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Low-density lipoprotein

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Each native LDL particle contains a single apolipoprotein B-100 molecule (Apo B-100, a protein with 4536 amino acid residues), which circulates the fatty acids, keeping them soluble in the aqueous environment. In addition, LDL has a highly-hydrophobic core consisting of polyunsaturated fatty acid known as linoleate and about 1500 esterified cholesterol molecules. This core is surrounded by a shell of phospholipids and unesterified cholesterol, as well as a single copy of B-100 large protein (514 kD). LDL particles are approximately 22 nm in diameter and have a mass of about 3 million daltons, but since LDL particles contain a changing number of fatty acids, they actually have a mass and size distribution.[2]

LDL subtype patternsEdit





Transport into the cellEdit



Medical relevance Edit

Because LDLs transport cholesterol to the arteries and can be retained there by arterial proteoglycans starting the formation of plaques, increased levels are associated with atherosclerosis, and thus heart attack, stroke, and peripheral vascular disease. For this reason, cholesterol inside LDL lipoproteins is often called "bad" cholesterol. This is a misnomer. The cholesterol transported on LDL is the same as cholesterol transported on other lipoprotein particles. The cholesterol itself is not "bad"; rather, it is how and where the cholesterol is being transported, and in what amounts over time, that causes adverse effects. [How to reference and link to summary or text]

Increasing evidence has revealed that the concentration and size of the LDL particles more powerfully relates to the degree of atherosclerosis progression than the concentration of cholesterol contained within all the LDL particles.[5] The healthiest pattern, though relatively rare, is to have small numbers of large LDL particles and no small particles. Having small LDL particles, though common, is an unhealthy pattern; high concentrations of small LDL particles (even though potentially carrying the same total cholesterol content as a low concentration of large particles) correlates with much faster growth of atheroma, progression of atherosclerosis and earlier and more severe cardiovascular disease events and death.

LDL is formed as VLDL lipoproteins lose triglyceride through the action of lipoprotein lipase (LPL) and become smaller and denser, containing a higher proportion of cholesterol.



Lowering LDLEdit




Niacin (B3), lowers LDL by selectively inhibiting hepatic diacyglycerol acyltransferase 2, reducing triglyceride synthesis and VLDL secretion through a receptor HM74 [7] and HM74A or GPR109A [8].

Tocotrienols, especially δ- and γ-tocotrienols, have been shown to be effective nutritional agents to treat high cholesterol in recent research programs. In particular, γ-tocotrienol appears to act on a specific enzyme called 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) and suppresses the production of this enzyme, which results in less cholesterol being manufactured by liver cells.[9] This decrease in hepatic (liver) LDL levels causes hepatic LDL receptor up-regulation, further decreasing plasma LDL levels as it is taken in by the liver.

Insulin induces HMG-CoA reductase activity, whereas glucagon downregulates it.[10] While glucagon production is stimulated by dietary protein ingestion, insulin production is stimulated by dietary carbohydrate. The rise of insulin is, in general, determined by the digestion of carbohydrates into glucose and subsequent increase in serum glucose levels. Glucagon levels are very low when insulin levels are high.


Lowering the blood lipid concentration of triglycerides helps lower the amount of LDL, because VLDL gets converted in the bloodstream into LDL.

Fructose, a component of sucrose as well as high-fructose corn syrup, upregulates hepatic VLDL synthesis.[11]

Importance of antioxidants Edit

Because LDL appears to be harmless until oxidized by free radicals,[12] it is postulated that ingesting antioxidants and minimizing free radical exposure may reduce LDL's contribution to atherosclerosis, though results are not conclusive.[13]

Measurement of LDL Edit

Chemical measures of lipid concentration have long been the most-used clinical measurement, not because they have the best correlation with individual outcome, but because these lab methods are less expensive and more widely available. However, there is increasing evidence and recognition of the value of more sophisticated measurements. To be specific, LDL particle number (concentration), and to a lesser extent size, have shown much tighter correlation with atherosclerotic progression and cardiovascular events than is obtained using chemical measures of total LDL concentration contained within the particles. LDL cholesterol concentration can be low, yet LDL particle number high and cardiovascular events rates are high. Also, LDL cholesterol concentration can be relatively high, yet LDL particle number low and cardiovascular events are also low. If LDL particle concentration is tracked against event rates, many other statistical correlates of cardiovascular events, such as diabetes mellitus, obesity, and smoking, lose much of their additional predictive power.[How to reference and link to summary or text]

