Good cholesterol lipoproteins

High density lipoproteins (HDL)—these collect cholesterol from the body s tissues, and bring it back to the liver. Sometimes referred to as the good cholesterol lipoprotein. 

Cholesterol is biosynthesized in the liver trans ported throughout the body to be used in a va riety of ways and returned to the liver where it serves as the biosynthetic precursor to other steroids But cholesterol is a lipid and isn t soluble in water How can it move through the blood if it doesn t dis solve in if The answer is that it doesn t dissolve but IS instead carried through the blood and tissues as part of a lipoprotein (lipid + protein = lipoprotein) The proteins that carry cholesterol from the liver are called low density lipoproteins or LDLs those that return it to the liver are the high-density lipoproteins or HDLs If too much cholesterol is being transported by LDL or too little by HDL the extra cholesterol builds up on the walls of the arteries caus mg atherosclerosis A thorough physical examination nowadays measures not only total cholesterol con centration but also the distribution between LDL and HDL cholesterol An elevated level of LDL cholesterol IS a risk factor for heart disease LDL cholesterol is bad cholesterol HDLs on the other hand remove excess cholesterol and are protective HDL cholesterol IS good cholesterol... 

Heterogeneous reaction (Section 6 1) A reaction involving two or more substances present in different phases Hydro genation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst Heterolytic cleavage (Section 4 16) Dissociation of a two electron covalent bond in such a way that both electrons are retained by one of the initially bonded atoms Hexose (Section 25 4) A carbohydrate with six carbon atoms High density lipoprotein (HDL) (Section 26 11) A protein that carries cholesterol from the tissues to the liver where it is metabolized HDL is often called good cholesterol Histones (Section 28 9) Proteins that are associated with DNA in nucleosomes... 

High-density lipoprotein (HDL) (Section 26.11) Aprotein that carries cholesterol from the tissues to the liver where it is metabolized. HDL is often called good cholesterol. ... 

Krieger, M, 1999. Charting the fate of the good cholesterol Identification and characterization of the high-density lipoprotein receptor SR-BI. Annu Rev Biochem 68, 523-558. 

Further research on serum cholesterol revealed that is exists in two forms, high and low density lipoprotein. The high density lipoprotein appears to consist of cholesterol that is being moved to the liver for metabolism while low density lipoprotein appears to be cholesterol that is likely to block arteries. This has led to the labelling of high density lipoprotein as good cholesterol and of low density lipoprotein as bad cholesterol. 

The recent years have seen the success of statins like Lipitor (atorvastatin) as hypolipidemic agents that help treating cardiovascular disease primarily by lowering low-density lipoproteins ( bad cholesterol ) levels. Another novel strategy is to tackle the same problem by elevating high-density lipoproteins (H D L or good cholesterol ) levels via inhibition of cholesteryl ester transfer protein (CETP). 

Cholesterol is carried in the bloodstream by hpoproteins such as low density lipoprotein (LDL, or bad cholesterol ) and high density lipoprotein (HDL, good cholesterol ). LDL carries cholesterol from the liver to other parts of the body. LDL attaches to receptors (see Chapter 2) on the cell surface and is taken into the cell interior. It is then degraded and the cholesterol is used as a component for the cell membrane. When there is excessive cholesterol inside the cell, it leads to a reduction in the synthesis of LDL receptors. 

HDL (high-density lipoprotein) a protein-lipid complex that carries cholesterol away from the tissues good cholesterol. ... 

High-density lipoproteins (HDLs) contain a different apolipoprotein form, Apolipoprotein A. These proteins are about half lipid and half protein by weight. Phospholipids and cholesterol esters are the most important lipid components. HDL is sometimes referred to as good cholesterol because a higher ratio of HDL to LDL corresponds to a lower rate of coronary artery disease. 

The relationship between elevated triglycerides (TGRs) as a risk factor for CAD is less clear. However, serum TGRs are often inversely related to high-density lipoprotein (HDL—the good cholesterol). Therefore, reduction in TGR levels are associated with a rise in HDL, which has a negative correlation (protective effect) with CAD. 

