processes Review Glycoalkaloids: Structure, Properties, and Interactions with Model Membrane Systems Bishal Nepal and Keith J. Stine * Department of Chemistry and Biochemistry, University of Missouri—Saint Louis, Saint Louis, MO 63121, USA * Correspondence: [email protected]; Tel.: +1-314-516-5346 Received: 22 June 2019; Accepted: 31 July 2019; Published: 5 August 2019 Abstract: The glycoalkaloids which are secondary metabolites from plants have proven to be of significant interest for their biological properties both in terms of their roles in plant biology and the effects they exhibit when ingested by humans. The main feature of the action of glycoalkaloids is their strong binding to 3β-hydroxysterols, such as cholesterol, to form complexes with the consequence that membrane structure is significantly perturbed, and leakage or release of contents inside cells or liposomes becomes possible. The glycoalkaloids have been studied for their ability to inhibit the growth of cancer cells and in other roles such as vaccine adjuvants and as synergistic agents when combined with other therapeutics. The glycoalkaloids have rich and complex physical behavior when interacting with model membranes for which many aspects are yet to be understood. This review introduces the general properties of glycoalkaloids and aspects of their behavior, and then summarizes their effects against model membrane systems. While there are many glycoalkaloids that have been identified, most physical or biological studies have focused on the readily available ones from tomatoes (α-tomatine), potatoes (α-chaconine and α-solanine), and eggplant (α-solamargine and α-solasonine). Keywords: glycoalkaloid; membrane; monolayer; liposome 1. Introduction A number of classes of compounds are known to disrupt cell membranes by interfering with bilayer structure, including dendrimers [1], chitosan [2], and membrane-active lytic peptides [3]. In this review, the focus is on the disruption of membrane structure by the class of compounds known as glycoalkaloids [4,5] which exert such effects by binding to cholesterol that is ubiquitously present in cell membranes [6], forming complexes that can aggregate and compromise the membrane structure. A particularly unique feature of these compounds is that in addition to their membrane-disrupting effects, they also interact with biological pathways in cells [7,8] and function as inhibitors of cholinesterase enzymes [9,10]. In particular, this review focuses on studies that have involved model membrane systems such as liposomes and monolayers at the water–air interface. Glycoalkaloids have been widely studied for many reasons, including their anti-cancer activities [11,12] either alone or in combination with various therapeutic agents [13–16], and their potential use as vaccine adjuvants [17–19]. Their study remains of major interest given their potential medicinal applications [20]. This review also surveys the biological effects of glycoalkaloids, as the connection between the physical interactions with membranes as studied primarily in model systems such as liposomes and monolayers, and the biological effects as primarily studied in cell lines poses an important question for future research. Glycoalkaloids are readily available natural products that may prove useful in therapeutic applications, alone or in combination with other biologically active agents. Processes 2019, 7, 513; doi:10.3390/pr7080513 www.mdpi.com/journal/processes Processes 2019, 7, x FOR PEER REVIEW 2 of 26 Processes 2019, 7, 513 2 of 26 2. Origin, Structure, and General Properties of Glycoalkaloids 2.1.2. Origin, Origin Structure,of Glycoalkaloids and General Properties of Glycoalkaloids Plants of the Solanaceae family include tomatoes, potatoes, and eggplants, and are well known to 2.1. Origin of Glycoalkaloids produce secondary metabolites known as glycoalkaloids [21]. The glycoalkaloids are produced in the leaves,Plants flowers, of the andSolanaceae roots of familythe plant. include In the tomatoes, case of potatoes, potatoes, they and are eggplants, also found and in are the well sprouts known [22]. to produceTomato plants secondary produce metabolites the glycoalkaloid known as glycoalkaloidsα-tomatine, potato [21]. Theplants glycoalkaloids produce both are α produced-chaconine in and the αleaves,-solanine, flowers, and andeggplants roots of produce the plant. both In the α-solamargine case of potatoes, and they α-solasonine are also found (see inFigure the sprouts 1). Green [22]. Tomatotomatoes plants can have produce up to the 500 glycoalkaloid mg kg−1 of tomatineα-tomatine, in the potato fruit, plantswhich producewill decrease both toα-chaconine near 5 mg kg and−1 uponα-solanine, ripening and [5]. eggplants The concentration produce both ofα