Phospholipid adipose tissue

Fat absorbed from the diet and lipids synthesized by the liver and adipose tissue must be transported between the various tissues and organs for utilization and storage. Since lipids are insoluble in water, the problem of how to transport them in the aqueous blood plasma is solved by associating nonpolar lipids (triacylglycerol and cholesteryl esters) with amphipathic hpids (phospholipids and cholesterol) and proteins to make water-miscible hpoproteins. 

Glycerol utilization depends on the tissue. Adipose tissue can t use glycerol. Nitrogen-containing phospholipids are made from diglycerides, while other phospholipids are made from phosphatidic acid (PA). PI = phosphatidylinosi-tol PC = phosphatidylcholine PE = phosphatidylethanolamine PS = phos-phatidylserine. 

The resulting triacylglycerol is stored in adipose tissue. In the liver, some is combined with protein and phospholipids to form a complex, known as very low density lipoprotein (VLDL), which is secreted from the liver into the blood. Details of the formation of VLDL are presented in Appendix 11.2. Failure to form VLDL or secrete can cause accu-... 

Lipogenesis in liver and possibly adipose tissue, leading to increased synthesis of phospholipids and triacylglycerol. 

Lipid metabolism in the liver is closely linked to the carbohydrate and amino acid metabolism. When there is a good supply of nutrients in the resorptive (wellfed) state (see p. 308), the liver converts glucose via acetyl CoA into fatty acids. The liver can also take up fatty acids from chylomicrons, which are supplied by the intestine, or from fatty acid-albumin complexes (see p. 162). Fatty acids from both sources are converted into fats and phospholipids. Together with apoproteins, they are packed into very-low-density lipoproteins (VLDLs see p.278) and then released into the blood by exocytosis. The VLDLs supply extrahepatic tissue, particularly adipose tissue and muscle. 

The liver converts fatty acids to triacyiglycerols, phospholipids, or cholesterol and its esters, for transport as plasma lipoproteins to adipose tissue for storage. Fatty acids can also be oxidized to yield ATP or to form ketone bodies, which are circulated to other tissues. 

In adipose tissue, TAG is stored in the cytosol of the cells in a nearly anhydrous form. It serves as "depot fat," ready for mobilization when the body requires it for fuel. Little TAG is stored in the liver. Instead, most is exported, packaged with cholesteryl esters, cholesterol, phospholipid, and protein (apolipoprotein B-100, see p. 229) to form lipoprotein particles called very low density lipoproteins (VLDL). Nascent VLDL are secreted into the blood where they mature and function to deliver the endogenously-derived lipids to the peripheral tissues. [Note Recall that chylomicrons deliver primarily dietary (exogenously-derived) lipids.] Plasma lipoproteins are discussed in Chapter 18, p. 225. 

PA is the precursor of many other phosphoglycerides. The steps in its synthesis from glycerol phosphate and two fatty acyl CoAs were illustrated in Figure 16.14, p. 187, in which PA is shown as a precursor of triacylglycerol. [Note Essentially all cells except mature ery-. throcytes can synthesize phospholipids, whereas triacylglycerol synthesis occurs essentially only in liver, adipose tissue, lactating mammary glands, and intestinal mucosal cells.]... 

Some effects of prostaglandins are mediated through cell surface G-protein coupled receptors (see Chapter ll).306 Some other prostanoids bind to and activate nuclear peroxisome proliferator-activated receptors.306 PGJ2 may inhibit fatty acid synthesis and fat deposition in adipose tissue through these receptors. Some of the prostanoid derivatives enter membranes and may become incorporated into phospholipids and exert their effects there. 

Phospholipids are considered to be involved in the transport of triglycerides through die liver, especially during mobilization from adipose tissue, Conditions which could be interpreted as interfering with phosphatidylcholine formation, such as deficiency of choline or its precursors, result in a pronounced increase 111 liver triglycerides. 

Data on the proportions of different fatty acids in plasma lipid esters (cholesteryl esters, phospholipids, free fatty acids, or triacylglycerol), erythrocyte membranes, or adipose tissue may provide a more objective and accurate path to evaluating dietary fatty acid composition (Arab, 2003 Baylin and Campos, 2006). The fatty acid composition in blood and body tissues reflects the fatty acid composition of the diet at different time points after ingestion. Short and medium-term changes in the composition of dietary fatty acid intake are reflected in plasma lipids and erythrocyte membranes, weeks and months after intake, respectively. The incorporation of fatty acids in adipose tissue reflects long-term changes in the diet (years) (Baylin and Campos, 2006 Katan et al., 1997 Ma et al., 1995 Zock et al, 1997). 

