Partial amino acid sequence of potato solanidine UDP-glucose glucosyltransferase purified by new anion-exchange and size exclusion media

Potato glycoalkaloids and metabolites: roles in the plant and in the diet

Potatoes, members of the Solanaceae plant family, serve as major, inexpensive low-fat food sources providing energy (starch), high-quality protein, fiber, and vitamins. Potatoes also produce biologically active secondary metabolites, which may have both adverse and beneficial effects in the diet. These include glycoalkaloids, calystegine alkaloids, protease inhibitors, lectins, phenolic compounds, and chlorophyll. Because glycoalkaloids are reported to be involved in host-plant resistance and to have a variety of adverse as well as beneficial effects in cells, animals, and humans, a need exists to develop a clearer understanding of their roles both in the plant and in the diet. To contribute to this effort, this integrated review presents data on the (a) history of glycoalkaloids; (b) glycoalkaloid content in different parts of the potato plant, in processed potato products, and in wild, transgenic, and organic potatoes; (c) biosynthesis, inheritance, plant molecular biology, and glycoalkaloid-plant phytopathogen relationships; (d) dietary significance with special focus on the chemistry, analysis, and nutritional quality of low-glycoalkaloid potato protein; (e) pharmacology and toxicology of the potato glycoalkaloids comprising alpha-chaconine and alpha-solanine and their hydrolysis products (metabolites); (f) anticarcinogenic and other beneficial effects; and (g) possible dietary consequences of concurrent consumption of glycoalkaloids and other biologically active compounds present in fresh and processed potatoes. An enhanced understanding of the multiple and overlapping aspects of glycoalkaloids in the plant and in the diet will benefit producers and consumers of potatoes.

Analysis of biologically active compounds in potatoes (Solanum tuberosum), tomatoes (Lycopersicon esculentum), and jimson weed (Datura stramonium) seeds

Potatoes and tomatoes, members of the Solanaceae plant family, serve as major, inexpensive low-fat food sources providing for energy, high-quality protein, fiber, vitamins, pigments, as well as other nutrients. These crops also produce biologically active secondary metabolites, which may have both adverse and beneficial effects in the diet. This limited overview, based largely on our studies with the aid of HPLC, TLC, ELISA, GC-MS, and UV spectroscopy, covers analytical aspects of two major potato trisaccharide glycoalkaloids, alpha-chaconine and alpha-solanine, and their hydrolysis products (metabolites) with two, one, and zero carbohydrate groups; the potato water-soluble nortropane alkaloids calystegine A3 and B2; the principal potato polyphenolic compound chlorogenic acid; potato inhibitors of digestive enzymes; the tomato tetrasaccharide glycoalkaloids dehydrotomatine and alpha-tomatine and hydrolysis products; the tomato pigments beta-carotene, lycopene, and chlorophyll; and the anticholinergic alkaloids atropine and scopolamine present in Datura stramonium (jimson weed) seeds that contaminate grain and animal feed. Related studies by other investigators are also mentioned. Accurate analytical methods for these food ingredients help assure the consumer of eating a good-quality and safe diet.

Effect of feeding solanidine, solasodine and tomatidine to non-pregnant and pregnant mice

The aglycone forms of three steroidal glycoalkaloids-solanidine (derived by hydrolytic removal of the carbohydrate side chain from the potato glycoalkaloids alpha-chaconine and alpha-solanine), solasodine (derived from solasonine in eggplants) and tomatidine (derived from alpha-tomatine in tomatoes)-were evaluated for their effects on liver weight increase (hepatomegaly) in non-pregnant and pregnant mice and on fecundity in pregnant mice fed for 14 days on a diet containing 2.4 mmol/kg of aglycone. In non-pregnant mice, observed ratios of % liver weights to body weights (%LW/BWs) were significantly greater than those of the control values as follows (all values in % vs matched controls+/-S.D.): solanidine, 25.5+/-13.2; solasodine 16.8+/-12.0; and tomatidine, 6.0+/-7.1. The corresponding increases in pregnant mice were: solanidine, 5.3+/-10.7; solasodine, 33.1+/-15.1; tomatidine, 8.4+/-9.1. For pregnant mice (a) body weight gains were less with the algycones than with controls: solanidine, -36.1+/-14.5; solasodine, -17.9+/-14.3; tomatidine, -11.9+/-18.1; (b) litter weights were less than controls: solanidine, -27.0+/-17.1; solasodine, -15.5+/-16.8; tomatidine, no difference; (c) the %LTW/BW ratio was less than that of the controls and was significant only for solasodine, -8.7+/-13.7; and (d) the average weight of the fetuses was less than the controls: solanidine, -11.2+/-15.2; solasodine, -11.4+/-9.4; tomatidine, no difference. Abortion of fetuses occurred in five of 24 pregnant mice on the solanidine and none on the other diets. To obtain evidence for possible mechanisms of the observed in vivo effects, the four glycoalkaloids (alpha-chaconine, alpha-solanine, solasonine and alpha-tomatine) mentioned above and the aglycones solanidine and tomatidine were also evaluated in in vitro assays for estrogenic activity. Only solanidine at 10 microM concentration exhibited an increase in the MCF-7 human breast cancer cell proliferation assay. Generally, the biological effects of solanidine differ from those of the parent potato glycoalkaloids. Possible mechanisms of these effects and the implication of the results for food safety and plant physiology are discussed.

