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Figure 3-14. Absorption of the saturated fatty acid in the sn 3 position (X refers to the chain length) of 1,2 dioleoyl, 3 acyl-sn-glycerols (the OOX series). OSO is 1-oleoyl, 2-steroyl, 3-oleoyl-glycerol. As the chain lengthens from 16 to 24 carbons, there is a sharp decline in the absorption of the saturated fatty acid. Oleate absorption was greater than 90% for all of the triacylglycerols. (Based on data of Redgrave, T.G., Kodali, D. R. and Small, D. M. 11988], The effect of triacyl-sn-glycerol structure on the metabolism of chylomicrons and triacylglycerol-rich emulsions in the rat. J Biol Chem 263:5118-5123.)

thus be absorbed. However, hydrolysis of OOS gives a mole of stearic acid or its acid-soap; these products have MPs above 50°C (see Fig. 3-13) and therefore solidify and are poorly absorbed.

If the fatty acid in the 1 or 3 position has a high MP and consequently its acid-soap has a high MP, then it is unlikely to be readily absorbed. This is shown in Figure 3-14, which illustrates the absorption and incorporation of fatty acids into lipids in chylomicrons in rats fed a series of stereospecific triacylglycerols containing oleic acid in the 1 and 2 positions and a saturated fatty acid in the 3 position. These 1,2-dioleoyl, 3 acyl-sn-glycerols all have melting points below body temperature (Fig. 3-15) and are oils in the intestine. After hydrolysis, the MP of the fatty acid-soaps (see Fig. 3-13) and the absorption of the fatty acid liberated from the sn-3 position depend on chain length. Absorption of myristate (C14) and palmitate (CI6) is quite good at about

80%, but the absorption of stearate (CI8) falls to 62% and that of C22 and C24 (behenic and lignoceric acids) falls precipitously so that only about 6% of the lignoceric acid released from l,2-dioleoyl-3-lignoceroyl-sn-glycerol is absorbed (Redgrave et al., 1988). Because the MP of the acid-soaps of these long-chain saturated fatty acids (see Fig. 3-13) is very high (greater than 65°C), they probably precipitate in the intestine and are lost in the feces.

Polymorphism of Fats and Oils

The relatively simple chemical structures of the fatty acids esterified to glycerol gives rise to very complicated physical properties of solid triacylglycerols. The capacity of a single molecular species to form several different solid crystalline forms (polymorphism) is a characteristic of glycerides and especially of triacylglycerols. This arises because there are many possible orientations of the chains in

Crystallisation Melting Point

Figure 3-15. Melting and crystallization points (Tf,TJ of 1,2-dioleoyl, 3-acyl-sn-glycerols (OOX series). The MPs are all below body temperature. The melting and crystallization points of OSO for comparison with those of OOS are 2S.2°C and - 7°C. (Based on data of Fahey, D„ Small, D. M„ Atkinson, D., Kodali, D., and Redgrave, T. [T985]. Structure and polymorphism of 1,2-dioleoyl-3-acyl-sn-glycerols, three- and six-layered structures. Biochemistry 24:3757-3764.)

3-Acyl Carbon Number

3-Acyl Carbon Number

Figure 3-15. Melting and crystallization points (Tf,TJ of 1,2-dioleoyl, 3-acyl-sn-glycerols (OOX series). The MPs are all below body temperature. The melting and crystallization points of OSO for comparison with those of OOS are 2S.2°C and - 7°C. (Based on data of Fahey, D„ Small, D. M„ Atkinson, D., Kodali, D., and Redgrave, T. [T985]. Structure and polymorphism of 1,2-dioleoyl-3-acyl-sn-glycerols, three- and six-layered structures. Biochemistry 24:3757-3764.)

layers, which may vary in tilt from the basal plane; for instance, structures may organize into bilayered structures (two layers of chains), trilayered structures, or even more complicated structures with six layers (Fahey et al., 1985; Small. 1986; Kodali et al-, 1989). The chains themselves may pack in a variety of different subcells, generally classified as (3, 3', and a. The combination of different layers, different angles of tilt, and different chain packing can give rise to many different crystalline forms in the same triacylglycerol.

A rather simple case is illustrated by 1,3-dioleoyl-2-stearoyl-glycerol (OSO), a compo nent of lard and butterfat, shown in Figure 3-16 (Kodali et al., 1987). The stablest form of this symmetrical triacylglycerol is the (3 phase (Small, 1991). It is a trilayer (three layers of chains) that repeats every 65 A. It melts at 25°C to a liquid oil. On rapid cooling of the liquid, an a phase, which is a bilayered phase with hexagonal packed chains, is formed. This can convert to another bilayered phase with 3' chain packing at - 7°C. The 3' phase transforms to the 3 phase at 11°C. Many other triacylglycerols have more complicated polymorphisms, and natural mixtures of triacylglycerols such as oils and fats may exist as a combination of polymorphic forms and stoichiometric ratios of molecules in the same crystal. It is striking, however, that many natural fats and oils behave in a general way like a single species of triacylglycerol in that they show the different polymorphic forms of a,

