APOLIPOPROTEINS

General Information for Lipoproteins and Apolipoproteins

Plasma lipoproteins classes can be defined according to the densities at which they are isolated, as high (HDL), low (LDL), intermediate (IDL), very low-density lipoproteins (VLDL), and the chylomicrons. In general, lipoprotein particles range in size from 10 to 1000 nm. They are composed of a hydrophobic core containing cholesteryl esters, triglycerides, fatty acids and fat-soluble vitamins. The surrounding hydrophilic layer is composed of various apolipoproteins, phospholipids and cholesterol.

Apolipoproteins can be isolated by delipidation from the lipoprotein, and a number of preparative methods, such as gel filtration or DEAE chromatography have been established.

1. Apolipoprotein AI (ApoAI)

ApoAI comprises approximately 70% of the protein moiety in HDL. It is a single polypeptide chain consisting of 243 amino acid residues without disulfide bound and with glutamic acid as the C-terminal residue and aspartic acid as the N-terminal residue. The molecular weight is reported to be 28 kDa (Brewer et al., 1978). ApoAI activates lecithin-cholesterol (LCAT) acyltransferase, which is responsible for cholesterol esterification in plasma. ApoAI levels may be inversely related to the risk of coronary disease.

2. Apolipoprotein AII (ApoAII)

ApoAII comprises 25% of ApoAI in HDL. It exists in human plasma as a dimer of 2 identical chains of 77 amino acid residues, joined by disulfide. The molecular weight is reported to be 8.7 kDa for a single chain (Brewer et al., 1972). Studies on mouse reported that apoAII may be proatherogenic (Warden et al., 1993); however, case-control Study in the large European Prospective Investigation demonstrated that plasma apoAII concentrations were strongly inversely correlated with CHD events (Birjmohun et al., 2007).

3. Apolipoprotein B (ApoB)

ApoB exists in human plasma in two isoforms, ApoB-48 (Chen et al., 1987) and ApoB-100 (Wei et al., 1985, Yang et al., 1986a; 1989a,b; 1990; Chen et al., 1986; Yang et al., 1990, Yang and Pownall 1992). ApoB-100 is the major physiological ligand for the LDL receptor. ApoB100 is a large monomeric protein, containing 4536 amino acids (m.w. 515 kDa, Yang et al., 1986b). ApoB-100 is synthesized in the liver and is required for the assembly of VLDL. It is found in LDL and VLDL after the removal of the apoA, E and C. ApoB-48 is present in chylomicrons and their remnants. It is essential for the intestinal absorption of dietary lipids. ApoB levels correlate with the risk of coronary disease. ApoB-48 is synthesized in the small intestine. It comprises approximately half of the N-terminal region of ApoB-100 and is the result of posttranscriptional mRNA editing by a stop codon in the intestine not found in the liver.

4. Apolipoprotein CI (ApoCI)

ApoCI contains 57 amino acid residues and the m.w. is 6.6 kDa (Jackson et al., 1974). ApoCI has been found to activate LCAT (Liu and Subbaiah 1993) and to inhibit cholesterol ester transferase that can potentially regulate several lipase enzymes (Poensgen, 1990, Conde-Knape et al., 2002; Berbee et al., 2005.)

5. Apolipoprotein CII (ApoCII)

ApoCII contains 78 amino acid residues. The m.w. is 8.5 kDa (Jackson et al., 1977). ApoCII activates lipoprotein lipase that hydrolyzes fatty acids from triacylglycerols in chylomicrons.

6. Apolipoprotein CIII (ApoCIII)

ApoCIII contains 79 amino acid residues. The m.w. is 8.7 kDa (Brewer et al., 1974). It may inhibit the activation of lipoprotein lipase by apoCIII. ApoCIII is a constituent of both apoB- and apoAI-containing lipoproteins in the circulation. ApoCIII plays a pivotal role in regulating the plasma metabolism of VLDL, IDL, and LDL, primarily by inhibiting receptor-mediated uptake of these lipoproteins by the liver (Sehayek and Eisenberg 1991, Aalto-Setala et al., 1992, Zheng et al., 2007)

7. Apolipoprotein E (ApoE)

ApoE contains 299 amino acid residues. It is a 34-37 kDa glycosylated protein (Rall et al., 1983). ApoE is involved with triglyceride, phospholipid, cholesteryl ester, and cholesterol transport in and out of cells and is a ligand for LDL receptors. ApoE has also been implicated in immune and nerve degeneration. It has been found to suppress lymphocyte proliferation. Late-onset familial and sporadic Alzheimer disease patients have been found to have a higher occurrence of one of the three common ApoE isoforms, ApoE4. The ApoE4 isoform has been detected in senile plaques and neurofibrillary tangles of Alzheimer disease patients. ApoE4 is associated with rapid chylomicron-remnant clearance and increased total cholesterol levels.

8. Apolipoprotein (a) [apo (a)]

The plasma concentration of human lipoprotein[a], Lp[a], is highly correlated with coronary artery disease. The protein moiety of Lp[a], apoLp[a], consists of two apoproteins, apo[a] and apoB-100, linked by one or more disulfide bonds(s). Apo[a], the protein unique to Lp[a], exists in polymorphic forms that exhibit different apparent molecular weights ranging from 419 kDa to 838 kDa (Gaubatz et al., 1983; 1990, 1993).

9. Plasminogen

Plasminogen contains 810 amino acid residues. It is a single chain glycoprotein with m.w. 90 kDa (Robbins et al., 1967), soluble in water; prepared from plasma that has been shown by certified test to be negative for HBsAg and for antibodies to HIV and HCV. Plasminogen is the inactive precursor of the protease plasmin. Plasminogen is activated by the action of either tissue plasminogen activator (tPA), which primarily activates the fibrinolytic (thrombolytic) activity of plasmin, or urokinase plasminogen activator (uPA), which is associated with extracellular matrix remodeling and cell migration.

10. C-Reactive Protein (CRP)

Human C-Reactive Protein (CRP) is an important biomarker for predicting of future cardiovascular events, such as heart attack and stroke (Koenig et al., 1999, Jenny et al., 2007, Kabagambe et al. 2011). CRP is an acute phase protein produced by the liver. It is a member of the pentraxin family of proteins with five identical nonglycosylated subunits of 206 amino acids each (m.w. 23 kDa) (Agrawal et al., 2009). Among other markers of inflammation, CRP has shown the strongest association with cardiovascular events (Marsik et al., 2008, Kones et al., 2010). Clinical studies demonstrated that coronary mortality among patients with unstable angina and elevated CRP is significantly higher comparing with the patients without elevated CRP. It is an important biomarker for detecting individuals at high risk of plaque rapture. Our series of CRP products including Human C-Reactive Protein (CRP), Rabbit Anti-Human-CRP, Goat ant-Human-CRP, and its modified conjugated antibodies, such as HRP, Biotin, and FITC products can be used for detecting CRP levels to evaluate the risk of heart attack.

11. Irisin

The exercise-inducible myokine Irisin (Boström, et al. 2012) is secreted by cleavage from the transmembrane precursor, FNDC5. Circulating Irisin govern a wide variety of cellular processes relevant to browning, angiogenesis, wound healing, bone mass, and metabolism.

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