پروتين دهني منخفض الكثافة

الپروتين الدهني منخفض الكثافة Low-density lipoprotein (LDL) هو أحد خمس مجموعات أساسية من الپروتين الدهنية المرتبة حسب الحجم، من الأكبر إلى الأصغر، پروتين دهني وضعي الكثافة، پروتين دهني متوسط الكثافة، الپروتين الدهني منخفض الكثافة، ثم پروتين دهني مرتفع الكثافة، والتي يمكنها نقل الدهون مثل الكوليسترول وثلاثي الگليسريدات مع المحلول المائي لتيار الدم. الجزء الأكبر من الكوليسترول في الدم يكون محمولا بواسطة البروتينات الدهنية منخفضة الكثافة. وهذا النوع من الكوليسترول يعتبر المصدر الأساسي لترسب الكوليسترول في الشرايين وضيقها وانسدادها، وبهذا، فكلما ارتفع تركيز كوليسترول البروتينات الدهنية منخفضة الكثافة في الدم كلما ارتفعت مخاطر الإصابة بأمراض القلب التاجية والنوبات القلبية. ويمكن تعيين نوع الكولسترول استنادًا إلى نوع الپروتين الدهني الذي يحمله. ويوجد البروتين الدهني المنخفض الكثافة في جدران شرايين القلب. ويعتقد بعض العلماء أن البروتينات الدهنية العالية الكثافة تساعد على إزالة الكولسترول من الأنسجة. ويوجد لدى الأشخاص بصفة عامة كولسترول البروتين الدهني المنخفض الكثافة أكثر من كولسترول البروتين الدهني العالي الكثافة.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

 الكيمياء الحيوية

 التركيب

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 (0.00000087 in.) 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 distribution of mass and size. Determining structure of LDL has been a tough task because of its heterogeneous structure. First structure of LDL at human body temperature in native condition has been recently found using cryo-electron microscopy and it has resolution of 16 Angstrom.

 الأنماط الفرعية للپروتين الدهني المنخفض الكثاف

LDL particles vary in size and density, and studies have shown that a pattern that has more small dense LDL particles, called Pattern B, equates to a higher risk factor for coronary heart disease (CHD) than does a pattern with more of the larger and less dense LDL particles (Pattern A). This is because the smaller particles are more easily able to penetrate the endothelium. Pattern I, for intermediate, indicates that most LDL particles are very close in size to the normal gaps in the endothelium (26 nm).

Some in the medical community have suggested the correspondence between Pattern B and CHD is stronger than the correspondence between the LDL number measured in the standard lipid profile test. Tests to measure these LDL subtype patterns have been more expensive and not widely available, so the common lipid profile test is used more commonly.

There has also been noted a correspondence between higher triglyceride levels and higher levels of smaller, denser LDL particles and alternately lower triglyceride levels and higher levels of the larger, less dense LDL.

With continued research, decreasing cost, greater availability and wider acceptance of other lipoprotein subclass analysis assay methods, including NMR spectroscopy, research studies have continued to show a stronger correlation between human clinically obvious cardiovascular event and quantitatively-measured particle concentrations.[citation needed]

 النقل إلى الخلايا

When a cell requires cholesterol, it synthesizes the necessary LDL receptors, and inserts them into the plasma membrane. The LDL receptors diffuse freely until they associate with clathrin-coated pits. LDL particles in the blood stream bind to these extracellular LDL receptors. The clathrin-coated pits then form vesicles that are endocytosed into the cell.

After the clathrin coat is shed, the vesicles deliver the LDL and their receptors to early endosomes, onto late endosomes to lysosomes. Here the cholesterol esters in the LDL are hydrolysed. The LDL receptors are recycled back to the plasma membrane.

 الأهمية الطبية

Because LDL particles can also transport cholesterol into the artery wall, retained there by arterial proteoglycans and attract macrophages that engulf the LDL particles and start the formation of plaques, increased levels are associated with atherosclerosis. Over time vulnerable plaques rupture, activate blood clotting and produce arterial stenosis, which if severe enough results in heart attack, stroke, and peripheral vascular disease symptoms and major debilitating events.

