A Facile Method to Determine the Native Contents of 4′-O-Methylpyridoxine and 4′-O-Methylpyridoxine-5′-glucoside in Ginkgo biloba Seeds

4'-O-Methylpyridoxine (MPN) and MPN-5'-glucoside (MPNG) are collectively known as ginkgotoxin, which are the main toxic ingredients of excessive consumption of Ginkgo biloba seeds. Water extraction is the generally adopted sample preparation method for high-performance liquid chromatography determination of ginkgotoxin. However, endogenous enzymes such as glycosidases in Ginkgo biloba seeds can hydrolyze MPNG to MPN in the process of water extraction, which will result in the measured contents of MPN and MPNG but not their natural contents in Ginkgo biloba seeds. In this work, inhibitors for the endogenous enzymes were first screened, and it was found that silver fluoride could effectively inhibit endogenous enzymes such as glucosidase and phosphatase. The optimized concentration of silver fluoride was 25 mmol/L, which could effectively inhibit the endogenous enzymes for more than 60 h. A new sample preparation method based on water extraction with 25 mmol/L silver fluoride addition was thus developed. This method was employed to determine the native contents of MPN and MPNG in the exotesta and kernel of five Ginkgo biloba seed cultivars. The result showed that the contents of MPNG in the exotesta and kernel of five cultivars were significantly higher than those of MPN. MPNG was present at high content in raw seeds, which was the main form of ginkgotoxin in seeds. The method established in this work is simple and effective and can be used to accurately quantify the native contents of MPN and MPNG.