Sterols and the phytosterol content in oilseed rape - Journal of Cell ...

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72 M. Kemal Gül and Samija Amar Introduction Sterols are natural, organic compounds and they are widely distributed in all eukaryotic organisms. They present a significant part of the organism membrane biomass, while their functional role is evident through the participation in the control of membraneassociated metabolic processes, such as: regulations of membrane permeability and fluidity, signal transduction events and the activity of membranebound enzymes (Piironen et al., 2000). Sterols are the precursors of steroid hormones and bile acids in humans, brassinosteroids - phytohormones in plants and, as the recent identifications of sterol mutants have shown (Lindsey et al., 2003), they are involved in important growth and developmental processes in living organisms (Hartmann, 1998). Plants have a variety of more than 40 well-identified and studied sterols (Law, 2000), which are termed phytosterols and are predominantly present in oilseed plants. Cereals are recognised as a significant source of phytosterols as well, whereas phytosterol content in vegetables and nuts is considerably lower (Piironen et al., 2000; Piironen et al., 2003; Normen et al., 1999). The most abundant phytosterols are: sitosterol, campesterol and stigmasterol. Other phytosterols like avenasterol and cycloartenol are synthesised earlier in the biosynthetic pathway and as sterol precursors they usually occur in relatively smaller amounts (Määttä et al., 1999). Phytosterols are, with respect to their physiological function and their chemical structure, similar to the major and only animal produced sterol – cholesterol. Increased scientific interest and economic importance, in the past few decades, in the most important oil crop in Europe – rapeseed, has been largely due to its improved quality of oil seeds, which can yield between 40 and 47 percentages of oil (Becker et al., 1999). Brassica napus L., known as rapeseed, rape, or in some cultivars, low in erucic acid and glucosinolate content, as canola, belongs to the genus B r a s s i c a , which is a member of the Brassicaceae (Cruciferae) f a m i l y. This family, of about 375 genera and 3200 species, includes crops, condiments and ornamentals but only genus Brassica is well known for their admirable phenotypical diversity: cabbage, cauliflower, broccoli, Brussels sprouts, kohl-rabi, turnip, black and white mustards, garden cress and so forth (Gomez-Campo, 1999). Like other vegetable oils, the rapeseed oil is the richest natural source of phytosterols. Apart from phytosterols rapeseed oil is predominantly composed of fatty acids, such as oleic acid, linoleic and linolenic acid (vitamin F complex), of phospho- and glycolipids and of tocopherols (vitamin E) and carotenoids - pro-vitamin A (Rehm and Espig, 1991). Therefore, it became reasonable to increase the rapeseed oil production. According to “Food Outlook” (FAO, 2004) from Food and Agriculture Organisation of the United Nations, global production of rapeseed crop rose more than 10 % from 1998 to 2002, with the estimation for 2004 that it will reach second place after soybean. Encouraged with the development of improved rape cultivars with high quality edible oils and most recent utilisation of rapeseed oil for bio-diesel (biogenic fuel) production (UFOP, 2004), the EU farmers expanded rapeseed planting, exceeding those of soybean, sunflower, groundnut and cottonseed (Kimber and McGregor, 1995). Literature review Phytosterols have been isolated from a large number of species and according to numerous publications (Grunwald, 1980; Gordon and Miller, 1997; Dutta and Normen, 1998; Piironen et al., 2000) they probably exist in all angiosperm and gymnosperm species. Although, there are more than 40 diff e r e n t phytosterols found in higher plants, sitosterol, campesterol and stigmasterol often predominate, while other phytosterols are usually typical only for certain plant family or even species. Brassicasterol is for example typical only for Brassicaceae family, and therefore it could be used for identification (Benveniste, 2002). Chemical structure and properties Sterols belong to a large group of hydrocarbons, o rganic chemical compounds known as polyisoprenoids with carbon skeletons structurally based and derived from multiple isoprene, five carbon unit -CH 2=C(CH 3)CH=CH 2. Sterols are, together with tocopherols, carotenoids and chlorophylls, formed by polymerisation of isoprene unit (Grunwald, 1980). The structural feature, which virtually all sterols have in common, is that they are derivatives of a tetracyclic perhydro-cyclopentano-phenanthrene ring system with a flexible side chain at the C-17 atom (Figure 1) and 3β-monohydroxy compounds and (Hartmann, 1998).
Animal cells contain only one major sterol, i.e. c h o l e s t e ro l (Figure 2a). Cholesterol also occurs, though only in a few percentage of the whole sterol content, in plants (Gordon and Miller, 1997). Chemically, it is an analogue to the phytosterols, differing only in the side chain. Fungal cells, together with some unicellular algae and lichens synthesise ergosterol - provitamin D2 (Grunwald, 1980; Rehm and Espig, 1991). Namely, ergocalciferol (vitamin D2) is produced by ultraviolet irradiation of provitamin D2 ( e rgosterol), which occurs in yeast and fungi (Figure 2b). In contrast to animal cells and fungi, plant cells synthesize complex array of phytosterol mixtures with the sterol profiles varying between species. The scientific names of phytosterols are given to them according to the number of C atoms in the C-17 side chain, the number and the position of the double Sterols and phytosterols in oilseed rape 73 Figure 1. Chemical structure of 5 α cholestan 3β-ol (adapted from Piironen et al., 2000) a. b. bond in the ring system and the side chain. Their scientific names are usually very complex, so the most common phytosterols are referred to by their trivial names. The trivial and the scientific names of the most important phytosterols are given in Table 1. According to the IUPAC recommendations from 1989, sterol molecules consist of four rings marked as A, B, C and D with standard carbon numbering (Figure 1). Three rings, A, B and C, have 6 carbons atom nonlinear structure and they are fused to one 5 carbons atom ring (D). The various phytosterols found in plants differ in number of C atom in the side chain at the C-17 atom and the position and the number of the double bonds in the ring system. T h e predominating phytosterols in plants are: campesterol sometimes referred to as 24-methylcholesterol (Figure 3a), sitosterol (Figure 3c) and stigmasterol (Figure 3d). Figure 2. Chemical structure of a. cholesterol and b. ergosterol (taken from Kyoto Encyclopedia of Genes and Genomes, 2004)
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