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. The healthiest pattern A, 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 B; 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. This video [5], examining autopsy specimens from an actual heart attack resulting in sudden death, shows the seqence. These videos, [6] and [7], illustrate the sequence of events and why, though the underlying process develops over decades, the symptoms are often of sudden onset.

LDL particles are formed as VLDL lipoproteins lose triglyceride through the action of lipoprotein lipase (LPL) and they become smaller and denser (i.e. fewer fat molecules with same protein transport shell), containing a higher proportion of cholesterol esters [8].

A hereditary form of high LDL is familial hypercholesterolemia (FH). Increased LDL is termed hyperlipoproteinemia type II (after the dated Fredrickson classification).

LDL particles pose a risk for cardiovascular disease when they invade the endothelium and becomes oxidized, since the oxidized forms are more easily retained by the proteoglycans. A complex set of biochemical reactions regulates the oxidation of LDL particles, chiefly stimulated by presence of necrotic cell debries [9] and free radicals in the endothelium.

 دورها في نظام المناعة الخلقي

LDL lipoproteins interfere with the quorum sensing system that upregulates genes required for invasive Staphylococcus aureus infection. The mechanism of antagonism entails binding Apolipoprotein B, to a S. aureus autoinducer pheromone, preventing signaling through its receptor. Mice deficient in apolipoprotein B are more susceptible to invasive bacterial infection.

 تخفيض البروتينات الدهنية المنهفضة الكثافة

The mevalonate pathway serves as the basis for the biosynthesis of many molecules, including cholesterol. The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) is an essential component in the pathway.

 دوائيا

Statins reduce high levels of LDL particles by inhibiting the enzyme HMG-CoA reductase in cells, the rate-limiting step of cholesterol synthesis. To compensate for the decreased cholesterol availability, synthesis of hepatic LDL receptors is increased, resulting in an increased clearance of LDL particles from the blood.

Ezetimibe reduces intestinal absorption of cholesterol, thus can powerfully reduce LDL particle concentrations when combined with statins.

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

Clofibrate is effective at lowering cholesterol levels, but has been associated with significantly increased cancer and stroke mortality, despite lowered cholesterol levels. Other, more recently developed and tested fibrates, e.g. fenofibric acid [10] have had a better track record and are primarily promoted for lowering VLDL particles (triglycerides), not LDL particles, yet can help some in combination with other strategies.

Some Tocotrienols, especially delta- and gamma-tocotrienols, are being promoted as statin alternative non-prescription agents to treat high cholesterol, having been shown in vitro to have an effect. In particular, gamma-tocotrienol appears to be another HMG-CoA reductase inhibitor, and can reduce cholesterol production. As with statins, this decrease in intra-hepatic (liver) LDL levels may induce hepatic LDL receptor up-regulation, also decreasing plasma LDL levels. As always, a key issue is how benefits and complications of such agents compare with statins—molecular tools that have been analyzed in large numbers of human research and clinical trials since the mid-1970s.

 غذائيا

The most effective approach has been minimizing visceral (in addition to total) body fat stores. Visceral fat, which is more metabolically active than subcutaneous fat, has been found to produce many enzymatic signals, e.g. resistin, which increase insulin resistance and circulating VLDL particle concentrations, thus both increasing LDL particle concentrations and accelerating the development of Diabetes Mellitus.

Insulin induces HMG-CoA reductase activity, whereas glucagon diminishes it. 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. In non-diabetics, glucagon levels are very low when insulin levels are high; however, those who have become diabetic no longer suppress glucagon output after eating.

A ketogenic diet may have similar response to taking niacin (lowered LDL and increased HDL) through beta-hydroxybutyrate, a ketone body, coupling the niacin receptor (HM74A).

