Ginsenoside is the main active component of ginseng, 1 an important triterpenoid compound, which has good physiological and pharmacological activities, including anti-tumor, anti-inflammatory, and anti-aging biological functions.2 According to the content, ginsenoside is divided into common and rare saponins. Common saponins (high content) account for more than 80% of the total saponins in ginseng root, such as Rb1 and Rb2. 3,4The content of Rb1 is the highest in common saponins, 5 while that of rare saponins such as F2 is low. 6,7 Most of the minor ginsenosides generally exhibit good pharmacological activities. Rare ginsenoside F2 has a certain therapeutic effect on skin inflammation as a new treatment method, and F2 can inhibit human gastric cancer cells. 8, 9 Ginsenoside CK shows a high absorption rate in blood and exerts pharmacological activity in the body. 10,11 As a drug, it has anti-cancer, anti-diabetic, anti-allergic, and liver protection effects and has received widespread attention. 12,13 However, rare ginsenosides F2 and CK are difficult to produce because of their low content or because they are non-existent in ginseng or other plants. 14 Ginsenoside Rb1 belongs to PPD-type ginsenosides and has the same core structure as ginsenoside F2 and CK. Therefore, the conversion of ginsenoside Rb1 into rare saponin F2 and CK is a significant and wise approach.
At present, the transformation of ginsenosides mainly includes physical, chemical, and biotransformation methods. 15 The basic principle of the conversion of common saponins to produce rare ginsenosides is that a series of deglycosylation reactions occur at specific positions of common saponins. Among these methods, the physical conversion of saponin has harsh conditions, high energy consumption, and low yield, making it unsuitable for industrial production. 16–18 Chemical preparation methods have some disadvantages such as production of by-products, violent reaction process, difficulty in purification, and environmental pollution. 19,20 In comparison with physical and chemical methods, biotransformation has quite good advantages. For example, biotransformation is a fast, convenient, efficient, and environmentally friendly method for obtaining rare saponins. Hence, biotransformation is a common method for ginsenoside transformation. 21,22 Many studies have focused on the production of rare ginsenosides by microbial enzyme. 23,24 Moreover, the rare ginsenoside CK can be prepared from the common saponins Rb1 by microorganisms or enzymes produced from microorganisms. Lactic acid bacteria isolated from kimchi or enzyme preparation for culturing Armillaria can convert common saponins into highly active ginsenoside CK. 25–27 Quan et al. 28 discovered that ginsenoside Rb1 can be biotransformed by leuconostoc mesenteroides DC102 to obtain the CK, producing many intermediate products such as prosapogenins, gypenoside XVII, ginsenoside Rd, and ginsenoside F2 in this conversion reaction. Ginsenoside CK can be obtained from substrate ginsenoside Rb1 by the transformation of microorganism Endophyte JG09 from Platycodon grandiflorum, 29 in which 168 h of culture, ginsenoside CK is rapidly produced. Although these studies provide a good solution on how to obtain rare ginsenoside CK, the biotransformation of ginsenoside Rb1 into F2 and CK encounters many problems, such as low conversion rate, long conversion time, production of many intermediate products, and difficult separation of product. In addition, the final F2 and ginsenoside CK is mainly used for disease treatment and drug development. Hence, the highly efficient bioconversion of ginsenoside F2 and CK is very important and innovative.
In the present study, the extracellular enzyme produced by the safe strain A. niger Wu-16 was extracted and used as catalyst to transform the main ginsenoside Rb1 to produce ginsenoside F2 and CK. The conditions of the enzymatic reaction, including temperature, pH, reaction time, and ratio of enzyme to substrate were optimized, and the influence of metal ions on the conversion reaction was explored. This research opens up a new path for the simultaneous and efficient production of ginsenoside F2 and CK and lays the foundation for the mass production of rare ginsenoside in the future.