Chapter 6
Potato Glycoalkaloids: Chemical, Analytical, and Biochemical Perspectives 1
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S. J. Jadhav , S. E. Lutz , G. Mazza , and D. K. Salunkhe 1
Food Processing Development Centre, Leduc, Aberta T9E 7C5, Canada Food Research Program, Research Centre, Agriculture and Agri-Food Canada, Summerland, British Columbia V0H 1Z0, Canada Department of Nutrition and Food Sciences, Utah State University, Logan, UT 84322
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The most recentfindingson the structural characteristics, extraction, separation, analysis, biosynthesis and metabolism of potato glycoalkaloids are discussed. Molecular structures of the predominant steroidal alkaloids are presented, and analytical methods including colorimetric, TLC, GLC, HPLC and ELISA techniques are discussed in detail. Glycoalkaloids are naturally occurring secondary metabolites of potato. These compounds are comprised of a steroidal-like alkaloid skeleton to which one to four sugars may be attached through a series of glycosidic linkages. Glycoalkaloids are toxic and thus are believed to be involved in pest resistance. These toxicants occur in potato tubers, peels, sprouts, and blossoms and their concentration in tubers depends on cultivar, maturity, environmental factors, and stress conditions (7,2). Concentrations may vary as a result of fungal or bacterial infection and usually increase in response to wound, apparently as a defense mechanism against potential disease. Glycoalkaloids, therefore, may function as phytoalexins (2,3). Potato tubers with more than 20 mg of glycoalkaloids per 100 g offreshweight exceed the upper safety limit for food purposes. Moreover, the glycoalkaoids are not destroyed under conventional heat processing regimes. In fact, the toxicity is highly variable among the different glycoalkaoids. Certain factors can induce variation of the toxic levels of the glycoalkaloids in potatoes. There is great genetic variability of the glycoalkaloid level in tubers, and breeding programs for improved cultivars may accidently introduce extensive glycoalkaloid accumulation and thereby increase the potential for toxicoses. Since the role of the glycoalkaloids in host defense to insects and pathogens is uncertain and the presence of these compounds in potato tubers can have a negative impact on quality, the need for these compounds in potatoes is questionable (4). In view of these questions, understanding of the chemical, analytical, and biochemical aspects of the glycoalkaloids in potatoes has become important. 94
© 1997 American Chemical Society In Antinutrients and Phytochemicals in Food; Shahidi, F.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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Structures and Chemistry The glycoalkaloids in potato possess a steroidal skeleton that incorporates nitrogen in the molecule by cyclization of the steroidal side chain. The chemistry of steroidal alkaloids of the Solamim genus has been extensively reviewed (5-8). Most alkaloids in potato occur as glycosides; upon removal of the sugars alkamines (aglycones) are formed, which have a close structural resemblance with cholesterol. Molecular structures of the predominant steroidal alkaloids of potato are shown in Figure 1. Sugars released by partial hydrolysis have indicated that the glycoalkaloids, asolanine and a-chaconine, have branched sugars. They are named P-solatriose and pchacotriose, respectively, and assigned the structures of 0-a-L-rhamnopyranosyl-(l-2 gal)-0-p-D-glucopyranosyl-(l-3 gal)-P-D-galactopyranose and O-a-L-rhamnopyranosyl-( 1-2 glu)-0-a-L-rhamnopyranosyl-( 1 -4 glu)-P-D-glucopyranose. The disaccharides of p-solanine and P-chaconine are as follows: O-P-D-glucopyranosyl(l-3)-P-D-galactopyranose (P-solabiose) and 0-a-L-rhamnopyranosyl-(l-4)-P-Dglucopyranose (P-chacobiose). Optical data and the partial syntheses of y-solanine (3PD-galactosidosolanidine) and y-chaconine (3P-D-glucosidosolanidine) have shown that the alkamines are bound P-glycosidically with D-sugars (7). The sugar portions of asolamarine and p-solamarine were shown to be identical with P-solatriose and Pchacotriose, respectively. Gas liquid chromatography (GLC), mass spectrometry (MS), and thin layer chromatography (TLC) have indicated that commersonine is closely related to demissine, a terminal glucose unit in the former being replaced by the terminal xylose in the latter. The tetrasaccharide moiety (P-lycotetraose) of demissine, like tomatine, possesses the 0-P-D-glucopyranosyl-(l-2 glu)-0-P-D-xylopyranosyl-(H3 glu)-0-P-D-glucopyranosyl-(l-4 gal)~P-D-galactopyranose linkage. The mass spectral fragmentation patterns of the glycoalkaloids were revealed by Price et al. (9). The potato alkaloids have been visualized as two different steroidal skeletons: solanidane (solanidine type), which contains the indolizidine system exemplified by solanines, chaconines, solanidine, leptines, leptinines, and demissidine, and spirosolane (solasodine type), which possesses oxa-azaspirodecane structure as represented by tomatidenol, a- and P-solamarines. The structure of solanidine has been proved to be solanid-5en-3p-ol (JO). Demissidine is identical with solanidan-3P-ol. The stereochemistry of solanidanes has been elucidated with the aid of IR spectroscopy and X-ray analysis of demissidine hydroiodide. All natural solanidanes possess 20S:22R:25S:NS-configuration. Hydrolysis of leptines I and n, by esterases or by mild alkaline treatment creates leptinines I and II which are 23-hydroxy a-chaconine and 23-hydroxy a-solanine, respectively. The removal of sugars from leptinines I and II affords the aglycone leptinidine. Leptinidine possesses a double bond and a hydroxyl group similar to A -3p-ol system. The position of the second hydroxyl in leptinidine was provided by the selenium dehydrogenation study and was further defined by IR and N M R as P-oriented (77). Leptinidine is therefore 23P-hydroxysolanidine. Tomatidenol (tomatid-5-en-3P-ol) is C(5,6) dehydro tomatidine and has been prepared from 16-ketopregnane following an analogous synthetic pathway as outlined by Kessar et al. (72). Schreiber (7) found a small amount of this spirosolenol in an acid hydrolyzate of a glycoalkaloid mixture extracted from potato sprouts. The hydrolysis products and sugar moieties of the glycoalkaloids are listed in Table I and the aglycones are shown in Figure 2. 5
In Antinutrients and Phytochemicals in Food; Shahidi, F.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
ANTINUTRIENTS AND PHYTOCHEMICALS IN FOOD
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CH
Figure 1. Molecular structures of a-solanine and a-chaconine.
In Antinutrients and Phytochemicals in Food; Shahidi, F.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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Table I. Glycoalkaloids and Their Hydrolysis Products (Aglycone + Sugar Moiety) Glycoalkaloid
Aglycone
Sugar Moiety
a-Solanine
Solanidine
-D-gal