Lowering the blood lipid concentration of triglycerides helps lower the concentration of Small LDL particles, because fat rich VLDL particles convert in the bloodstream into Small LDL particles.

Fructose, a component of sucrose as well as high-fructose corn syrup, upregulates lilhepatic VLDL synthesis.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

 أهمية مضادات الأكسدة

Because LDL particles appear harmless until they are within the blood vessel walls and oxidized by free radicals, it is postulated that ingesting antioxidants and minimizing free radical exposure may reduce LDL's contribution to atherosclerosis, though results are not conclusive.

 تقدير جسميات البروتينات المنخفضة في الكوليسترول

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.

The lipid profile does not measure LDL particles directly but instead estimates them using the Friedewald equation by subtracting the amount of cholesterol associated with other particles, such as HDL and VLDL, assuming a prolonged fasting state, etc.:

$H\approx C-L-kT$

where H is HDL cholesterol, L is LDL cholesterol, C is total cholesterol, T are triglycerides, and k is 0.20 if the quantities are measured in mg/dl and 0.45 if in mmol/l.

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 inaccurate. If both total cholesterol and triglyceride levels are elevated then a modified formula, with quantities are in mg/dl, may be used

$L=C-H-0.16T$

This formula provides an approximation with fair accuracy for most people, assuming the blood was drawn after fasting for about 14 hours or longer.

However, the concentration of LDL particles, and to a lesser extent their size, has far tighter correlation with individual clinical outcome than the amount of cholesterol within LDL particles, even if the LDL-C estimation is about correct. There is increasing evidence and recognition of the value of more targeted and accurate measurements of LDL particles. Specificly, LDL particle number (concentration), and to a lesser extent size, have shown much tighter correlation with atherosclerotic progression and cardiovascular events than obtained using chemical measures of the amount of cholesterol carried by the LDL particles.[11] LDL cholesterol concentration can be low, yet LDL particle number high and cardiovascular events rates are high. Correspondingly, 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.

 النطاقات الطبيعية

In the USA, the American Heart Association, NIH, and NCEP provide a set of guidelines for fasting LDL-Cholesterol levels, estimated or measured, and risk for heart disease. As of about 2005, these guidelines were:

Level mg/dL	Level mmol/L	Interpretation
25 to <50	<1.3	Optimal LDL cholesterol, levels in healthy young children before onset of atherosclerotic plaque in heart artery walls
<70	<1.8	Optimal LDL cholesterol, corresponding to lower rates of progression, promoted as a target option for those known to clearly have advanced symptomatic cardiovascular disease
<100	<2.6	Optimal LDL cholesterol, corresponding to lower, but not zero, rates for symptomatic cardiovascular disease events
100 to 129	2.6 to 3.3	Near optimal LDL level, corresponding to higher rates for developing symptomatic cardiovascular disease events
130 to 159	3.3 to 4.1	Borderline high LDL level, corresponding to even higher rates for developing symptomatic cardiovascular disease events
160 to 199	4.1 to 4.9	High LDL level, corresponding to much higher rates for developing symptomatic cardiovascular disease events
>200	>4.9	Very high LDL level, corresponding to highest increased rates of symptomatic cardiovascular disease events

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, was the most effective strategy for reducing cardiovascular death rates in one large double blind, randomized clinical trial of men with hypercholesterolemia; 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. This low level of less than 70 mg/dL was recommended for primary prevention of 'very-high risk patients' and in secondary prevention as a 'reasonable further reduction'. Lack of evidence for such a recommendation is discussed in an article in the Annals of internal medicine. It should also be noted that statin drugs involved in such clinical trials have numerous physiological effects beyond simply the reduction of LDL levels.

It has been estimated from the results of multiple human pharmacologic LDL lowering trials 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, 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.

The feasibility of these figures has been questioned by sceptics, claiming that many members of the AHA and NIH are heavily associated with pharmaceutical companies giving them bias towards lowering cholesterol levels and such guidelines giving rise to increased use of cholesterol lowering medicine such as statins.

