Chylomicron characteristics

Three disorders of lipoprotein metabolism share these characteristics familial hypobetal-ipoproteinemia, chylomicron retention disease, and ABL (Table 27-2). The presence or absence of specific plasma apoB lipoproteins, as well as their mode of inheritance, can be useful when attempting to differentiate between these disorders. Symptoms associated with familial hypo-betalipoproteinemia are usually milder than for the other two and are inherited as dominant traits, that is, symptoms are observed in at least one parent of an affected offspring. Chylomicron retention disease is an autosomal recessive disorder with a severe phenotype commonly presenting soon after birth. Plasma lipoprotein analysis from affected individuals shows a specific absence of chylomicrons (apoB48) but normal amounts of VLDL and LDL (apoB 100). In our patient, evidence of recessive inheritance and absence of all apoB-containing lipoproteins implicates ABL as the most likely diagnosis. 

Ito cells (T. Ito, 1951) are also known as fat-storing cells, hepatic stellate cells or lipocytes. These long-lived cells, 5-10 im in size with long thin strands, lie in Disse s space (s. figs. 2.8, 2.9) and contain numerous cytoplasmic fat droplets as well as an abundance of vitamin A (= retinol ester). The retinol esters of the chylomicrons are absorbed by the hepatocytes and hydrolyzed into retinol. The latter is either passed to the blood by means of RBP or transported to Ito cells and stored. In the fat droplets of Ito cells, about 75% of the liver retinoids are present in the form of retinol esters. These fat droplets are characteristic of Ito cells they represent vacuolized... 

Approximately 95% of total lipids of LD from bovine heart are constituted by TAG. The amount of protein present in these LD is about 5% of total mass, and the amount of phospholipids varies from 3 to 7% of total lipid. The major phospholipids forming the monolayer of these LD are phosphatidylcholine (ca. 50%) and phosphatidylethanolamine (30-40%). The NEFA content is very low. The chemical composition of LD in beef heart with their high TAG content and the rather small amount of phospholipids resembles the composition of chylomicrons. A striking difference, however, is the lack of cholesterol and cholesteryles-ters in LD from beef heart whereas these lipid species occur at approximately 1-2% of total lipid in chylomicrons. Furthermore, the protein content of LD is two to three times higher than of chylomicrons. In contrast to bovine heart LD, those of stellate cells from the rat liver consist of retinylesters, TAG, free cholesterol and a small amount of phospholipids [142]. A general characteristic of LD regardless of the cell type appears to be the content of approximately 5% phospholipids of the total mass (reviewed in Ref. [143]). 

The triacylglycerols are incorporated into a heterogeneous population of spherical lipoprotein particles known as chylomicrons (diameter, 75-600 nm) that contain about 89% triacylglycerol, 8% phospholipid, 2% cholesterol, and 1 % protein. Phospholipids of the chylomicron arise by de novo synthesis (Chapter 19) or from reacylation of absorbed lysolecithin. Cholesterol is supplied by de novo synthesis (Chapter 19) or is absorbed. The protein apolipoprotein B-48 (apo B-48) forms a characteristic protein complement of chylomicrons and is synthesized in the enterocyte. Synthesis of apo B-48 is an obligatory step in chylomicron formation. Absence of apo B-48 synthesis, as in the rare hereditary disease abetalipoproteinemia,... 

Tsujita and Okuda demonstrated in vitro that lipoprotein lipase is capable of catalyzing FAEE synthesis (Tsujita. 1994a). Chang and Borensztajn supported this observation by demonstrating that FAEE synthesis in an isolated rat heart perfused with chylomicrons and ethanol is mediated by lipoprotein lipase (Chang, 1997). The basic characteristics of enzymes involved in FAEE synthesis is presented in Table 1. 

Apolipoproteins of the C group are peptides of small molecular weights, present in chylomicrons, VLDL, IDL, and HDL. These polypeptides have common characteristics they are regulators (activators or inhibitors) of the lipid metabolism of lipoproteins, particularly the catabolism of triglyceride-rich hpoproteins [1]. The distribution of the different Apo C in lipoprotein classes varies between normal subjects and hypertriglyceridemics and in relation to fasting and the type of diet. 

Normal concentrations in the blood plasma are in the range 1200-2200 mg/1. Some 30 per cent of this is in the free state, the remainder being bound to lipoproteins. These are complexes of proteins and lipids held together by non-co-valent bonds. Each has a characteristic size, molecular weight, chemical composition and density. They are classified on the basis of their density. The five classes, of which one, the chylomicrons, occurs only in the post-absorptive state, are shown in Table 3.5. 

This phenomenon was first observed in a case of diabetic acidosis by Heyl (1880) and quite certainly only reflects the degree of plasma turbidity resulting from increase in VLDLP and/or chylomicrons. The fundus in lipemia retinalis is characteristic and can only be imitated to some degree by leukemias. The retinal vessels appear flattened and show increased light reflexes arteries and veins are difficult to distinguish on a salmon colored background. Vision is usually not impaired, and the case of de Rosa (1952) with blindness of the left eye was due to other changes (atrophy of the left optic nerve with massive lipid deposits lateral to the optic disc.)... 

Absorption of fatty adds in the small intestine follows a complex process that varies in efficiency depending on certain qualitative characteristics of dietary fat. Unsaturated fatty acids tend to be absorbed at a higha- effidraicy than saturated fatty adds. Fatty acids with slO carbons are directly absorbed into the portal drculation, while fatty acids with al2 carbons are absorbed into the lymphatic system packaged as chylomicrons. Fat digestion products incorporated into micelles occur largely through passive diffusion. Absorption of micellar components into intestinal mucosal cells depends on the penetration of the micelles across the unstirred water layer that separates the intestinal content from the brush border of the small intestine. ... 

Figure 3. Metabolic characteristics of DAG in comparison with TAG. Both TAG and 1,2-DAG break down in the small intestine to form 2-MAG before being re-esterified in gut epithelial cells to re-form TAG, which circulate in lymph chylomicrons. 1,3-DAG is broken down to form l(or 3)-MAG or glycerol, which are poorly re-esterified compared with 2-MAG.

It is desirable that the fat particles in an artificial fat emulsion have the same biological properties as those of the natural chylomicrons in respect of their transport in the blood and distribution in the body. It is believed that the clearance of fat emulsions by the reticulo-endothelial system and their subsequent fate in the body is dependent on the surface characteristics of the droplets, in particular the nature of the emulsifying agent [248]. 

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