P', and p. These properties are important in producing palatable and attractive foods. Polymorphic transitions from [3' to 3 forms that produce white films on chocolate or grainy textures in shortenings are rather unacceptable.

p-Phase p-Phase

Triolein Packing

Figure 3-16. Polymorphic forms of 1,3-dioleoyl-2-stearoyl-sr\-glycerol IOSO), a common component of pork fat. The MP is given above the arrow, and the enthalpy of the transition is given in kcal/mol below the arrow. The AAAAA lines indicate the zigzag shown in Figure 3-9. In p packing, the most stable form, the chains are all parallel. In p'

packing, every other chain is twisted 9CP to the next, so that AAAA/\ and_alternate. In the a-phase, all the chains are packed in a loose hexagonal chain packing, and all are drawn as__In the melt, the chains have movement and are drawn as squiggly lines ^V\/. The most stable structure, p, melts at 25°C and is a trilayer (3 layers of chains): 2 layers of oleates with the 9 to 70 double bond and a separate layer of stearates (middle layer). All other structures are bilayers (two layers). (From Kodali, D. R. Atkinson, D„ Redgrave, T. G. and Small, D. M. [1987], Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: Effect of unsaturation and substitution in the 2-position. J Lipid Res 28:403-413.)

Bilayer 52A

Figure 3-16. Polymorphic forms of 1,3-dioleoyl-2-stearoyl-sr\-glycerol IOSO), a common component of pork fat. The MP is given above the arrow, and the enthalpy of the transition is given in kcal/mol below the arrow. The AAAAA lines indicate the zigzag shown in Figure 3-9. In p packing, the most stable form, the chains are all parallel. In p'

packing, every other chain is twisted 9CP to the next, so that AAAA/\ and_alternate. In the a-phase, all the chains are packed in a loose hexagonal chain packing, and all are drawn as__In the melt, the chains have movement and are drawn as squiggly lines ^V\/. The most stable structure, p, melts at 25°C and is a trilayer (3 layers of chains): 2 layers of oleates with the 9 to 70 double bond and a separate layer of stearates (middle layer). All other structures are bilayers (two layers). (From Kodali, D. R. Atkinson, D„ Redgrave, T. G. and Small, D. M. [1987], Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: Effect of unsaturation and substitution in the 2-position. J Lipid Res 28:403-413.)

The Surface Orientation of Different Triacylglycerols at the Water Interface

Lipolytic enzymes work at water/lipid interfaces of intestinal fat emulsions, plasma lipoproteins, and membranes. Thus the interfacial conformation of triacylglycerols must be recognized by lipases so that they can react with the ester bond and water to effect hydrolysis.

Triacylglycerols have a different conformation at an aqueous/oil interface than in the bulk liquid. In bulk liquid, triacylglycerols have "tuning fork" conformation, with one chain pointing in one direction and the other two chains pointing in the opposite direction (Fig. 3-17). In general, at an aqueous interface, when the three fatty acid chains are long, all three ester groups protrude into the aqueous phase, and the three chains point upward in the oil (Fig. 3-17). The area of a triacylglycerol at the interface in the expanded liquid state is about 100 to 120 À2 per molecule. When the surface becomes solid—that is, when the molecules freeze on the surface—the molecule occupies about 60 À2, which is about 20 Á2 per acyl chain, the area occupied by a crystalline solid. At a condensed liquid surface such as might occur between a fat droplet and water, an interfacial area of —75 Â2 is found. Lipolytic enzyme activity tends to be lowest when the interfacial substrate is in the crystalline state and highest when the substrate is in the liquid condensed state. As surface triacylglycerols are hy-drolyzed, the products (fatty acids and 2-monoacylglycerols) are removed, and other triacylglycerol molecules move from the bulk liquid to the interface.

In butterfat, one chain is quite short (2 to 6 carbons). In this case, only the 2 long chains lie pointing up from the water into the oil while the short chain is submerged in the aqueous milieu, as shown in Figure 3-17 (Fa-hey and Small, 1986). This may be an important factor in the rapid hydrolysis of the short chains of fats that have short chains (such as butyric acid) at the sn-3 position. When one chain is a medium-chain fatty acid of 8 to 11 carbons, it cannot enter the water because it is too hydrophobic; it points into the oil phase and causes the melting of the two other longer-chain fatty acids by creating a partial void (due to its shortness), thus depressing the MP (Fahey and Small, 1988). This can be seen in the depression of the surface

1 Short Chain

2 Long Chains

1 Short Chain

2 Long Chains

Water

Solid

Condensed Liquid

Liquid

Condensed Liquid

Water

Solid

Condensed Liquid

Liquid

Condensed Liquid

Area of Molecule at Interface

State of Monolayer

Figure 3-17. Surface orientation of long and of long- and short-chain triacylglycerols, as found in dairy fat All three ester bonds are anchored in the water interface. The short chains (e.g., butyric, hexanoic) are in the water phase, whereas the long-chain fatty acyl chains all point into the oil phase. When the chains crystallize at the surface, they form a rigid 2D solid monolayer that is tightly packed in a small area (20 A2/hydrocarbon chain). If the temperature is raised, the solid monolayer will melt to a condensed liquid, and at higher temperatures or lower surface pressures to an extended liquid. These interfaces are potentially present in any fat/water system (e.g., fat globules in adipose tissue, fat in the stomach and intestine, and fat in lipoprotein particles in plasma). These interfaces are probably the substrates for many lipases (Small, 1997). Note that these conformations are quite different from those in the bulk liquid phase (see Fig. 3-16).

