Predominantly phospholipids

Two main types of lipids occur in biological membranes phospholipids and sterols. The predominant phospholipids in most membranes are phosphoglycerides, which are phosphate esters of the three-carbon alcohol, glycerol. A typical structure is that of phosphatidylcholine (lecithin) ... 

Almost all body cells contain phospholipids. The common animal phospholipids are made of sphingomyelin, phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), and other glycerol phospholipids of complex fatty acid composition. PC, formerly referred to as lecithin, PE, formerly referred to as cephalin, and PS are by far the most predominant phospholipids from most animal sources. As constituents of cell walls and active participants in metabolic processes, they appear to be essential to life (8). 

The phospholipid content of the plasma membrane Increased from 30 to 42 mol% following cold acclimation, although the relative proportions of the individual phospholipids did not change appreciably. The predominant phospholipids were phosphatidylcholine (PC) and phosphatidylethanolamine (PE), which increased from 14 to 19.3 mol% and from 10.3 to 15.5 mol% total lipid, respectively. Phosphatldylglycerol (PG), phosphatidyl serine (PS), and phosphatidylinositol (PI) were present in lesser amounts, each constituting <2 mol% total lipid. 

Because all food derives from biological tissues and because all tissues contain biological membranes, all foods contain, to a greater or lesser extent, structural fats. These are predominantly phospholipids and glycolipids (Chapter 6) with, in animal tissues, cholesterol and in plant tissues, the plant sterols. Thus, the lean part of meat contains the structural lipids, phospholipids and cholesterol of the muscle membranes and minor quantities of gly colipids. Consumption of tissues like brain would introduce larger quantities of animal sphingolipids into the diet. Dairy products contribute small amounts of structural lipids because of the presence of the fat globule membrane. The structural lipids of plants that are important in... 

The lipids consist of lecithin (phosphatidylcholine), triglycerides (ordinary fats), fatty acids, and cholesterol (see Section 4.3 for lipid chemistry). The phospholipid seems to predominate. Phospholipid bilayers... 

Biochemical studies have demonstrated the presence of surface-active components, predominantly phospholipids, in airway secretions (42). Bernhard et al. (43,44) studied the conductive airway phospholipids in the tracheobronchial secretions of adult pigs and compared their results with analyses of bronchoalveolar lavage (B AL) fluid, tracheobronchial epithelium, and lung parenchyma. The composition of the PC and PL molecular species of the tracheal aspirates was similar... 

Figure 25-3. Metabolic fate of chylomicrons. (A, apolipoprotein A B-48, apolipoprotein B-48 , apolipoprotein C E, apolipoprotein E HDL, high-density lipoprotein TG, triacylgiycerol C, cholesterol and cholesteryl ester P, phospholipid HL, hepatic lipase LRP, LDL receptor-reiated protein.) Only the predominant lipids are shown.

The choline-containing phospholipids, PC and Sph, predominate in the outer leaflet and the amino-containing phospholipids (PE and PS) in the inner leaflet. 

Two possible pathways for the biosynthesis of 2-AG have been proposed (1) a phospholipase C (PLC) hydrolysis of membrane phospholipids followed by a second hydrolysis of the resulting 1,2-diacylglycerol by diacylglycerol lipase or (2) a phospholipase Ai (PLA,) activity that generates a lysophospholipid, which in turn is hydrolyzed to 2-AG by lysophospholipase C (Fig. 5) (Piomelli, 1998). Alternative pathways may also exist from either triacylglycerols by a neutral lipase activity or lysophosphatidic acid by a dephosphorylase. The fact that PLC and diacylglycerol lipase inhibitors inhibit 2-AG formation in cortical neurons supports the contention that 2-AG is, at least predominantly, biosynthesized by the PLC pathway (Stella, 1997). However, a mixed pathway may also be plausible. 

Fig. 6.9 Characteristic structures of biological membranes. (A) The fluid mosaic model (S. J. Singer and G. L. Nicholson) where the phospholipid component is predominant. (B) The mitochondrial membrane where the proteins prevail over the phospholipids...

Normal lungs, however, produce a chemical substance referred to as pulmonary surfactant. Made by alveolar type II cells within the alveoli, surfactant is a complex mixture of proteins (10 to 15%) and phospholipids (85 to 90%), including dipalmitoyl phosphatidyl choline, the predominant constituent. By interspersing throughout the fluid lining the alveoli, surfactant disrupts the cohesive forces between the water molecules. As a result, pulmonary surfactant has three major functions ... 

Free arachidonic acid, along with diacylglycerols and free docosahexaenoic acid, is a product of membrane lipid breakdown at the onset of cerebral ischemia, seizures and other forms of brain trauma. Because polyunsaturated fatty acids are the predominant FFA pool components that accumulate under these conditions, this further supports the notion that fatty acids released from the C2 position of membrane phospholipids are major contributors to the FFA pool, implicating PLA2 activation as the critical step in FFA release [1,2] (Fig. 33-6). 

Many of the globulins act as transport proteins. Of particular interest are those proteins which are combined with lipids, themselves synthesized in the liver, to form lipoprotein complexes. High density lipoprotein (HDL), which contains predominantly apoproteins A and C combined with mainly phospholipids (most of the cholesterol found in mature HDL is added later) and very low density lipoprotein... 

It can be seen from Figure 1 that the choline-containing phospholipids, phosphatidylcholine and sphingomyelin are localized predominantly in the outer monolayer of the plasma membrane. The aminophospholipids, conprising phosphatidylethanolamine and phosphatidylserine, by contrast, are enriched in the cytoplasmic leaflet of the membrane (Bretcher, 1972b Rothman and Lenard, 1977 Op den Kamp, 1979). The transmembrane distribution of the minor membrane lipid components has been determined by reaction with lipid-specific antibodies (Gascard et al, 1991) and lipid hydrolases (Biitikofer et al, 1990). Such studies have shown that phosphatidic acid, phosphatidylinositol and phosphatidylinositol-4,5-fc -phosphate all resemble phosphatidylethanolamine in that about 80% of the phospholipids are localized in the cytoplasmic leaflet of the membrane. 

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