It is helpful to view arachadonic acid in the coiled conformation shown in the shaded box. Leukotriene A is a precursor to other leukotriene derivatives by epoxide opening reactions.
The prostaglandins are given systematic names that reflect their structure. The initially formed peroxide PGH 2 is a common intermediate to other prostaglandins, as well as thromboxanes such as TXA 2.
Compounds classified as terpenes constitute what is arguably the largest and most diverse class of natural products.
A majority of these compounds are found only in plants, but some of the larger and more complex terpenes e. Terpenes incorporating most of the common functional groups are known, so this does not provide a useful means of classification.
Instead, the number and structural organization of carbons is a definitive characteristic. Terpenes may be considered to be made up of isoprene more accurately isopentane units, an empirical feature known as the isoprene rule. Because of this, terpenes usually have 5n carbon atoms n is an integer , and are subdivided as follows:. Classification Isoprene Units. Isoprene itself, a C 5 H 8 gaseous hydrocarbon, is emitted by the leaves of various plants as a natural byproduct of plant metabolism.
Next to methane it is the most common volatile organic compound found in the atmosphere. Examples of C 10 and higher terpenes, representing the four most common classes are shown in the following diagram. The initial display is of monoterpenes; larger terpenes will be shown by clicking the " Toggle Structures " button under the diagram. Most terpenes may be structurally dissected into isopentane segments. To see how this is done click directly on the structures in the diagram.
The isopentane units in most of these terpenes are easy to discern, and are defined by the shaded areas. In the case of the monoterpene camphor, the units overlap to such a degree it is easier to distinguish them by coloring the carbon chains.
This is also done for alpha-pinene. In the case of the triterpene lanosterol we see an interesting deviation from the isoprene rule. This thirty carbon compound is clearly a terpene, and four of the six isopentane units can be identified. However, the ten carbons in center of the molecule cannot be dissected in this manner. Evidence exists that the two methyl groups circled in magenta and light blue have moved from their original isoprenoid locations marked by small circles of the same color to their present location.
This rearrangement is described in the biosynthesis section. Similar alkyl group rearrangements account for other terpenes that do not strictly follow the isoprene rule. To see a model of the monoterpene camphor Click Here.
Polymeric isoprenoid hydrocarbons have also been identified. Rubber is undoubtedly the best known and most widely used compound of this kind.
It occurs as a colloidal suspension called latex in a number of plants, ranging from the dandelion to the rubber tree Hevea brasiliensis. Bromine, hydrogen chloride and hydrogen all add with a stoichiometry of one molar equivalent per isoprene unit. Pyrolysis of rubber produces the diene isoprene along with other products.
The double bonds in rubber all have a Z -configuration, which causes this macromolecule to adopt a kinked or coiled conformation.
This is reflected in the physical properties of rubber. Despite its high molecular weight about one million , crude latex rubber is a soft, sticky, elastic substance.
Chemical modification of this material is normal for commercial applications. Gutta-percha structure above is a naturally occurring E -isomer of rubber. Here the hydrocarbon chains adopt a uniform zig-zag or rod like conformation, which produces a more rigid and tough substance. Uses of gutta-percha include electrical insulation and the covering of golf balls. To see a model of the rubber chain Click Here. The important class of lipids called steroids are actually metabolic derivatives of terpenes, but they are customarily treated as a separate group.
Steroids may be recognized by their tetracyclic skeleton, consisting of three fused six-membered and one five-membered ring, as shown in the diagram to the right.
The substituents designated by R are often alkyl groups, but may also have functionality. The R group at the A:B ring fusion is most commonly methyl or hydrogen, that at the C:D fusion is usually methyl.
The substituent at C varies considerably, and is usually larger than methyl if it is not a functional group. Ring A is sometimes aromatic. Since a number of tetracyclic triterpenes also have this tetracyclic structure, it cannot be considered a unique identifier. Steroids are widely distributed in animals, where they are associated with a number of physiological processes.
Examples of some important steroids are shown in the following diagram. Different kinds of steroids will be displayed by clicking the " Toggle Structures " button under the diagram. Norethindrone is a synthetic steroid, all the other examples occur naturally. A common strategy in pharmaceutical chemistry is to take a natural compound, having certain desired biological properties together with undesired side effects, and to modify its structure to enhance the desired characteristics and diminish the undesired.
This is sometimes accomplished by trial and error. With the exception of C-5, natural steroids generally have a single common configuration. This is shown in the last of the toggled displays, along with the preferred conformations of the rings. Chemical studies of the steroids were very important to our present understanding of the configurations and conformations of six-membered rings.
Substituent groups at different sites on the tetracyclic skeleton will have axial or equatorial orientations that are fixed because of the rigid structure of the trans-fused rings. This fixed orientation influences chemical reactivity, largely due to the greater steric hindrance of axial groups versus their equatorial isomers.
Thus an equatorial hydroxyl group is esterified more rapidly than its axial isomer. To see a model of the steroid cholesterol Click Here. It is instructive to examine a simple bicyclic system as a model for the fused rings of the steroid molecule. Decalin, short for decahydronaphthalene, exists as cis and trans isomers at the ring fusion carbon atoms.
