Tomatine structure

Fig. 2.1 Strucnires of some common saponins. The strucnires of some of the saponins described in the text are shown, including aescin from horse chesnut, avenacin A-1 and avenacoside A from oat, and a-tomatine from tomato. The glucose molecule enclosed in square brackets in the structure of avenacoside A highlights the glucose moiety that is cleaved off by hydrolysis by glycosidases in disrupted oat leaf tissue, leading to the fungitoxic 26-desglucosyl avenacoside A. Redrawn from [94]...

Tomatine is a steroid alkaloid glycoside consisting of an aglycone moiety (tomatidine), and a tetrasaccharide moiety (p-lycotetraose) composed of two molecules of glucose and one each of galactose and xylose the four monosaccharides form a branched structure that is attached at the C-3 position of the aglycone (reviewed by Roddick [7], see also references therein). The structure of tomatine is shown in Fig (1). 

Table 1.- Structural characteristics of partially hydrolyzed a-tomatine.

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

A second major mechanism of resistance to tomatine involves enzymatic detoxification by tomatinases. Although saponins are very numerous and widely distributed in the plant kingdom, detailed studies on saponin detoxification by fungi have been restricted to pathogens of a few plant species, principally to oat and the Solanaceous tomato and potato. This is because structures and antifungal properties of oat, tomato and potato saponins are well established, and that saponin profiles of these plants are relatively simple, in contrast to other plants like alfalfa, where over 20 predominant different saponins have been identified. 

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]. 

The structural relationship of tomatinase from B. cinerea to other tomatinases is unknown, but its molecular mass (70 kDa) is different from the other two enzymes mentioned before, 50 kDa from F. oxysporum [35, 38] and 110 kDa from S. lycopersici [33]. Moreover, when Quidde et al., attempted cloning of the tomatinase gene from B. cinerea using the tomatinase from S. lycopersici as a probe, they isolated a gene with high sequence homology, whose product had not tomatinase activity but was able to detoxify avenacin A-l [97], the saponin from oats related to some extent to tomatine. 

Steroidal alkaloids, such as solanine and tomatine which are present in many members of the Solanaceae, can form complexes with the cholesterol and other lipids present in biomembranes. Important for this interaction is the presence of a lipophilic portion of the molecule (given by the steroidal moiety) and a hydrophilic portion (provided by the sugar side chain). Whereas the lipophilic moiety "dives" into the lipophilic interior of the membrane and interacts with the structurally similar cholesterol, the hydrophilic side chain remains outside and binds to external sugar receptors. Since phospholipids are in a continuous motion (spinning around their axis and horizontal movements), a tension easily builds up which leads to membrane disruption i.e. transient "holes" form in the biomembrane rendering the cell leaky. Since particular steroidal alkaloids can specifically interact with receptors, ion channels or transmitter... 

Tomatine is tomatidine-3/3-lycotetrose. The lycotetrose moiety has a galactose attached to the 3-hydroxyl-group of tomatidine that is substituted on its 4-hydroxyl by a glucose. This glucose is further substituted at its 2-hydroxyl by another glucose and at its 4-hydroxyl by a xylose. Structures for muldamine and 225,25R- solanidanes (representing solanidanes both saturated and unsaturated at C-S, C-6) are shown below. 

Freeze-fracture E.M. studies of natural and synthetic membranes treated with filipin, digitonin and tomatine revealed characteristic protuberances on the surfaces of membranes (52,53) but how these structures relate to the chemical complexes or to the formation of pores is not understood. 

Alkaloids such as solanidine and tomatidine are related to diosgenin in terms of structure. These cholesterol-derived metabolites usually occur as the 3-(7-glycosides (e.g., sola-nine and tomatine) (Mann, 1987) (see Chapter 36). 

Fig. 7.22 The six most frequent glycoalkaloids of the genus Solanum further solanaceous detections solasonine in Cestrum parqui, solanine in Capsicum annuum, tomatine in Lycianthes ran-tonnetii. Dotted lines separate the structures given together with their trivial names above) from related congeners (below)...

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