Tomato glycoalkaloids: role in the plant and in the diet

Tomatoes, a major food source for humans, accumulate a variety of secondary metabolites including phenolic compounds, phytoalexins, protease inhibitors, and glycoalkaloids. These metabolites protect against adverse effects of hosts of predators including fungi, bacteria, viruses, and insects. Because glycoalkaloids are reported to be involved in host-plant resistance, on the one hand, and to have a variety of pharmacological and nutritional properties in animals and humans, on the other, a need exists to develop a better understanding of the role of these compounds both in the plant and in the diet. To contribute to this effort, this integrated review presents data on the history, composition, and nutrition of tomatoes, with special focus on the assessment of the chemistry, analysis, composition, nutrition, microbiology, and pharmacology of the tomato glycoalkaloids comprising alpha-tomatine and dehydrotomatine; their content in different parts of the tomato plant, in processed tomato products, and in wild and transgenic tomatoes; their biosynthesis, inheritance, metabolism, and catabolism; plant-microbe relationships with fungi, bacteria, viruses, insects, and worms; interactions with ergosterol and cholesterol; disruption of cell membranes; tomatine-induced tomatinases, pantothenate synthetase, steroid hydroxylases, and cytokines; and inhibition of acetylcholinesterase. Also covered are tomato-human pathogen relationships and tomatine-induced lowering of plasma cholesterol and triglycerides and enhancement of the immune system. Further research needs in each of these areas are suggested. The overlapping aspects are discussed in terms of general concepts for a better understanding of the impact of tomato glycoalkaloids in the plant in general and in food in particular. Such an understanding can lead to the creation of improved tomatoes and to improved practices on the farm and in the consumption of tomatoes.

Tracer studies on the incorporation of [2-14C]-DL-mevalonate into chlorophylls a and b, alpha-chaconine, and alpha-solanine of potato sprouts

Chlorophyll and glycoalkaloids are synthesized in different parts of the potato plant including leaves, tubers, and sprouts. Although light stimulates the biosynthesis of both constituents, the question of whether the two biosynthetic pathways are under the same genetic control has not been resolved. This study investigated the dynamics of incorporation of labeled [2-(14)C]-DL-mavalonate into chlorophyll a, chlorophyll b, and the glycoalkaloids alpha-chaconine and alpha-solanine in potato sprouts after 7 and 14 days of storage in the light and in the dark. No chlorophyll synthesis occurred in the dark. Fractionation of the "glycoalkaloid" extract followed by high-performance liquid chromatography produced four peaks. The fractions were collected and analyzed for radioactivity. About 80% of the radioactivity resided in fraction 1, the composition of which is unknown. Two of the fractions, with 1-14% of the original label, were alpha-chaconine and alpha-solanine. The radioactivity derived from mevalonate largely resides in unidentified compound(s) eluting as a single peak on the HPLC column before the peaks associated with the glycoalkaloids. The specific radioactivity of alpha-chaconine and alpha-solanine increased approximately 2-fold in going from 7 to 14 days of exposure in the light and in the dark. These and additional observations point to the near identity of the dynamics of biosynthesis of the two glycoalkaloids. These data also implicate a non-mevalonate pathway for the synthesis of both chlorophylls and the glycoalkaloids and are consistent with independent genetic control of the concurrent formation of the two classes of compounds during greening of potatoes.