toma-solamarginetine in the and vinesα-solasonine will stay near (see Figure500 mg1). kg Green−1. The tomatoes amount 1 1 ofcan α have-tomatine up to in 500 red mg tomatoes kg− of consumed tomatinein by the humans fruit, whichis 10–30 will mg decrease kg−1, but to if near green 5 mg tomatoes kg− upon are 1 consumedripening [5 ].then The the concentration level will ofbe tomatine 200–500 in mg the vineskg−1. willIn eggplants, stay near500 the mg average kg− . Theglycoalkaloid amount of 1 concentrationα-tomatine in redfor tomatoes21 varieties consumed was found by humans to vary is 10–30from mg0.625–20.5 kg− , but mg if greenkg−1 [23]. tomatoes The glycoalkaloid are consumed 1 contentthen the of level potatoes will bevaries 200–500 widely mg with kg− potato. In eggplants, type and conditions the average of cultivation glycoalkaloid [24] concentration and will increase for 1 in21 damaged varieties wasplants found [25]. to The vary glycoalkaloid from 0.625–20.5 content mg of kg normal− [23]. potato The glycoalkaloid tubers is reported content as of 12–20 potatoes mg varieskg−1 while widely green with tubers potato can have type 250–280 and conditions mg kg−1 and of cultivationgreen skin 1500–2200 [24] and willmg kg increase−1 [26]. The in damaged enzymes 1 plantsinvolved [25 in]. Thethe natural glycoalkaloid synthesis content of the of oligosacchar normal potatoides tubersof the isglycoalkaloids reported as 12–20 have mgbeen kg reviewed− while 1 1 [27].green tubers can have 250–280 mg kg− and green skin 1500–2200 mg kg− [26]. The enzymes involved in the natural synthesis of the oligosaccharides of the glycoalkaloids have been reviewed [27]. 2.2. Structure of Glycoalkaloids 2.2. Structure of Glycoalkaloids The glycoalkaloids consist of a nitrogen-containing steroidal aglycone and an attached oligosaccharide.The glycoalkaloids The prefix consist α- refers of ato nitrogen-containingthe compound with steroidalfully intact aglycone oligosaccharide, and an attachedwhereas removaloligosaccharide. of one sugar The prefix wouldα- refersbe denoted to the compound by a β- prefix, with fullyremoval intact of oligosaccharide, two sugars by whereasa γ- prefix, removal and removalof one sugar of three would for be a denoted tetrasaccharide by a β-prefix, by a removalδ- prefix. of The two aglycones sugars by aareγ-prefix, referred and to removal as tomatidine, of three solanidine,for a tetrasaccharide and solasidine, by a δ-prefix. respectively. The aglycones The branched are referred tetrasaccharide to as tomatidine, of solanidine,α-tomatine and is solasidine,known as lycotetraose.respectively. TheThe branchedbranched tetrasaccharide trisaccharide chacotriose of α-tomatine constitutes is known asα-chaconine lycotetraose. when The linked branched to solanidine,trisaccharide and chacotriose α-solamargine constitutes when linkedα-chaconine to solasidine. when Th linkede branched to solanidine, trisaccharide and solatrioseα-solamargine forms whenα-solanine linked when to solasidine. linked to solanidine The branched and α trisaccharide-solasonine when solatriose linked forms to solasidine.α-solanine The when structures linked toof thesolanidine glycoalkaloids and α-solasonine from tomato, when potato, linked and to solasidine. eggplant are The shown structures in Figure of the glycoalkaloids1. The solubility from of tomatinetomato, potato, in water and decreases eggplant with are shown pH, being in Figure reported1. The as solubility 6 mM at of pH tomatine 5, 1 mM in at water pH decreases6, 0.040 mM with at pH,pH 7, being and reported0.030 mM as at 6 pH mM 8 at[28]. pH 5, 1 mM at pH 6, 0.040 mM at pH 7, and 0.030 mM at pH 8 [28]. Figure 1. Cont. Processes 2019, 7, 513 3 of 26 Processes 2019, 7, x FOR PEER REVIEW 3 of 26 Figure 1.1. Structures of thethe glycoalkaloidsglycoalkaloids αα-tomatine,-tomatine, αα-chaconine,-chaconine, αα-chaconine,-chaconine, α-solamargine,-solamargine, andand α-solasonine.-solasonine. 2.3. General Properties of Glycoalkaloids 2.3. General Properties of Glycoalkaloids These compounds are important for the defense of the plant against pests and pathogens [29,30] These compounds are important for the defense of the plant against pests and pathogens [29,30] and have numerous biological and medicinal effects if ingested by humans [31]. While α-tomatine and have numerous biological and medicinal effects if ingested by humans [31]. While α-tomatine can help to protect the tomato plant against fungal pathogens and inhibit fungal growth [32], a number of pathogens express an enzyme that can detoxify tomatine by hydrolysis of sugars [33]. For Processes 2019, 7, 513 4 of 26 can help to protect the tomato plant against fungal pathogens and inhibit fungal growth [32], a number of pathogens express an enzyme that can detoxify