Determination of free apolipoprotein(a) in serum by immunoassay and its significance for risk assessment in patients with coronary artery disease

This paper describes a new enzyme-linked ligand sorbent assay (ELLSA) to qu antify free apolipoprotein(a) (apo(a)). The new test immobilizes free apo(a ) utilizing a specific peptide that carries the amino acid sequence of a no n-covalent apo(a) binding site on apoB(3375-3405) (ligand-peptide). The lig and-peptide coupled to Sepharose was used in affinity chromatography to sep arate free apo(a) from whole serum. Isolated free apo(a) consisted of full length apo(a) and smaller apo(a). Additionally, free apo(a) levels determin ed by ELLSA as well as by electroimmunodiffusion correlated moderately well . Significantly increased serum concentrations of free apo(a) were found in coronary artery disease. The mean value of free apo(a) was three times hig her in patients than in controls while the lipoprotein(a) (Lpla) concentrat ion was doubled. Utilizing receiver operating characteristic diagrams, it w as shown that the free apo(a)-ELLSA had a better diagnostic test performanc e in atherosclerotic risk assessment than the Lp(a)-test: specificity free apo(a)ELLSA 0.77, Lp(a)-test 0.81 [with (a:a)-enzyme immunoassay (EIA)] to 0.83 [with (a:B)-EIA]; sensitivity free apo(a)-ELLSA 0.57, Lp(a)-test 0.36 to 0.40. In conclusion, the new free apo(a)-ELLSA allows for the specific q uantification of free apo(a). This provides an interesting indicator for at herosclerotic risk assessment.