Quantification of apolipoprotein C-III in serum by a noncompetitive immunoenzymometric assay.

We used a noncompetitive immunoenzymometric assay to measure the concentration of total apolipoprotein C-III in human sera. Affinity-purified antibodies to apolipoprotein C-III were adsorbed to the surface of microtiter plates. After washing, this solid-phase antibody was incubated with antigen (serum from fasting subjects), washed, and then incubated with peroxidase-labeled purified antibodies to apolipoprotein C-III. After a last washing, the bound label was assayed, providing a direct measurement of the antigen. Optimized technical conditions for the assay yielded assay CVs of 3.5 and 5.6% for within- and between-run precision, respectively. Analytical recovery of apolipoprotein C-III added to a serum was quantitative (97%). This noncompetitive assay can be used to measure apolipoprotein C-III in different lipoprotein fractions (very-low or high-density fractions) and yields values that compare favorably with those obtained by electroimmunoassay (r = 0.94). The assay offers several advantages over existing techniques--sensitivity, specificity, simplicity, and no use of radioisotopes--and hyperlipemic samples can be used.