Plectranthus amboinicus (Spreng.) Semi-purified Fractions with Selective β-Glucuronidase Inhibition Elucidated with gas chromatography-mass spectrometry and in silico docking

Background: Plectranthus amboinicus (Spreng.) is an herb commonly used in folk medicine and food by several Asian countries. The bioactivities of this medicinal plant have so far not been linked to a specific enzyme target. Bacterial beta-glucuronidases (β-GUS) expressed by human gut microbiota affect xenobiotic processing by reactivating toxic substances (e.g., anticancer drugs, nonsteroidal anti-inflammatory drugs, and food carcinogens) in the gut lumen. Objectives: An approach to alleviating the toxic effects of these compounds is by inhibiting bacterial β-GUS. Materials and Methods: We determined the Escherichia coli β-GUS inhibitory activity of P. amboinicus leaves using a bioassay-guided purification approach. The P. amboinicus chloroform extract was purified using normal-phase column chromatography to produce several fractions. The fractions were screened for E. coli β-GUS inhibitory activity using the 4-Methylumbelliferyl glucuronide (4-MUG) assay. Fractions with high activity were assayed further to determine toxicity against E. coli and selectivity compared to human β-GUS. Highly-active and highly-selective fractions were further characterized using gas chromatography-mass spectrometry (GC-MS) and in silico docking to identify specific compounds. Results: Assay-guided purification of the crude chloroform extract with β-GUS inhibitory activity (IC50 = 57.8 μg/mL) yielded four fractions with high activity: Fraction-543W (IC50 = 16.24 μg/mL), Fraction-5231 (IC50 = 3.087 μg/mL), and Fraction-52335A (IC50 = 12.93 μg/mL). The crude extract and fractions exhibited high selectivity for E. coli β-GUS against human β-GUS (P