Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2002
Publication Date: 12/15/2002
Citation: GU, L., TAO, G., GU, W., PRIOR, R.L. Determination of Soyasaponins in Soy with LC-MS Following Structural Unification by Partial Alkaline Degradation. JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY. 2002. v. 50(24). p. 6951-6959.
Interpretive Summary: Soyasaponins are a class of chemical compounds that exist in soy protein. Some health promoting properties have been reported for this class of compounds, however, there has not been a good analytical method for their quantitation. The chemical structural diversity of soyasaponins hinders the separation and determination of the individual compounds in soy. Since these compounds will likely be found in infant formulas and other diets containing soy protein, it is important to be able to determine the amounts contained in the diet. This manuscript describes for the first time, a method whereby the many different conjugated soyasaponins are hydrolyzed, using mild basic conditions, to a single well-defined structure that can be separated and quantitated. The content of soyasaponins in soy was found to be comparable to the quantities of another class of compounds found in soy, the isoflavones. Using this analytical method, it will be possible to know and control for the quantities of the soyasaponins in the diet of clinical studies using soy or soy extracts.
Technical Abstract: Soyasaponins are another classes of phytochemicals in soy coexisting with the isoflavones. They serve as the primary dietary sources of saponins from foods. Multiple health promoting properties has been reported for them. Due to their structural diversities, there is no method so far to quantify the individual soyasaponins with accuracy. High-Performance Liquid Chromatography coupled with electrospray ionization mass spectrometer was used to study the soyasaponins in soy. We found that each soyasaponin belonging to group A existed mainly in their genuine acetylated forms. The partially to fully deacetylated structures coexisted in various proportions. Likewise, the soyasaponins belonging to group B in soy were detected both as 2,3-dihydro-2,5-dihydroxy-6-methyl-4-pyrone (DDMP) conjugated forms and non-DDMP forms. The structural diversity of soyasaponins hinders the separation and determination of the individual compounds in soy. In the present studies, the soyasaponins extracted from soy were treated with sodium hydroxide under mild conditions to cleave the acetyl groups from soyasaponins in group A as well as the DDMP from soyasaponins in group B, while the glycoside structures remained unaffected. By doing so, all soyasaponins originating from the same initial structures were unified into well-defined structures and then quantified individually using the selective ion recording of their [M-H]- ions. The pure deacetyl and non-DDMP soyasaponins were used as the external standards. The quantification limits of soyasaponins in group A and group B were 1.74 ng and 1.89 ng injected on column with recovery rates of 94.1 plus/minus 4.2% and 96.9 plus/minus 2.9%, respectively. The content of soyasaponins in soy is comparable to that of the isoflavones. Since these two classes of compounds have almost the same polarity, they cannot be separated unless a complicated chromatographic method is used. In most clinical studies using soy or soy extracts, the amount of soyasaponins in the formula was not controlled. While the soyasaponin itself has physiological effects, the interaction between soyasaponins and isoflavones may also be important. The method here provides for the first time a method to quantify the soyasaponins in the diet.