Apolipoprotein[a] is not associated with apolipoprotein B in human liver

The aim of this research was to determine whether apolipoprotein[a] (apo[a]) is linked to apolipoprotein B (apoB) in human liver. Four ELISAs were developed: 1) a competition assay that measures apoB; 2) a competition assay that measures apo[a]; 3) a capture assay based on capture of apo[a] by a polyclonal antibody and detection of co-immobilized apoB using a monoclonal antibody; and 4) a capture assay based on capture of apo[a] using a polyclonal antibody and detection of immobilized apo[a] using a monoclonal antibody. Assays 2 and 4, therefore, measure apo[a] either free or in complex with other proteins, while assay 3 measures apo[a] associated with apoB. The levels of apo[a] ranged from 25 to 440 micrograms/g liver in nine individual liver samples. There was no significant difference between apo[a] levels in individual human liver samples measured using ELISA 1 or 3; however, it was not possible to detect apo[a]/apoB using assay 3. ApoB was present in human liver homogenates at levels ranging from 90 to 700 micrograms/g measured using assay 1. These results suggest, therefore, that apo[a] is not coupled to apoB in the liver and may be secreted in the free form to bind with low density lipoprotein (LDL) in the extracellular fluid or plasma.