Moreover, there are publications regarding the risks of low-LDL cholesterol too.

 القياس المباشر لتركيز البروتينات الدهنية المنخفضة الكثافة

There are several competing methods for measurement of lipoprotein particle size although the evidence in favor of their superiority to existing methods is weak, even by the statements of proponents. Direct LDL particle measurement by NMR was mentioned by the ADA and ACC, in a 28 March 2008 joint consensus statement, as having advantages for predicting individual risk of atherosclerosis disease events, but the statement noted that the test is not widely available and is more expensive than existing tests. Furthermore the authors also said it is "...unclear whether LDL particle size measurements add value to measurement of LDL particle concentration." Since the later 1990s, because of the development of NMR measurements, it has been possible, to clinically measure lipoprotein particles at lower cost [under $100 US (including shipping) versus the previous costs of >$400 to >$5,000] and high accuracy. There are also other (less expensive) homogeneous assays for LDL.

Using NMR, the total LDL particle concentrations, in nmol/L plasma, are typically subdivided by percentiles referenced to the 5,382 men and women, not on any lipid medications, who participated in the MESA trial.

 النطاقات المثلى

The LDL particle concentrations are typically categorized by percentiles, <20%, 20-50%, 50th-80th%, 80th-95% and >95% groups of the people participating and being tracked in the MESA trial, a medical research study sponsored by the United States National Heart, Lung, and Blood Institute.

MESA Percentile	LDL particles nmol/L	Interpretation
0-20%	<1,000	Those with lowest rate of cardiovascular disease events & low, Optimal LDL particle concentration
20-50%	1,000-1,299	Those with moderate rate of cardiovascular disease events & moderate LDL particle concentration
50-80%	1,300-1,599	Those with Borderline-High rate of cardiovascular disease events & higher LDL particle concentration
89-95%	1,600 to 2,000	Those with High rate of cardiovascular disease events & High LDL particle concentration
>95%	>2,000	Those with Very High rate of cardiovascular disease events & Highest LDL particle concentrations

The lowest incidence of atherosclerotic events over time occurs within the <20% group, with increased rates for the higher groups. Multiple other measures, including particle sizes, small LDL particle concentrations, total and large HDL particle concentrations, along with estimations of Insulin resistance pattern and standard cholesterol lipid measurements (for comparison of the plasma data with the estimation methods discussed above) are also routinely provided.

 انظر أيضا

• كاتيكين
• Category:Low density lipoprotein receptor gene family
• كوليسترول
• Lysosomal acid lipase deficiency
• Cholesteryl ester storage disease
• Coenzyme Q10
• فلافونويد
• گلوتاثيون
• التأثيرات الصحية للشاي
• پروتين دهني مرتفع الكثافة
• LDL receptor
• بروتين دهني (أ)
• بروتين دهني إكس
• ملتونين
• Polyphenol antioxidant
• دهون مشبعة
• Stanol ester
• Sterol ester
• Triglyceride
• فيتامين أ
• فيتامين ج
• فيتامين إي

 هوامش

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

 مصادر ووصلات خارجية

• Fat (LDL) Degradation: PMAP The Proteolysis Map-animation
• Adult Treatment Panel III Full Report
• ATP III Update 2004
• Cromwell WC, Otvos JD (2004). "Low-density lipoprotein particle number and risk for cardiovascular disease". Current Atherosclerosis Reports. 6 (5): 381–7. doi:10.1007/s11883-004-0050-5. PMID 15296705. {{cite journal}}: Unknown parameter |month= ignored (help)
• O'Keefe JH, Cordain L, Harris WH, Moe RM, Vogel R (2004). "Optimal low-density lipoprotein is 50 to 70 mg/dl: lower is better and physiologically normal". Journal of the American College of Cardiology. 43 (11): 2142–6. doi:10.1016/j.jacc.2004.03.046. PMID 15172426. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)