed by agar diffusion and minimum inhibitory concentration (MIC) assays. Cytotoxic effects were determined using the MTT assay on HepG-2, HT-29, and BNL-CL2 cell lines, while antioxidant activity was evaluated using DPPH and ABTS radical scavenging assays. A total of thirty-three saponins were tentatively identified, with quinovic acid derivatives predominating. Biocatalysis led to the formation of eight new rare saponins and the loss of two pre-existing ones. Both TS and TSM demonstrated selective antifungal activity, with TSM producing broader inhibition zones and over 70% MIC reduction in most strains, except against Penicillium marneffei. Both fractions exhibited weak cytotoxicity (IC50 > 300 µg/mL) against the tested cell lines. Antioxidant activity declined post-biocatalysis as DPPH values dropped from 40.91 ± 1.87 to 19.90 ± 0.22 µM Trolox equivalents/mg, and ABTS values from 309.66 ± 11.10 to 165.78 ± 4.62 µM Trolox equivalents/mg. Biocatalytic treatment with A. niger caused pronounced structural modifications in Z. decumbens saponins, resulting in enhanced selective antifungal activity accompanied by diminished antioxidant capacity. These results highlight microbial biocatalysis as an effective approach for generating structural diversity in plant-derived compounds and modulating their pharmacological properties.