Remnant lipoproteins

Chylomicrons and VLDL are metabolized by hydrolysis of their triacylglycerol, and lipoprotein remnants are left in the circulation. These are taken up by liver, but some of the remnants (IDL) resulting from VLDL form LDL which is taken up by the liver and other tissues via the LDL receptor. 

Chappell DA, Medh JD Receptor-mediated mechanisms of lipoprotein remnant catabolism. Prog Lipid Res 1998 37 393. 

Substantial evidence indicates that high plasma levels of lipoprotein remnants and LDL are atherogenic, while high levels of HDL are atheroprotective. Therefore, the class of lipoproteins that is increased or decreased will determine the clinical feature of a patient. Besides the influence on atherosclerosis, high levels of chylomicrons lead to acute pancreatitis, while markedly decreased levels of VLDL and LDL lead to retinal and neurologic disease, probably due to vitamin E deficiency. 

The receptors can be recycled, whereas the lipoprotein remnants in the vesicle are transferred to lysosomes and degraded by lysosomal (hydrolytic) enzymes, releasing free cholesterol, amro acids, fatty acids, and phospholipids. These compounds can be reutilized by the cell. [Note Rare autosomal recessive deficiencies in the ability to hydrolyze lysosomal cholesteryl esters (Wolman disease), or to transport unesterified cholesterol out of the lyso some (Niemann-Pick disease, type C) have been identified.]... 

The disease is characterized in part by increased plasma cholesterol and triglycerides, and the concentrations of the two lipids are about the same when expressed in milligrams per deciliter. p-VLDL present in type III has been shown to contain both apo B-lOO and B-48, and is therefore related to triglyceride-rich lipoprotein remnants of both hepatic and intestinal origins. Both LDL and HDL cholesterol are lower than normal in these patients. 

Intermediate-Density (Remnant) Lipoproteins Remnant lipoproteins include the lipolytic products of catabolism of the triglyceride-rich lipoproteins, VLDL and chylomicrons, occurring in the VLDL and LDL ranges. A traditionally defined fraction at the lighter end of the LDL density range, the IDL portion comprises the 1.006 to 1.019 g/mL fraction, which is obtained by sequential ultracentrifugation for quantitation, generally in terms of cholesterol content. 

III Broad beta disease Increased (3-VLDL levels Imperfect clearance of lipoprotein remnants... 

Hodis, TIN. (1999) Trigylceride-Rich Lipoprotein Remnant Particles and Risk of Atherosclerosis, Circulation 99,2852-2854. 

Fig. 2. Assay method for unbound lipoproteins [remnant-like particle (RLP)]. Abbreviation TG, triglycerides.

C-l II Chylomicrons, VLDL, HDL 8.75 Liver 12-15 Inhibitor of LPL, involved in lipoprotein remnant uptake... 

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