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Course of LABORATORY MEDICINE
Lipids (essentially triglycerides and cholesterol) are insoluble in water and are transported in the blood in the form of complexes with soluble proteins, called lipoproteins. Lipoproteins fall in different categories, based on their density (as measured with the ultracentrifuge), their electrophoretic mobility and their metabolic origin and destination.
Chylomicrons are the lowest density lipoproteins. They are of alimentary origin, and are synthesized by the enterocytes with triglycerides and some cholesterol. The apolipoprotein subunits of chylomicrons (i.e. the proteic component of the particle) are called A-I, A-II, B-48, C-I, C-II, C-III and E: each is coded by a specific gene and presents its own aminoacid sequence. The density of chylomicrons, measured by ultracentrifugation is <0.95 g/mL.
Chlomicrons are released both in the portal venous system and in the lymphatic vessels. Chlomicrons in the porta vein are removed by the liver which transfers their lipidic contento to other types of lipoproteins. Chylomicrons released in the intestinal lymph ("chyle") reach the thoracic duct that drains into the brachiocephalic vein and from this into the superior vena cava. They are thus distributed to all tissues that can extract triglycerides for their metabolic needs. The major consumers of triglycerides are the skeletal muscle and the heart. Adipocytes are able to extract and store the lipidic content of chylomicrons (and other lipoproteins) if in excess for the metabolic demands, Removal of triglycerides requires the enzyme lipoprotein lipase, which is activated by apolipoprotein C-II. As triglycerides are removed, the particles shrink and are enriched in cholesterol, forming the so-called chylomicron remnants, that are taken up by the liver.
Very low density lipoproteins (VLDL) are composed of triglycerides, cholesterol and colesteryl-esters, bound to apolipoprotein components B-100, C-I, C-II, C-III and E. Their density is 0.95-1.006 g/mL. VLDLs are synthesized by the liver and their lipid component is degraded by lipoprotein lipase as it occurs for chylomicrons. As VLDLs shrink, they loose some apolipoprotein components and are successively converted to IDLs and LDLs.
Intermediate density lipoproteins (IDL) are composed of triglycerides, cholesterol and colesteryl-esters, bound to apolipoprotein components B-100,C-III and E. Their density is 1.006-1.019 g/mL. They originate from the shrinking of VLDLs and have a similar fate.
Low density lipoproteins (LDL) are composed of triglycerides, cholesterol and colesteryl-esters, bound to apolipoprotein component B-100. Their density is 1.019-1.063 g/mL. LDLs are rich in cholesterol and are absorbed by the cells via receptor mediated endocytosis, thanks to a specific LDL receptor residing on the cell membrane. The endocytic vesicle is degraded by fusion with lysosomes and the B-100 apolipoprotein component is digested by proteases. Cholesterol and triglycerides are used by the cell either for membrane turnover or for energy roduction (limited to the beta oxidation of fatty acids).
High density lipoproteins (HDL) carry essentially cholesterol and its esters and are composed by the apolipoprotein components A-I, A-II, C-I, C-II, C-III, D and E. Their density is 1.063-1.210 g/mL. Their function is the transport of cholesterol from peripheral tissues to the liver: i.e. in the opposite direction of other lipoproteins. The liver requires substantial amounts of cholesterol for the biosynthesis of bile salts (cholate and deoxycholate). Bile salts are secreted in the intestine where they emulsify fat of alimentary origin and allow its digestion by lipases; they are reabsorbed by the intestine and re-utilized (on average 7-times), then are finally lost in the faeces. The main way of eliminating cholesterol from the organism is via the bile salts (i.e. the bile salts are the terminal metabolytes of cholesterol).
The essential clinical information about lipoproteins can be summarized as follows: chylomicrons essentially carry dietary triglycerides, are synthesized by the enterocytes, and are the largest and less dense lipoproteins; their proteic component is relatively complex and involves many different subunits. All other lipoproteins are synthesized by the liver and modified by the adipocytes. LDL, IDL and VLDL strongly resemble each other; they carry both cholesterol and triglycerides, and their apoliprotein component alweys involves apoliproptein B-100. Their affinity for cholesterol is relatively low and they may release some of their cholesterol content to the endothelial wall (so called "bad" cholesterol, associated to increased risk of atherosclerosis). HDL carry essentially cholesterol, have a very specific apolipoprotein composition, different from that of VLDL, IDL, and LDL. They bind cholesterol with high affinity and may be able to remove it from the endothelial wall (so called "good" cholesterol, not associated to atherosclerosis).
triglycerides are used mainly for energy production and their fatty acids are oxidized by the beta-oxidation and the Krebs cycle: they are thus expelled as CO2 and H2O. The skeletal muscle and the heart are among the organs that use mostly fatty acids for their catabolic metabolism. triglycerides are also be used for the turnover of cell membrane components, after conversion to phospholipids.
