Lipoprotein isoforms

Webb, NR, Connell, PM, Graf, GA, Smart, EJ, de Villiers, WJ, de Beer, FC, and van der Westhuyzen, DR, 1998. SR-BII, an isoform of the scavenger receptor BI containing an alternate cytoplasmic tail, mediates lipid transfer between high density lipoprotein and cells. J Biol Chem 273, 15241-15248. 

Standardization problems (L4) arise from the polymorphic nature of apo(a) and from its linkage to apo-B within the Lp(a) lipoprotein. A combination of an anti-apo(a) as capture antibody with an anti-apo-B for detection enables the expression of the Lp(a) concentration as lipoprotein particles. The size of the apo(a) isoforms becomes critical in assays using only apo(a) antibodies, so that the problem of the units of mass for Lp(a) has not been solved yet. 

Although most assays perform well with regard to specificity and reproducibility, the major problem remains their standardization (A9, Dl, K30, L4). There is currently no internationally accepted standard, and the selection of a reference material raised many problems (A8, G5, K30, L4). A number of questions have not been solved Should the standard consist of several apo(a) isoforms Can the reference material be lyophilized Should results be expressed as mass or as moles of apoprotein or lipoprotein How should the protein mass of the primary standard be determined What are optimal storage conditions for the secondary standard Which method can be used as a reference method Can recombinant apo(a) represent an alternative for a primary standard These problems came to light in the course of the international surveys whose results were presented at the Lp(a) Workshop in New Orleans (1992) (L4). 

K15. Klausen, I. C., Berg Schmidt, E., Lervang, H. H., Gerdes, L. U., Ditzel, J., and Faergeman, O., Normal lipoprotein(a) concentrations and apolipoprotein(a) isoforms in patients with insulin-dependent diabetes mellitus. Eur. J. Clin. Invest. 22, 538-541 (1992). 

M12. Marcovina, S. M., Zhang, Z. H., Gaur, V. P., and Albers, J. J., Identification of 34 apo-lipoprotein(a) isoforms Differential expression of apolipoprotein(a) allelles between American blacks and whites. Biochem. Biophys. Res. Commun. 191, 1192-11% (1993). 

Rl. Rader, D. J., Cain, W., Zech, L. A., Usher, D., and Brewer, H. B., Jr., Variation in lipoprotein(a) concentration among individuals with the same apolipoprotein(a) isoforms is determined by the rate of lipoprotein(a) production. J. Clin. Invest. 91, 443-444 (1993). 

Tl. Taddei-Peters, W. C., Butman, B. T., Jones, G. R., Venetta, T. M., Macomber, P. F., and Ransom, J. H., Quantification of lipoprotein(a) particles containing various apolipoprotein(a) isoforms by a monoclonal anti apo(a) capture antibody and a polyclonal anti-apolipoprotein B detection antibody sandwich enzyme immunoassay. Clin. Chem. (Winston-Salem, NC) 39, 1382-1389 (1993). 

Production of LDL from VLDL in the plasma With these modifications, the VLDL is converted in the plasma to LDL. An intermediate-sized particle, the intermediate-density lipoprotein (IDL) or VLDL remnant, is observed during this transition. IDLs can also be taken up by cells through receptor-mediated endocytosis that uses apo E as the ligand. [Note Apolipoprotein E is normally present in three isoforms, E2, E3, and E4. Apo E2 binds poorly to receptors, and patients who are homozygotic for apo E2 are deficient in the clearance of chylomicron remants and IDLs. The individuals have familial type III hyperlipoproteinemia (familial dysbetalipoproteinemia, or broad beta disease), with hypercholesterolemia and premature atherosclerosis. Not yet understood is the fact that the E4 isoform confers increased susceptibility to late-onset Alzheimer disease.]... 

Schmitz, G., Assmann, G., Rail, S. C., Jr., and Mahley, R. W., Tangier disease Defective recombination of a specific Tangier apolipoprotein A-I isoform (pro-apo A-I) with high density lipoproteins. Proc. Natl. Acad. Sci. U.S.A. 80, 6081-6085 (1983). 

Chylomicron remnants and very low density lipoprotein (VLDL) remnants are rapidly removed from the circulation by receptor-mediated endocytosis. ApoE, the major apolipoprotein of the chylomicron in the brain, binds to a specific receptor and is essential for the normal catabolism of triglyceride-rich lipoprotein constituents. Defects in apolipoprotein E result in familial dysbetalipoproteinemia, or type III hyperlipoproteinemia (HLP III), in which increased plasma cholesterol and triglycerides are the consequence of impaired clearance of chylomicron and VLDL remnants (Mahley et al., 1999). In the brain, lipidated apoE binds aggregated in a isoform-speciflc manner, apoE4 being much more effective than the other forms,... 

ADMA is degraded by the enzyme dimethylarginine dimethylaminohydrolase (DDAH), which hydrolyzes ADMA to L-citrulline and dimethylamine [70,83]. Two isoforms of this enzyme have been characterized and cloned to date. DDAH I predominates in tissues that express neuronal NOS and DDAH II predominates in tissues expressing endothelial NOS [70,84]. Activity of DDAH has been shown to be decreased by oxidized low density lipoprotein (LDL) or tumor necrosis factor-a (TNF-a) in vitro yielding increased levels of ADMA. Plasma levels of ADMA were found elevated in hyperhomo-cysteinemia, hypercholesterolemia and in hypertensive patients on a high salt diet [70,72,73]. 

Fig. 8. Ability of apolipoprotein E-dimyristoylphosphatidylcholine (DMPC) complexes of the three major isoforms to compete with human I-labeled LDL for binding to normal human fibroblasts. Cells incubated in medium containing 10% human lipoprotein-deficient serum received 1 ml of the same medium with 2 of 1-labeled...

With what is known regarding the properties of the three common isoforms, we can begin to understand the basis for the differences in cholesterol levels among the three alleles. In the case of apoE2, remnant lipoproteins are cleared from circulation at a slower rate than normal and the conversion of VLDLs to LDLs appears to be retarded. This defect in remnant clearance leads to the up-regulation of hepatic LDL receptors, which contribute to a further lowering of plasma LDL concentrations (Davignon et al., 1988). It is known that E2/2 subjects with... 

The function of apoE in lipoprotein metabolism is reviewed in the sixth chapter by Karl Weisgraber. The three-dimensional structure of a 22-kDa fragment of human apoE (34.2 kDa) has been solved by X-ray crystallography the relation of this structure to the role of apoE in lipoprotein metabolism is discussed in detail, together with a critical and extensive examination of the chemistry and biology of this apolipoprotein which plays such a central role in lipoprotein metabolism. Apolipoprotein E has three major isoforms in the human population which affect lipoprotein metabolism differently, resulting in different levels of the plasma lipoproteins. The impact of structure on function and how plasma lipid concentrations are affected by the different apoE isoforms are the themes of this important chapter. 

Apolipoprotein E, which has a direct influence on the development of cardiovascular and neurodegenerative diseases, exhibits no change in immunoreactivity upon freezedrying provided a bicarbonate buffer is used as the matrix [23]. As can be seen from Fig. 2.8, the immunological behaviour of various dilutions of reconstituted freeze-dried recombinant apo E, (one of the isoforms of apolipoprotein E) in relation to the frozen recombinant apo E the human VLDL (very low density lipoprotein) purified apo E, and fresh human serum samples, as determined using turbidimetric and ELISA methods, is similar. The stability of freeze-dried recombinant apo E, is acceptable, without any major alterations of its immunological reactivity. 

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