Regular ArticleCancer chemotherapy has been of limited success because of intrinsic or acquired resistance of cancer cells to a broad range of chemically and functionally distinct anticancer agents, a phenomenon termed multidrug resistance (MDR). Tumor cells develop drug resistance through various mechanisms, such as overexpression of drug efflux transporters like P-glycoprotein (P-gp), changes in topoisomerase activity, modifications to glutathione S-transferase, and altered expressions of apoptosis-associated protein, Bcl-2, and tumor suppressor protein, p53. Of these, overexpression of P-gp is the critical factor. P-gp (ABCB1), a 170-kDa plasma glycoprotein encoded by the human MDR1 gene and murine mdr1a, mdr1b, and mdr2 genes, belongs to the ATP-binding cassette family of transporters.1) It is an integral membrane protein with two homologous halves, each consisting of one hydrophobic loop with six transmembrane domains (TMDs) and one hydrophilic nucleotide-binding domain (NBD). Binding and transport of substrates is mediated by TMD using energy derived from the hydrolysis of ATP at the NBD. 2,3) Numerous studies suggest that the principal physiological role of P-gp is to protect the organism from toxic substances. This efflux pump is also present in many normal tissues including the epithelium of the gastrointestinal tract, renal proximal tubules, canalicular surface of hepatocytes, and the endothelial cell surfaces comprising the blood-brain barrier. P-gp in the intestines, liver, and kidneys may play important roles in the absorption, distribution, or excretion of drugs. 4)P-gp acts as an energy-dependent efflux pump with a broad specificity for chemically unrelated hydrophobic compounds, such as paclitaxel, anthracyclines, Vinca alkaloids, dexamethasone, lidocaine, erythromycin, and protease inhibitors. Therefore, its overexpression is a major cause of failure in human cancer chemotherapy.MDR cells can be sensitized to anticancer drugs when treated with a P-gp inhibitor, otherwise known as a chemosensitizer. Compounds such as verapamil, dihydropyridine analogs, quinidine, and cyclosporin A are capable of suppressing P-gp function due to their inhibitory properties.Meanwhile, these compounds have an intrinsic toxicity because they are pharmacologically active. They cannot be used safely at dosages required for MDR reversal. The search for chemosensitizers, which have the advantage of being a non-transportable inhibitor without side effects, has led to a great deal of research in plant-derived flavonoids.Flavonoids are constituents of fruits, vegetables, and plantderived beverages, such as coffee, tea, and red wine as well as components of herbal-containing dietary supplements (e.g., Silybum marianum, Alpina officinarum, and Hypericum perforatum). More than 4000 naturally occurring flavonoids have been discovered, 5) arising from the various combinations of multiple hydroxyl and methoxyl group substituents of the basic flavonoid skeleton. The classes of flavonoids include chalcones, flavones, flavonols, fla...

Ganoderic acid A (GAA) is a bioactive triterpenoid isolated from the medicinal fungus Ganoderma lucidum. Our previous study showed that the Bacillus subtilis ATCC (American type culture collection) 6633 strain could biotransform GAA into compound (1), GAA-15-O-β-glucoside, and compound (2). Even though we identified two glycosyltransferases (GT) to catalyze the synthesis of GAA-15-O-β-glucoside, the chemical structure of compound (2) and its corresponding enzyme remain elusive. In the present study, we identified BsGT110, a GT from the same B. subtilis strain, for the biotransformation of GAA into compound (2) through acidic glycosylation. BsGT110 showed an optimal glycosylation activity toward GAA at pH 6 but lost most of its activity at pH 8. Through a scaled-up production, compound (2) was successfully isolated using preparative high-performance liquid chromatography and identified to be a new triterpenoid glucoside (GAA-26-O-β-glucoside) by mass and nuclear magnetic resonance spectroscopy. The results of kinetic experiments showed that the turnover number (kcat) of BsGT110 toward GAA at pH 6 (kcat = 11.2 min−1) was 3-fold higher than that at pH 7 (kcat = 3.8 min−1), indicating that the glycosylation activity of BsGT110 toward GAA was more active at acidic pH 6. In short, we determined that BsGT110 is a unique GT that plays a role in the glycosylation of triterpenoid at the C-26 position under acidic conditions, but loses most of this activity under alkaline ones, suggesting that acidic solutions may enhance the catalytic activity of this and similar types of GTs toward triterpenoids.