Among these, the ginsenosides have been well characterized for their functionality, and are thus regarded as the principal components responsible for the pharmacological and biological activities of ginseng . Ginsenosides are composed of a dammarane
backbone with several side chains, including glucose, arabinose, xylose, and rhamnose side chains . Thus far, more than 50 types SCH727965 in vivo of ginsenosides have been isolated and identified from Panax ginseng Meyer . Based on the differences in their chemical constitutions, the ginsenosides are generally classified into three types: protopanaxadiol (PPD), protopanaxatriol, and olenolic acid. Among those thus far identified, six major ginsenosides (Rb1, Rb2, Rc, Rd, Re, and Rg1) have been determined to account for 90% of the total ginsenoside content of P. ginseng Meyer . In particular, ginsenoside Rb1 is present in greater abundance (usually >20% of total ginsenosides) than any other ginsenosides in P. ginseng, Panax quinquefolius, Panax japonicum and Panax notoginseng selleck . Earlier reports have shown that the major PPD-type ginsenosides (Rb1, Rb2, Rc, Rd) are metabolized by intestinal bacteria after oral administration to minor ginsenosides such as Rg3, Rh2, F2, and compound K (CK) . In recent years, it has been demonstrated
that the minor ginsenosides possess remarkable pharmaceutical activity and can be readily absorbed by the human body . For example, ginsenoside Rg3 induces tumor cell apoptosis, inhibits tumor cell proliferation and attenuates tumor invasion and metastasis  and . In addition, Rg3 serves as a natural cytoprotective agent against environmental carcinogens . Therefore, a variety of studies have focused on the conversion of major ginsenosides to the more active minor ginsenosides via methods such as heating , acid treatment , alkali treatment , and enzymatic conversion  and . Chemical transformation induces side reactions including epimerization, hydroxylation,
and hydration, and also generates more environmental pollution ADAMTS5 . By contrast, microbial or enzymatic approaches have arisen as the predominant conversion modalities, owing to their marked selectivity, mild reaction conditions, and environmental compatibility. Some studies have involved attempts to find suitable microbes or enzymes that can transform Rb1 into minor ginsenosides such as Rd, F2, Rg3, and compound K , , ,  and . However, the majority of the microorganisms employed in these experiments are not of food-grade. Aspergillus niger strain has been known to be one of the most popular fungi in fermentation of the crops such as soybean and in brewing industry due to its production of various hydrolyzing exoenzymes . In particular, production of glucosidase by using A. niger as a good producer has been recently studied by many researchers .