Cholesterol has three main functions: it is used in the cell membrane, as the precursor of steroid hormones, and as the precursor of bile salts (that also constitute its main elimination pathway).
Dislipidemias of clinical relevance are always associated with increased serum concentrations of lipids and lipoproteins, as these conditions may favor atherosclerosis and other disturbances; on the contrary, lower than normal levels of blood lipids are not considered clinically relevant, unless they are associated to other conditions (e.g. hyponutrition, anorexia, malabsorption, etc.). The usual classification of hyperlipoproteinemias is due to Fredrickson, but has been greatly improved over time. It is summarized in the Table below.
Analytical methods. Cholesterol, triglycerides and phosphoglycerids can be extracted from the human serum using organic solvents (usually mixtures of hexane and 2-propanol), and their concentration can be measured by chemical or enzymatic methods (after dilution in the appropriate buffers). Examples of the methods used to quantitate Cholesterol and Triglycerids are as follows. Cholesterol can be measured using the enzyme cholesterol oxidase that uses molecular oxygen to convert cholesterol to cholestenone; the reaction produces one mol of hydrogen peroxide per mol of cholesterol, which is easily measured by with standard assays (e.g. using horseradish peroxidase and the fluorescent probe o-dianisidine). Triglycerides should be hydrolysed after extraction, to yield glycerol and fatty acids (this can be obtained either chemically or enzymatically, with lipiprotein lipases); the concentration of glycerol is then measured using the microbial enzymes glycerol kinase (that uses ATP to produce Glycerol-1-phosphate) and Glycerol-1P oxidase (that uses O2 and yields dihydroxyacetone phosphate and hydrogen peroxide, measured using standard methods).
The lipoproteins can be submitted to electrophoresis and separated in their components; however their quantitation requires specific procedures because they co-migrate with other protein fractions and stain poorly with the usual protein-specific dyes, due the relatively modest amount of protein (by weight). To quantitate the lipoproteins, the electrophoretic preparation must be stained using lipid-specific dyes (e.g. Sudan black). These do not react with the protein fractions and only stain the lipid content of lipoproteins.
The metabolic syndrome is a complex clinical condition that causes increased risk of myocardial infarction and other caridovascular diseases. It has the following markers
(i) A large waistline. This also is called abdominal obesity or "having an apple shape." Excess fat in the stomach area is a greater risk factor for heart disease than excess fat in other parts of the body, such as on the hips.
(ii) A high triglyceride level (or you're on medicine to treat high triglycerides). Triglycerides are a type of fat found in the blood.
(iii) A low HDL cholesterol level (or you're on medicine to treat low HDL cholesterol). HDL sometimes is called "good" cholesterol. This is because it helps remove cholesterol from your arteries. A low HDL cholesterol level raises your risk for heart disease.
(iv) High blood pressure (or you're on medicine to treat high blood pressure). Blood pressure is the force of blood pushing against the walls of your arteries as your heart pumps blood. If this pressure rises and stays high over time, it can damage your heart and lead to plaque buildup.
(v) High fasting blood sugar (or you're on medicine to treat high blood sugar). Mildly high blood sugar may be an early sign of diabetes.
The metabolic syndrome depends on a genetic predisposition, and environmental factors.
HUNGER AND BODY WEIGHT
The sensation of hunger is regulated by hormones, the most important being the Y polypeptides produced by the hypothalamus. Increased hypothalamic production of Y polypeptides and hormones causes hunger.
The production of hunger hormones by the hypothalamus is regulated by the adipose tissue in a negative feedback loop. The adipose tissue produces a protein hormone called LEPTIN that inhibits the production and secretion of Y polypeptides. Genetic defects of leptin are known that cause hereditary obesity (the gene coding for leptin is called Ob because it was identified in strains of genetically obese rats).
Excess (or a satisfactory amount of) fat tissue produces excess leptin and inhibits production of Y peptides, causing the sensation of satiation. Unfortunately, the hypothalamus may become adapted to the higher level of leptin, and may stop responding, causing obesity.
ENDOCRINE FUNCTIONS OF ADIPOSE TISSUE
Following the discovery of leptin, several endocrine functions have been attributed to the adipose tissue. The protein factors secreted by the adipose tissue include cytokines, adiponectin, proteins of the renin-angiotensin system, and resistin. Moreover, the adipose tissue participates to the metabolism of steroid homrones and is involved in fertility (especially in women).
The endocrine functions of the adipose tissue help explaining the relationships between obesity and hypertension or diabetes. Indeed, obesity shares some endocrine responses of chronic inflammatory diseases (because of the production of cytokines).
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