Postharvest changes in glycoalkaloid content of potatoes

Potatoes contain antinutritional and potentially toxic compounds including inhibitors of digestive enzymes, hemagglutinins, and glycoalkaloids. Solanum glycoalkaloids are reported to inhibit cholinesterase, disrupt cell membranes, and induce teratogenicity. In this overview, we describe the role of potatoes in the human diet, reported changes in glycoalkaloid content of fresh and processed potatoes during storage, under the influence of light and radiation, following mechanical damage, and as a result of food processing. Also covered are safety aspects and suggested research needs to develop a protocol that can be adopted by the potato producers and processors to minimize post-harvest synthesis of glycoalkaloids in potatoes. Reducing the glycoalkaloid content of potatoes will provide a variety of benefits extending from the farm to processing, shipping, marketing, and consumption of potatoes and potato products. A commercially available ELISA kit is described which permits rapid assay of glycoalkaloid content of parts of the potato plant including leaves, tubers, and peel, as well as processed potato products including french fries, chips, and skins. Understanding the multiple overlapping aspects of glycoalkaloids in the plant and in the diet will permit controlling postharvest glycoalkaloid production for the benefit of the producer and consumer.

Cloning and expression of transaldolase from potato

We have isolated a cDNA encoding transaldolase, an enzyme of the pentose-phosphate pathway, from potato (Solanum tuberosum). The 1.5 kb cDNA encodes a protein of 438 amino acid residues with a molecular mass of 47.8 kDa. When the potato cDNA was expressed in Escherichia coli a 45 kDa protein with transaldolase activity was produced. The first 62 amino acids of the deduced amino acid sequence represent an apparent plastid transit sequence. While the potato transaldolase has considerable similarity to the enzyme from cyanobacteria and Mycobacterium leprae, similarity to the conserved transaldolase enzymes from humans, E. coli and Saccharomyces cerevisiae is more limited. Northern analysis indicated that the transaldolase mRNA accumulated in tubers in response to wounding. Probing the RNA from various potato tissues indicated that the transaldolase mRNA accumulation to higher levels in the stem of mature potato plants than in either leaves or tubers. These data are consistent with a role for this enzyme in lignin biosynthesis.

Multiple secondary plant product UDP-glucose glucosyltransferase genes expressed in cassava (Manihot esculenta Crantz) cotyledons

Six different putative UDP-glucose glucosyltransferase clones were isolated from a cassava cotyledon cDNA library probed with an Acc I-Bgl II restriction fragment from a UDP-glucose flavonoid 3-O-glucosyltransferase from Antirrhinum majus. The heterologous probe contained a glucosyltransferase consensus signature amino acid sequence which was also present in the cassava cDNA clones. Nucleotide and derived amino acid sequences are presented for two of the clones. Northern analysis showed different patterns of expression for the six genes in developing seedling tissues, indicating temporal and tissue-specific regulation. A comparative analysis was made of the six cassava clone derived amino acid sequences and other reported UDP-glucosyltransferase genes. Highly conserved residues in plant genes from three species allow redefinition of essential residues within the signature sequence for secondary plant product metabolism glucosyltransferase genes.

Absence of genotoxicity of potato alkaloids alpha-chaconine, alpha-solanine and solanidine in the Ames Salmonella and adult and foetal erythrocyte micronucleus assays

To assess whether reported toxicities of potato-derived glycoalkaloids could be the result of interactions with cellular DNA, the genotoxic effects of alpha-solanine, alpha-chaconine and solanidine were studied, using the Ames test (Salmonella strains TA98 and TA100), the mouse peripheral blood micronucleus test and the mouse transplacental micronucleus test. The Ames test for mutagenicity with alpha-solanine was weakly positive in TA100 with S-9 activation (29 revertants per millimole per plate). However, pooled data from duplicate tests gave a negative effect. Pooled data from two experiments with alpha-chaconine gave a weak positive response in TA98 without microsomes (17 revertants per millimole per plate). The micronucleus tests for clastogenicity using male mouse and foetal blood were negative. The absence of mutagenicity and clastogenicity suggests lack of damage to intracellular DNA for potato alkaloid toxicity.