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Analyte: Apolipoprotein AI
Specimen Type: Urine
Optimum Volume: 0.5 mL
|6 days||1 month||2.8 years|
Reporting units: ng/mL or ng/mg Creatinine
Biological or Clinical Significance:
Human Apolipoprotein A-I (apo A-I) comprises about 70% of the high-density lipoproteins (HDL) protein mass, while apo A-II makes up another 15 – 20%. Apo A-I, a 243-amino acid molecule that contains a series of highly homologous amphipathic (polar and non-polar properties) alpha-helices, is a 28-kDa single polypeptide that lacks glycosylation or disulfide linkages. About 5 – 10% of human plasma apo A-I exists in a lipoprotein-unassociated state. Apo A-I appears to play a major role in the inhibition of atherosclerosis via its role in reverse cholesterol transport, and perhaps, because of its inherent antioxidant and anti-inflammatory properties or its central role in organizing HDL, which may possess additional antioxidant and anti-inflammatory properties. Oxidation of specific amino acid residues in apo A-I may contribute to atherogenesis by impairing cholesterol efflux from macrophages or by allowing the prolongation of free-radical oxidation, which leads to the spread of low-density lipoprotein (LDL) oxidation.
A majority of HDL functionality is derived from the capacity of apo A-I to sequester phospholipid and cholesterol and interact with plasma enzymes and cellular receptors. During reverse cholesterol transport, HDL interacts with lecithin:cholesteryl acyltransferase (LCAT) and cellular receptor class B type I in an ordered fashion that is reflected by HDL particle lipid composition. A high-affinity HDL receptor for apo A-I is in the beta-chain of ATP synthase on the surface of hepatocytes.
The plasma concentration of apo A-I is an excellent biomarkers of susceptibility to atherosclerosis and risk of cardiovascular disease.
In the kidney, apo A-I and some HDL particles may be filtered through the glomerulus and scavenged in the proximal tubule by binding to a receptor called cubulin. Once bound, the particles are internalized and catabolized. The HDL particle itself seems to have a higher affinity for cubulin than pure apo A-I, and HDL lipid composition influences HDL affinity. Some apo A-I and HDL must escape uptake in the tubule and can be measured in the urine (see reference 1).
Principle of Test Method:
The A-I assay is an Enzyme-Linked Immunosorbent Assay (ELISA) designed for detection of human apo A-I in urine and cell culture supernatant. This assay employs a quantitative sandwich enzyme immunoassay technique.
1. A. P. Simonelli and D. S. Dresback. Principles of Formulation of Parenteral Dosage Forms (Stability Considerations), Kenneth A. Connors et al. Chemical stability of pharmaceuticals: a handbook for pharmacists, Canada. Wiley Interscience Publication, 1979, pg. 9-19.