Figure 3-18. Surface and bulk MPs of 1,2 dipalmitoyl, 3 acyl-sn-glycerols. Bulk MPs were determined for the most stable crystalline forms (•). As the chain at the 3-position lengthens to about 5 carbons, the MP falls. It then remains at about 40 to 5CPC until the chain reaches 12 carbons, where the MP rises again. Melting of the triacylglycerol/water interface (') is different. When the chain reaches 5 or greater, the surface MP becomes low and is below ETC for the 8- and 10-carbon acids. This is due to the void left in the surface when shorter and longer chains stand side-by-side.

3-Acyl Carbon Number

Figure 3-18. Surface and bulk MPs of 1,2 dipalmitoyl, 3 acyl-sn-glycerols. Bulk MPs were determined for the most stable crystalline forms (•). As the chain at the 3-position lengthens to about 5 carbons, the MP falls. It then remains at about 40 to 5CPC until the chain reaches 12 carbons, where the MP rises again. Melting of the triacylglycerol/water interface (') is different. When the chain reaches 5 or greater, the surface MP becomes low and is below ETC for the 8- and 10-carbon acids. This is due to the void left in the surface when shorter and longer chains stand side-by-side.

MP compared with the bulk value in the series of l,2-dipalmitoyl-3-acyl-sr?-glycerols, as depicted in Figure 3-18.

Using 13C-enriched triolein, it was shown by nuclear magnetic resonance (NMR) that the same interfacial conformation (3 chains side-by-side with all ester groups at the aqueous interface) occurs in a monolayer of phospholipid in which a few molecules of triacyl-glycerol are imbedded (Hamilton and Small, 1981; Hamilton et al., 1983). The solubility of triacylglycerol in phospholipid monolayers is about 3% (3 mol triacylglycerol/100 mol total lipid). The orientation shows that the 1,3 ester groups are slightly more deeply embedded in the water layer than the sn-2 ester group, but all three are exposed to the aqueous phase, and the 3 acyl chains lie side by side, parallel to the chains of the phospholipid. Thus in fat emulsions, membranes, or lipoproteins, triacylglycerol can have an interfacial orientation in which lipolytic enzymes will have ready access to the 1 and 3 ester linkages of triacylglycerol, which are the sites of hydrolysis of triacylglycerols by the gut and plasma lipases.

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Redgrave, T. G., Kodali, D. R. and Small, D. M. (1988) The effect of triacyl-sn-glycerol structure on the metabolism of chylomicrons and triacylglycerol-rich emulsions in the rat. J Biol Chem 263:5118-5123.

Salen, G., Shefer, S„ Batta, A. K, Tint, G. S., Xu, G„ Honda, A., Irons, M. and Elias, E. R. (1996) Abnormal cholesterol biosynthesis in the Smith-Lemli-Opitz syndrome. J Lipid Res 37:1169-1180.

Small, D. M. and Zoeller R. A. (1991) Lipids. In: Encyclopedia of Human Biology, Vol. 4, pp. 725-748. Academic Press, New York.

Small, D. M. (1991) The effects of glyceride structure on absorption and metabolism. Annu Rev Nutr 11:413-434.

Small, D. M. (1992) Structure and metabolism of the plasma lipoprotein. In: Plasma Lipoproteins in Coronary Artery Disease. (Kreisberg, J. I. and Segrest, J. P., eds.), pp. 57-85. Blackwell Scientific Publishing, Ltd., Oxford Press, New York.

Small, D. M. (1988) Progression and regression of atherosclerotic lesions: Insights from lipid physical biochemistry. Arteriosclerosis 8:103-129.

Small, D. M. (1986) The Physical Chemistry of Lipids from Alkanes to Phospholipids. Handbook of Lipid Research Series (Hanahan, D., ed.), Vol. 4. Plenum Press, New York.

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RECOMMENDED READINGS

Small, D. M. and Zoeller, R. A. (1991) Lipids. In: Encyclopedia of Human Biology, Vol. 4, pp. 725-748. Academic Press, Inc., New York.

Small, D. M. (1991) The effects of glyceride structure on absorption and metabolism. Annu Rev Nutr 11:413-434.

Small, D. M. (1992) Structure and metabolism of the plasma lipoprotein. In: Plasma Lipoproteins in Coronary Artery Disease. (Kreisberg, and Segrest, eds.), pp. 57-85. Blackwell Scientific Publishing, Ltd., Oxford Press, New York.

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