The substance itself isn t soluble in water or, on a more practical level, in the bloodstream. So cholesterol is transported through the body in a variety of envelopes made of fat and protein called lipoproteins. Cholesterol transported in a low-density lipoprotein envelope tends to be deposited in damaged regions of our arteries. That s a bad thing, so we call LDL (low-density lipoprotein) cholesterol bad. But in the body s system of balances, we also have cholesterol transported out of those arteries and removed from the bloodstream by HDL (high-density lipoprotein). We call that combination the good cholesterol. 

Lipids are the main component in lipoproteins known as carriers of cholesterol (2). HDL (high-density lipoprotein, which is the good cholesterol) transports cholesterol and its esters from cells to liver for recirculation, whereas LDL (low-density lipoprotein, which is the bad cholesterol) carries cholesterol and cholesterol esters to the cells. The ratio of the two carrier particles partly determines one s risk for diseases such as atherosclerosis, although the understanding and the overall view of the related issues is still incomplete in many ways. 

High serum levels of cholesterol cause disease and death by contributing to the formation of atherosclerotic plaques in arteries throughout the body. This excess cholesterol is present in the form of the low density lipoprotein particle, so-called "bad cholesterol. "The ratio of cholesterol in the form of high density lipoprotein, sometimes referred to as "good cholesterol," to that in the form of LDL can be used to evaluate susceptibility to the development of heart disease. For a healthy person, the LDL/HDL ratio is 3.5. 

For men and women, the use of male steroids (androgens) - either the hormone testosterone or the synthetic anabolic steroids - may also increase the risk of coronary artery disease. These drugs lower high-density lipoprotein (HDL) (the good) cholesterol levels, increase low-density lipoprotein (LDL) (the bad) cholesterol levels, and cause high blood pressure. All of these effects may contribute to having a heart attack at an early age or to having a stroke. What effects the use of anabolic steroids early in life has later in life are unclear. 

Scientific reports have confirmed the relationship between trans fat and an increased risk of coronary heart disease. Trans fat, like saturated fat and dietary cholesterol, can raise low-density lipoprotein (LDL or bad cholesterol) levels in the blood. An elevated LDL cholesterol level increases the risk of developing coronary heart disease. Unlike saturated fat, trans fat also lowers high-density lipoprotein (HDL or good ) cholesterol in the blood. 

High density lipoprotein is sometimes referred to as good cholesterol, HDL functions as a shuttle that moves cholesterol throughout the body. HDL binds and esterifies cholesterol released from the peripheral tissues and then transfers cholesteryl esters to the liver or to tissues that use cholesterol to synthesize steroid hormones. A specific receptor mediates the docking of the HDL to these tissues. The exact nature of the protective effect of HDL levels is not known however, a possible mechanism will be exammed on page 747. 

HDL High-density lipoprotein synthesized in the liver, the HDL serves as a source of apoUpoproteins for other lipoproteins, as the site of action for the conversion of cholesterol to cholesterol ester in the plasma by the enzyme lecithin-cholesterol acyltransferase (LCAT), and delivers cholesterol esters derived from peripheral membranes to the liver. It is commonly called the good cholesterol. ... 

The major classes of lipoproteins are chylomicrons (lowest density), very-low-density lipoproteins (VLDLs), intermediate-density hpoproteins (IDLs), low-density lipoproteins (LDLs), and high-density lipoproteins (HDLs) (considered the good cholesterol ). 

Niacin is a vitamin that is used in high doses to treat hypercholesterolemia. Niacin acts to decrease VLDL and LDL plasma levels. Its mechanism of action is not clearly nnderstood but probably involves inhibition of VLDL secretion, which in tnm decreases the production of LDL. Niacin inhibits the release of free fatty acids from adipose tissue which leads to a decrease of free fatty acids entering the liver and decreased VLDL synthesis in the liver. This decreases the availability of VLDL for conversion to LDL (containing cholesterol esters). Niacin also increases high-density lipoprotein (HDL) (the good cholesterol ) by an nnknown mechanism. 

In the popular literature HDL is frequently referred to as good cholesterol and LDL as bad cholesterol. Blood chemistry reports often state total cholesterol levels, even though LDL and HDL values (or their ratios) would probably be more relevant to a patient s clinical evaluation. Total serum cholesterol levels, normally reported in milligrams/deciliter (mg%) do not indicate the fraction that is free, esterified, or bound up in the various lipoprotein fractions. 

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