Standardization of Apolipoprotein Immunoassays by an Isotope Dilution Method
Plasma lipoproteins consist of a mixture of lipid-protein particles differing in hydrated density, size and chemical composition (Nichols, 1967). The recognized existence of at least nine well-characterized apolipoproteins and their distribution throughout the entire density spectrum are further indications of the complex nature of this macromolecular system (Osborne and Brewer, 1977; Olofsson et al. 1978). As distinct constituents, the apolipoproteins represent suitable markers for the identification and differentiation of lipoprotein particles. To assess the physiological role of apolipoproteins in lipid transport processes and to evaluate the potential use of apolipoprotein profiles in clinical practice, various immunochemical procedures have been developed for the quantitative determination of almost all known plasma apolipoproteins (Alaupovic et al. 1978; Albers 1978). Although all three immunoassays, i.e., radioimmunoassay, radial immunodiffusion and electroimmunoassay, display a high degree of specificity, sensitivity and precision, the standardization of individual assays and the assessment of accuracy remain unsolved problems. To establish a standard for accuracy, we have applied a simple isotope dilution method to determine the A-II content in HDL3. This method also furnishes a standard for electroimmunoassay, not as a delipidized polypeptide but as a lipoprotein.
KeywordsAmino Acid Analysis Plasma Lipoprotein Radial Immunodiffusion Macromolecular System Plasma Apolipoprotein
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