A cDNA-dependent scintillation proximity assay for quantifying apolipoprotein A-I.

We have developed a cDNA-dependent scintillation proximity assay (SPA) for rabbit apolipoprotein A-I that follows a classic radioimmunoassay scheme, in that antiserum and radiolabeled ligand are used in a process to quantify a source containing unlabeled ligand. To synthesize radiolabeled ligand we isolated a full-length rabbit apolipoprotein A-I (apoA-I) cDNA, transcribed the corresponding RNA in vitro, and synthesized radiolabeled apoA-I by including tritiated leucine in an in vitro translation reaction. Assay conditions were established which allowed quantification of unlabeled apoA-I over a range of 0.2 to 4 nanograms with intra- and interassay coefficients of variation of 5% and 10%, respectively. Purified rabbit apoA-I, apoA-I in rabbit liver parenchymal cell conditioned media, and apoA-I contained in rabbit plasma all generated parallel titration curves. Quantification of rabbit plasma apoA-I was not affected when sheep anti-rabbit apoA-I serum was mixed with sheep anti-rabbit apoB or apoE serum; thus, the antibody need not be specific to quantify the ligand of interest. To show utility of the assay, apoA-I mass was quantified in in vitro and in vivo models displaying altered apoA-I levels. In each model apoA-I values from the cDNA-dependent SPA and the established methodologies of Western blotting and electroimmunodiffusion were highly correlated. The approach outlined in this report should permit rapid development of scintillation proximity assays for other proteins given the widespread availability of full-length cDNAs.