Planar representations of these isomers are drawn at the top of the following diagram, with corresponding conformational formulas displayed underneath.
The numbering shown for the ring carbons follows IUPAC rules, and is different from the unusual numbering used for steroids. For purposes of discussion, the left ring is labeled A colored blue and the right ring B colored red.
In the conformational drawings the ring fusion and the angular hydrogens are black. Each chair is fused to the other by equatorial bonds, leaving the angular hydrogens H a axial to both rings. Note that the bonds directed above the plane of the two rings alternate from axial to equatorial and back if we proceed around the rings from C-1 to C in numerical order.
The bonds directed below the rings also alternate in a complementary fashion. Conformational descriptions of cis- decalin are complicated by the fact that two energetically equivalent fusions of chair cyclohexanes are possible, and are in rapid equilibrium as the rings flip from one chair conformation to the other.
In each of these all chair conformations the rings are fused by one axial and one equatorial bond, and the overall structure is bent at the ring fusion. In the conformer on the left, the red ring B is attached to the blue ring A by an axial bond to C-1 and an equatorial bond to C-6 these terms refer to ring A substituents.
In the conformer on the right, the carbon bond to C-1 is equatorial and the bond to C-6 is axial. Each of the angular hydrogens H ae or H ea is oriented axial to one of the rings and equatorial to the other. This relationship reverses when double ring flipping converts one cis-conformer into the other. Cis-decalin is less stable than trans-decalin by about 2.
This is due to steric crowding hindrance of the axial hydrogens in the concave region of both cis-conformers, as may be seen in the model display activated by the following button.
This difference is roughly three times the energy of a gauche butane conformer relative to its anti conformer. Indeed three gauche butane interactions may be identified in each of the cis-decalin conformations, as will be displayed by clicking on the above conformational diagram. These gauche interactions are also shown in the model. Steroids in which rings A and B are fused cis, such as the example on the right, do not have the same conformational mobility exhibited by cis-decalin. The fusion of ring C to ring B in a trans configuration prevents ring B from undergoing a conformational flip to another chair form.
This is too great a distance to be bridged by the four carbon atoms making up ring C. Consequently, the steroid molecule is locked in the all chair conformation shown here. After the high—stearic acid diets, fasting total, LDL, and HDL cholesterol; plasma triacylglycerol, glucose, and insulin concentrations; and HOMA-IR did not differ significantly from values after the low—stearic acid diet or between randomized and unrandomized shea blends Table 3. Postprandial changes in plasma triacylglycerol after the test meals at the end of each high—stearic acid dietary intervention period are shown in Figure 2.
The maximum increase occurred at 4 h: The postprandial increases in total fatty acid and plasma stearic, palmitic, oleic, and linoleic acid concentrations did not differ between unrandomized and randomized shea blends data not shown. Similarly, there were no significant differences in the proportions of fatty acids in the chylomicron triacylglycerol between shea blends mean of values from 2 to 6 h , as shown in Table 4.
The proportion of palmitic, oleic, and linoleic acids in the chylomicron triacylglycerol largely reflected those in the shea blends; however, stearic acid was significantly lower in the chylomicron triacylglycerol after the randomized mean of values from 2 to 6 h: No differences were observed in postprandial serum cholesterol total, HDL, and LDL cholesterol , glucose, or insulin concentrations after the randomized or unrandomized shea blends data not shown.
Postprandial fatty acid composition of the venous chylomicron triacylglycerol TG and proportions of fatty acids in the sn -2 position of the postprandial chylomicron TG and shea blends consumed 1. In the follow-up study, plasma triacylglycerol concentrations increased to a lesser extent after the unrandomized shea blend than after the high—oleic acid sunflower oil, but the pattern of response was not significantly different between meals Figure 3.
The proportion of fatty acids in the chylomicron triacylglycerol mean of values from 2 to 6 h after the high—oleic sunflower oil reflected that of the test fat data not shown. However, there was a significantly lower proportion of stearic acid in the chylomicron triacylglycerol after consumption of the unrandomized shea blend mean of values from 2 to 6 h: No significant differences were observed in postprandial serum cholesterol concentrations total, HDL, and LDL cholesterol after the unrandomized shea blend and the high—oleic acid sunflower oil data not shown.
Comparisons between meals were made by using a paired t test. Full blood counts did not differ postprandially between the test fats for both randomized compared with unrandomized shea blends and for high—oleic acid sunflower oil compared with unrandomized shea blend; data not shown. No other significant differences were noted. The aim of this study was to test the hypothesis that the randomization of a fat consisting mainly of the triacylglycerol species SOS would decrease postprandial lipemia and the associated increase in FVIIa concentration.
However, the results of the present study differ from those of previous studies, which used randomized cocoa butter, randomized short- and long-chain triacylglycerols, or a blend of randomized totally hydrogenated sunflower oil and unhydrogenated high—oleic acid sunflower oil.
