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a Also called peanut.

b Castor oil contains 90% of ricinoleic acid (see Table 2.4). c Evening primrose oil contains about 10% of y-linolenic acid (see Table 2.3).

a Also called peanut.

b Castor oil contains 90% of ricinoleic acid (see Table 2.4). c Evening primrose oil contains about 10% of y-linolenic acid (see Table 2.3).

* The vegetable oils are listed according to whether their main commercial use is in (A) foods, (B) chemicals. Some crops mainly used in foods also contribute to the chemical industry (e.g. use of palm oil in making 'biodiesel', a vehicle fuel made from fatty acid esters). Category (C) includes the fat-rich fruit, avocado, two varieties of safflower, which have been much used in nutrition studies and three crops for which there is increasing specialist interest: evening primrose for its high y-linolenic acid content, Cuphea species for their high content of medium-chain fatty acids and rice bran for its good balance of oleic and linoleic acids. The numbers in parentheses after each crop name represent world production in 1997 in millions of tonnes.

brane distension and eventually immature oil bodies (IOB) bud off. These IOB are bounded by a stabilizing phospholipid monolayer derived from the endoplasmic reticulum. The immature oil bodies subsequently become encircled by rough endoplasmic reticulum and begin to acquire a surface coat of proteins called 'oleosins', leading finally to the formation of mature oil bodies (MOB) with a monomolecular oleosin layer.

Oleosin is the name given to proteins that are specific to seeds and exclusively associated with the surfaces of oil bodies in plants. They are made up of three similarly sized but structurally distinct domains comprising a relatively polar C-terminal region, a central hydrophobic domain and an amphipathic N-terminal fragment. The two outer regions are located on the surface of the oil body while the central P-strand domain intrudes through the phospholipid monolayer into the triacylglycerol core. The relatively small size and great abundance of oleosins in oilseeds, coupled with their structural amphipathic characteristics should make them useful as model systems for structural studies, once they can be removed and purified from their

Oil Bodies And Oleosins Seeds

Fig. 3.4 Ultrastructure of oil bodies in plant tissues. The figure shows transmission electron micrographs of sections from (A) mesocarp of a mature avocado pear (Persea americana); (B) mature pollen of rape (Brassica napus); (C) mature seed of rape. L, oil (lipid) bodies; PB, protein bodies; V, vacuoles; PC, pollen coat; CW, cell wall. Note that, whereas the pollen and seed oil bodies are about 0.3-0.5 ^m in diameter the fruit oil bodies are 10-15 ^m in diameter. Reprinted from Progress in Lipid Research, 32, 247-280, 1993 with permission from Elsevier Science.

Fig. 3.4 Ultrastructure of oil bodies in plant tissues. The figure shows transmission electron micrographs of sections from (A) mesocarp of a mature avocado pear (Persea americana); (B) mature pollen of rape (Brassica napus); (C) mature seed of rape. L, oil (lipid) bodies; PB, protein bodies; V, vacuoles; PC, pollen coat; CW, cell wall. Note that, whereas the pollen and seed oil bodies are about 0.3-0.5 ^m in diameter the fruit oil bodies are 10-15 ^m in diameter. Reprinted from Progress in Lipid Research, 32, 247-280, 1993 with permission from Elsevier Science.

natural milieu without denaturation. The model proposed in Fig. 3.5 is based on the finding that oleosin gene expression occurs relatively late in embryo development compared with genes for lipid biosynthesis.

It is notable that the morphology of oil bodies in seeds is quite different from that in fruit mesocarp (e.g. palm, olive, avocado). The diameters of oil bodies in seeds tend to be small (often less than 1 ^m), whereas fruit oil bodies often have diameters exceeding 20 ^m. Although the pathways of lipid biosynthesis in both types of oil storage body appear to be identical, fruits lack oleosins, again underlining the principle that oil body synthesis can proceed independently of oleosin formation. The function of oleosins does not seem, therefore, to be in the initial formation of oil bodies but in the stabilization of small oil bodies in seeds and pollen. Without oleosins the immature oil bodies in fruit mesocarp coalesce until the cells contain just a few large oil droplets.

In the animal kingdom, there is a close parallel to seed oil bodies, namely the cells of the brown adipose tissue which, as described earlier (Section 3.3.1), are packed with small oil droplets. These cells also contain a large number of specialized mitochondria, adapted for oxidizing the fatty acids from the oil droplets that they surround. Similarly, seed oil bodies are surrounded by glyoxysomes. During seed germination, these receive fatty acids from the oil bodies for oxidation prior to the synthesis of carbohydrates, which occurs actively at this time.

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