The lipid profile does not measure LDL level directly but instead estimates it using the Friedewald equation [14][4] using levels of other cholesterol such as HDL:


There are limitations to this method, most notably that samples must be obtained after a 12 to 14 h fast and that LDL-C cannot be calculated if plasma triglyceride is >4.52 mmol/L (400 mg/dL). Even at LDL-C levels 2.5 to 4.5 mmol/L, this formula is considered to be inaccurate.[15] If both total cholesterol and triglyceride levels are elevated then a modified formula may be used



Normal rangesEdit


Level mg/dL Level mmol/L Interpretation
100 to 129 2.6 to 3.3 Near optimal LDL level
130 to 159 3.3 to 4.1 Borderline high LDL level
160 to 189 4.1 to 4.9 High LDL level

These guidelines were based on a goal of presumably decreasing death rates from cardiovascular disease to less than 2% to 3% per year or less than 20% to 30% every 10 years. Note that 100 is not considered optimal; less than 100 is optimal, though it is unspecified how much less.

Over time, with more clinical research, these recommended levels keep being reduced because LDL reduction, including to abnormally low levels, has been the most effective strategy for reducing cardiovascular death rates in large double blind, randomized clinical trials;[16] far more effective than coronary angioplasty/stenting or bypass surgery.

For instance, for people with known atherosclerosis diseases, the 2004 updated American Heart Association, NIH and NCEP recommendations are for LDL levels to be lowered to less than 70 mg/dL, unspecified how much lower. It has been estimated from the results of multiple human pharmacologic LDL lowering trials[How to reference and link to summary or text] that LDL should be lowered to about 50 to reduce cardiovascular event rates to near zero. For reference, from longitudinal population studies following progression of atherosclerosis-related behaviors from early childhood into adulthood[How to reference and link to summary or text], it has been discovered that the usual LDL in childhood, before the development of fatty streaks, is about 35 mg/dL. However, all the above values refer to chemical measures of lipid/cholesterol concentration within LDL, not LDLipoprotein concentrations, probably not the better approach.

See also Edit


  1. LDL and HDL Cholesterol: What's Bad and What's Good?
  2. Segrest, J. P. et al (September 2001ture of apolipoprotein B-100 in low density lipoproteins). {{{title}}}. Journal of Lipid Research 42: 1346–1367.
  3. Superko HR, Nejedly M, Garrett B (2002). Small LDL and its clinical importance as a new CAD risk factor: a female case study. Prog Cardiovasc Nurs 17 (4): 167–73.
  4. 4.0 4.1 Warnick et al. Estimating low-density lipoprotein cholesterol by the Friedewald equation is adequate for classifying patients on the basis of nationally recommended cutpoints. URL accessed on 2007-12-04.
  5. Not All Calories Are Created Equal, Author Says. Talk of the Nation discussion of the book "Good Calories, Bad Calories", by Gary Taubes. National Public Radio, 2 Nov 2007.
  6. [1] PMID:6147641
  7. [2] PMID:15529025
  8. 8.0 8.1 [3] PMID:17238156
  9. Song, B.L., DeBose-Boyd, R.A. (2006). Insig-Dependent Ubiquitination and Degradation of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Stimulated by Delta- and Gamma-Tocotrienols. J. Biol. Chem. 281 (35): 25054–25601.
  10. Regulation of Cholesterol Synthesis
  11. Fructose, insulin resistance, and metabolic dyslipidemia
  13. Effect of antioxidants on oxidative modification of LDL. Esterbauer H, Puhl H, Dieber-Rotheneder M, Waeg G, Rabl H. Ann Med. 1991;23(5):573-81.
  14. Friedewald WT, Levy RI, Fredrickson DS (1972). Estimation of the concentration of low-density lipoprotein cholesterol in plasma, without use of the preparative ultracentrifuge. Clin. Chem. 18 (6): 499–502.
  15. Sniderman AD, Blank D, Zakarian R, Bergeron J, Frohlich J (2003). Triglycerides and small dense LDL: the twin Achilles heels of the Friedewald formula. Clin. Biochem. 36 (7): 499–504.
  16. Shepherd J, Cobbe SM, Ford I, Isles CG, Lorimer AR, MacFarlane PW, McKillop JH, Packard CJ (1995). Prevention of coronary heart disease with pravastatin in men with hypercholesterolemia. West of Scotland Coronary Prevention Study Group. N Engl J Med. 333 (30): 1301–1307.

References & External LinksEdit



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