This finding supports the view that stearic acid—rich fats should not be regarded as cholesterol raising As predicted, randomization resulted in differences in the proportions of stearic acid in the sn -2 position of the chylomicron triacylglycerol. It was surprising, therefore, that there were no significant differences between the randomized and unrandomized shea blends on postprandial plasma triacylglycerol concentrations.
However, comparison of the unrandomized shea blend with the high—oleic acid sunflower oil showed that it decreased lipemia and failed to increase FVIIa postprandially. This effect was unlikely to have been a consequence of malabsorption because fecal fat excretion remained within the normal range, and other reports indicate that both symmetrical unrandomized and randomized stearic acid—rich fats are well absorbed 15 — Consequently, this would not explain the low postprandial response observed after both types of shea blends.
Furthermore, the substantially higher proportion of fats with a high melting point in the randomized shea blend did not appear to have an adverse effect on absorption. Another possible explanation could be that the rates of absorption and chylomicron secretion are slower or the rate of clearance of these particles is accelerated with these stearic acid—rich fats without any significant decline in digestibility. The prolonged postprandial lipemia observed after the shea blends and the delayed appearance of stearic acid in the chylomicron triacylglycerol compared with oleic acid supports this view.
A novel observation of this study, which requires confirmation, is that HL activity was greater after the unrandomized shea blend than after the high—oleic acid sunflower oil. This might indicate an adaptive response to cope with a lower substrate specificity for stearic acid—rich triacylglycerol in extrahepatic tissues compared with triolein. It could also indicate a faster rate of chylomicron removal from the circulation and partially explain the lower postprandial response after the shea blends.
However, LPL activities were not significantly different between meals, an observation also reported by Tholstrup et al 19 after meals rich in oleic and stearic acids. Additional studies are needed to investigate the mechanisms involved. Neither randomized nor unrandomized shea blends resulted in postprandial increases in FVIIa concentrations in contrast with the significant increase observed after the consumption of high-oleic acid sunflower oil.
This finding is consistent with that observed with other stearic acid—rich triacylglycerols, ie, decreased postprandial lipemia 7 , 9. The mechanism responsible for the postprandial activation of FVII is unclear. Previous studies have not found a clear relation between postprandial lipemia measured as the area under the curve or peak lipemia and the extent of the postprandial increase in FVIIa. Activation of FVII may occur via interaction with a charged surface, such as those provided by membrane lipids.
The large increase in FVIIa after the high—oleic acid meal may have been due to activation of the ABC-A1 transporter protein, which may be stimulated by unsaturated fatty acids, resulting in the expression of charged phospholipids on the outer surface of leukocytes and platelets Because postprandial lipemia is accompanied by an increased outflow of lymph, we hypothesized that changes in postprandial leukocyte counts might explain the increase in FVIIa.
However, leukocytes counts increased substantially after the test meals, regardless of their ability to increase FVIIa. Additional studies are required to examine the mechanisms leading to FVII activation. To investigate the differences in postprandial lipemia, we analyzed the physical properties of the test fats.
It is proposed that fats that contain crystalline solids at body temperature may affect micelle formation and retard the process of absorption and consequently result in reduced postprandial lipemia. These differences may have implications regarding the risk of cardiovascular disease. In conclusion, the present study indicated no adverse effects of randomized stearic acid—rich fats on cardiovascular disease risk factors.
We thank Britannia Food Ingredients, Anglia Oils, and Unilever Research for providing and processing the test fats; David Howarth for conducting the coagulation assays; Peter Lumb for conducting the lipase assays; Roy Sherwood for conducting the insulin and lipoprotein assays; L Chen for conducting the fecal analysis; and Robbie Gray for technical help.
The authors had no financial or commercial interest in any company or organization involved with this study. Lavery H. Differential thermal analysis of fats. Melting behavior of some pure glycerides. J Am Oil Chem Soc ; 35 : — Google Scholar. Yang LY , Kuksis A. Apparent convergence at 2-monoacylglycerol level of phosphatidic acid and 2-monoacylglycerol pathways of synthesis of chylomicron triacylglycerols. J Lipid Res ; 32 : — Positional specificity of purified milk lipoprotein lipase. J Biol Chem ; : — 7.
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J Lipid Res ; 40 : — 8. Atherosclerosis ; : — Influence of a stearic acid-rich structured triacylglycerol on postprandial lipemia, factor VII concentrations, and fibrinolytic activity in healthy subjects.
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Simple and direct procedure for the evaluation of triglyceride composition of cocoabutters by high performance liquid chromatography—a comparison with the existing TLC-GC method. Fette Seifen Anstrichmittel ; 85 : — 8. Quantitation of activated factor VII levels in plasma using a tissue factor mutant selectively deficient in promoting factor VII activation. Blood ; 81 : — Lepage G , Roy CC.
In fruit, the content does not exceed 1. It is present in trace amounts in legumes the highest content is present in chickpeas, 0. It is abundant in castor beans. In the USA the principal source of stearic acid is coconut oil, and to a lesser extent, palm oil, while in third world countries are more commonly used the other plant sources. It is used as ingredient in making candles, soaps, plastics, oil pastels, lubricant, cosmetics and as a softener in chewing gum base and for suppositories.
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