Consequently, a more objective way to measure the habitual intake of milk fat would be the fatty acid composition of adipose tissue. However, this is not routinely performed in larger cohort studies, due to cost and that the procedure is invasive and less tolerated by study participants. Analysis of plasma fatty acid composition is thus a more feasible option for examination to determine dairy intake in the study population. While some groups have separated plasma into its constituent phospholipids and cholesterol esters to analyze serum 15 0 and 17 0 as markers of dairy intake (Smedman et al., 1999), Baylin et al. (2005) found that plasma that was not separated into its constituent cholesteryl ester, phospholipids, and triacylglycerols was still able to reflect habitual dairy intakes comparably to adipose tissue. Thus, whole plasma is an acceptable alternative to fractionated plasma in the absence of adipose tissue for analysis to reflect habitual dairy intakes and may be a cost effective option for consideration when conducting future intervention studies to assess the affect of dairy products on health outcomes. 

In considering the distribution of particular lipids, it must be emphasized that there is wide variation in the lipid composition of various cells (see section entitled An Excursion into the Complexities of Phospholipids Found in Certain Cells Defining the Problem ). Of the three classes of lipids depicted in Figure 1-1, triglycerides (triacylglycerols) form the chief lipid constituent of adipose tissue in the mammal and also are found in plasma. 

In the capillaries of adipose tissue and muscle, fatty acids of chylomicrons are removed from the triacylglycerols hy the action of lipoprotein lipase (LPL) LPL is present on the surface of the endothehal cells of the capillaries. Chylomicron apo-C-II activates LPL in the presence of phospholipid. 

Poulin and Theil have developed a mechanistic model for estimating the Vd based on physiologically based pharmacokinetics (PBPK). For this method, the tissue plasma partition coefficient for each organ of the body is calculated by consideration of the volume fraction of neutral and phospholipids and water found in the tissues of a particular organ. For example, the volume fraction of neutral lipids in human adipose tissue is 0.79 whereas the volume fraction of neutral lipids in cardiac tissue is 0.0115. By contrast the volume fraction of water in adipose and heart are 0.18 and 0.76 respectively. Combined with the P, these volume fractions are used to estimate the distribution of a drag molecule into each tissue. Summation of the product of tissue volume and tissue/plasma partition coefficient produces the estimate of Vd. ... 

Diol lipids have been reported only recently when techniques for their stmcture elucidation were developed. Small quantities of diol lipids, such as diacylpropane-l,3-diol, diesters of butane-1,3-diol, and butane-1,4-diol are found in mammalian and fish liver, mammalian adipose tissues, egg yolk, com seed, and yeast (20). From Actynomyces olivaceus, mixed acyl- and alk-l-enyl derivatives of simple diols have been isolated (21). An acylated diol phospholipid has been produced by the yeast Lypomyces starkeyi when grown on propane-1,2-diol (22). 

Fatty acids are physiologically important as (1) components of phospholipids and glycolipids, (2) hydrophilic modifiers of proteins, (3) fuel molecules, and (4) hormones and intracellular messengers. They are stored in adipose tissue as triacylglycerols (neutral fat). 

Phospholipids in liver, kidney, muscle, and other tissues. Organ meats such as liver, kidney, and muscles are a major source of dietary phospholipids. The reader is referred to Kuksis (16) for the distribution of various phospholipid classes in the liver, kidney, muscles (heart and skeletal), spleen, lung, blood cells, bile, and adipose tissue of different animal species. Compositional data of fatty acids for these tissues and fluids are also given. 

PhosphattdykhoHne (PC) and phosphatidylethanolamine (PE) are the major phospholipids of cell membranes. Table 6.1 lists the amount of lipid in cell membranes expressed as micrograms of lipid per milligram of membrane-bound protein. The membranes were isolated from cells of adipose tissue from rats that had been raised on a diet containing sunflower oil (100 g oil/kg diet) as the source of lipids. The diet was essentially free of cholesterol and phospholipids because plants do not contain cholesterol and the phospholipids of vegetable oils are removed during the refining process. 

The nature of the fatty adds in the TGs stored in adipose tissue and those in the phospholipids in membranes can be influenced by the diet, The rat study reported in Table 6.3 assessed the plasma membranes of the liver. Diets containing 10% oil or fat by weight were fed to young and rapidly growing rats. Rapidly growing... 

The fact that animals do not need to eat continuously throughout the course of the day indicates that chylomicrons are not necessarily actively distributing nutrients during the entire day Free fatty acids ate supplied to tissues via the chylomicrons, but also from the adipose tissue, which supplies energy to the body throughout the day. The dry matter of adipose tissue consists of 5% protein and over 90% lipids. About 95% of the lipids ate TGs the nest consists of 1-2% diglycerides, 0-25% phospholipids, and 0.25% cholesterol- Adipose tissue may contain 5-30% water by weight-... 

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