Tomatine glycoalkaloid

Stine KJ et al (2006) Interaction of the glycoalkaloid tomatine with DMPC and sterol mono-layers studied by surface pressure measurements and brewster angle microscopy. J Physical Chem B 110 22220... 

The Indians of the Peruvian Altiplano eat potatoes with a dip of clay and a mustard-like herb. They say the clay removes the bitter taste and prevents stomach pains or vomiting after eating large amounts of potatoes. The people who eat clay intend detoxication. This may explain how Indians started to utilize and domesticate wild potatoes. Indeed, experiments have shown that four different types of edible clay adsorb the glycoalkaloid tomatine under simulated... 

Fig. 1A-C. Examples of different classes of saponins A the triterpenoid saponin avenacin A-1 from roots of Avena spp B the steroidal saponin gracillin, from Costus speciosus C the steroidal glycoalkaloid a-tomatine from tomato (Lycopersicon spp.)...

Friedman, M., Fitch, T. E., Yokoyama, W. E. (2000b). Lowering of plasma LDL cholesterol in hamsters by the tomato glycoalkaloid tomatine. Food Chem. Toxicol., 38, 549-553. 

Figure 14.3 Potato glycoalkaloids. (a) is tomatine, a spirosolane and (b) solanine, a solanidane.

Due to its antifungal properties it has been suggested that this glycoalkaloid may be important in resistance to fungal pathogens. Nevertheless, evidence for the role of tomatine as a barrier against microbial attack is inconclusive, and some confronting observations make... 

Fig. (1). Structure of the steroidal glycoalkaloid a-tomatine showing the aglycone moiety (tomatidine) and the tetrasaccharide moiety (p-lycotetraose).

Analysis of tomatine contents in tomato plants showed that the glycoalkaloid is present in all organs [7, 40-42]. There is a significant variation in the reported tomatine content of particular plant organs. For example, Tukalo (1958) found, in terms of dry weight, 0.86-1.9% tomatine in leaves, 0.3-0.6% in stems and roots and 0.93-2.2% in fully... 

This primary mode of action of tomatine, that involves the formation of complexes with membrane sterols is similar to that described for polyene antibiotics [2, 4], and results in pore formation and loss of membrane integrity. This mode of action is supported by the reduced activity of tomatine on sterol-free bacteria and Oomycete fungi such as Pythium and Phytophthora [15, 28], and the strongly reduced toxicity of hydrolysis products of the glycoalkaloid which fail to bind sterols [57]. 

More recently, Blankemeyer et al. studied the effect of tomatine and tomatidine on frog embryos and frog skin. They found that tomatine increased membrane permeability in frog embryos and decreased sodium-active transport in frog skin, in contrast to the essentially negative results with tomatidine [62]. This reinforces the hypothesis that the carbohydrate side-chain is essential for the glycoalkaloid activity. 

Although several authors claimed that the glycoalkaloid tomatine is not present in sufficient concentrations in roots or stems to play a major role in resistance to Fusarium oxysporum f. sp. lycopersici [7, 57, 88], the fact that tomatinase is induced in tomato plants during infection, strongly suggests that the enzyme may have a specific role in pathogenicity of tomato plant by F. oxysporum f. sp lycopersici. 

The function of these tomatinase in formae speciales that do not pathogenise tomato is unknown. One possible explanation could be the presence of tomatine or similar saponins in their host plant species. However, (i) tomatine has not yet been reported in these plants [2, 7, 9] and (ii) although some of these species contain small amount of tomatine and other saponins structurally related to tomatine (e.g. potato contains a- solanine and a-chaconine [2, 4, 9, 90]), these are inactive as inducers of tomatinase and, moreover, tomatinase cannot use any of these glycoalkaloids as substrate [89]. In addition, it is clear that tomatinase is not required for pathogenicity in these isolates, at least in the case of F. oxysporum f. sp. melonis, where some strains that are fully pathogens on muskmelon lack tomatinase activity [89]. 

One of the isolates used by Quidde et al., was unable to metabolize tomatine and was more sensitive to the glycoalkaloid. In addition, this isolate was not able to induce disease symptoms on detached tomato leaves, but could pathogenize bean leaves, indicating that the ability to degrade tomatine may be required for pathogenicity of B. cinerea on tomato [36]. It would be very interesting to transformed this strain with one tomatinase gene to confirm this hypothesis. 

The occurrence of a tomatine-detoxifying enzyme in F. solani, was studied in liquid medium with or without tomatine. No tomatinase activity was detected in the absence of tomatine, whereas a remarkable increase of tomatinase activity was observed after 12 h of growth in the presence of the glycoalkaloid, the maximal tomatinase activity being observed after 72 h of incubation [37]. Detection of tomatinase activity in F. solani contrasts with the results reported by Defago and Kern [70],... 

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