Lipids in potato tubers

The membrane lipids, in particular phospholipids and galactolipids, make up the predominant fraction of lipids in potato tubers, and only trace amounts of triacylglycerols are present. For example, in the tuber of cultivar Desiree, the membrane lipids constitute 92.8 mol-% of total lipids, whereas triacylglycerol amounts to only 1.1 mol-% (Klaus et al., 2004). 

Galliard, T. (1968). Aspeets of lipid metabolism in higher plants-I. Identification and quantitative determination of the lipids in potato tubers. Phytochemistry, 7, 1907-1914. 

Galliard T. The enzymatic breakdown of lipids in potato tuber by phospholipid and galactolipid acyl-hyrolase activities and by lipoxygenase. Phytochemistry 1970 9 1725-1734. 

Faueonnier, M. L., Welti, R., Blee, E., Marlier, M. (2003a). Lipid and oxylipin profiles during aging and sprout development in potato tubers (Solanum tuberosum L.). Biochim. Biophys. Acta, 1633,118-126. 

The patatin family consists of various glycoproteins in plants making up more than 40% of the total soluble protein in potato tubers. Patatins serve as storage proteins and it has been demonstrated that they exhibit both lipid acyl hydrolase and acetyl transferase activities, which might be involved in tissue wounding responses. Further, recent studies report on antioxidant activities of the major potato allergen Sola t 1 (Seppala et al. 2000). 

Lipids are not evenly distributed in starch granules. A certain proportion resides on the surfaces of granules (endosperm), and usually a much higher proportion resides as internal starch lipids. All of them are monoacyl glycerides, free fatty acids, and lysophospholipids. Cereal materials contain mainly internal starch lipids. Lipids of potato tubers reside in amyloplasts and are... 

Evidence for true lipases in other plants is limited. Hasson and Laties (1976a) have recently separated true lipase activity from other lipid acyl hydrolase enzymes in potato tubers. 

The action of hydrolases released on physical damage has been discussed earlier. The results of such action may be rapid and extensive, as shown with bean leaves and potato tubers (see above). To illustrate this, it can be calculated that, if all the endogenous substrates (membrane phospholipids and ga-lactolipids) in potato tuber were available to the acyl hydrolase enzyme, then at pH 6 all the lipid could be deacylated within 1 s at 25°C. Hence the need for care in the isolation of lipids or of subcellular fractions from such tissues. Free fatty acids may be respiratory substrates in storage tissue slices the liberation of free fatty acids by acyl hydrolase activity on cutting the tissue can provide the substrate, as shown by Hasson and Laties (1976a) in potato tuber slices. 

Acetate is frequently used to label the fatty acyl moieties of phospholipids. Galliard (1972) employed this property in preparing radioactive phospholipids in potato tuber slices PC and neutral lipids were the major fractions labeled, and the fatty acids labeled in PC were 16 0 (22.5%), 18 0 (11.4%), 18 1 (30.4%), and 18 2 (32.6%) after a 2-h incubation at 25°C. 

Recent progresses in the analysis of membrane lipids have shown the great variety of molecular species componing the central lipid bilayer of membranes. As an example, table 1 shows some twenty different molecular species of phospholipids found in potato tuber membranes. 

The composition and content of the different tocopherol components in plant tissue vary considerably, ranging from extremely low levels found in potato tubers to high levels found in oil seeds. a-Tocopherol is the predominant form in photosynthetic tissues and is mainly localized in plastids. The particular enrichment in the chloroplast membranes is probably related to the ability of tocopherols to quench or to scavenge reactive oxygen species and lipid peroxy radicals by physical or chemical means. In this way, the photosynthetic apparatus can be protected from oxygen toxicity and lipid peroxidation. In nonphotosynthetic tissues, 7-tocopherol frequently predominates and can be involved in the prevention of autoxidation of polyunsaturated fatty acids. 

Lipids can be identified and quantified using thin-layer chromatography (TEC) and gas chromatography (GC) (Galliard, 1968). Extraction of lipids is achieved by homogenizing potato tubers with isopropanol in a blender, followed by a series of filtrations and extractions with chloroform-methanol (2 1). Chloroform is removed by rotary evaporation and the residue is redissolved in benzene-ethanol (4 1). This extract is passed through a DEAE-cellulose column, and the fractions collected are subjected to TEC on 250 p,m layers of silica gel G, using three solvent systems. Fatty acid methyl esters for GC analysis are prepared by transmethylation of the parent lipids, or by diazomethane treatment of the free fatty acids released by acid... 

To place the available carbohydrate of potato, which is the main focus of this chapter, into a nutritional context, the nutrient composition of a typical commercial potato variety is shown in Table 13.1. An important aspect of the nutrient composition of potato tubers is that they are predominantly water (about 80%), and on a dry weight basis they consist mostly of available carbohydrate in the form of starch, with virtually no lipid. 

True lipases from plants will hydrolyze these partial glycerides, but other enzymes that attack monoacylglycerols (but not triacylglycerols) have been described. However, in most cases the full substrate specificities of these enzymes have not been studied. In one case a lipolytic acyl hydrolase from potato tubers was shown to hydrolyze mono- and diacylglycerols in addition to a range of polar lipids. Thus, to avoid introducing a class of hydrolytic... 

The lipid acyl hydrolase from potato tubers (described above) will transfer fatty acids from a donor lipid to a suitable alcohol acceptor. This effect is illustrated in Fig. 3 which shows the results obtained on adding increasing amounts of methanol to a system containing a lipid substrate and the enzyme at 30% methanol, the major reaction product is fatty acid methyl ester, and a relatively small amount of free fatty acid is produced. The affinity of the enzyme for methanol is approximately 10 times that for water, hence the acyltransferase activity is greater than the acyl hydrolase activity. 

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