Investigation of the association of intracellular apolipoprotein (a) with apolipoprotein B in human liver.

Plasma lipoprotein-a(Lp-a) consist of LDL-like particles in which apolipoprotein-a (apo-a) is linked by a disulphide bond to apolipoprotein B (apo-B). There is strong evidence that apo-a is synthesized in the liver. However, little is known of the intracellular transit of apo-a and the site and mechanism of its linkage to apo-B. In this investigation we have addressed the primary question--does apo-a become linked covalently to apo-B intracellularly in human liver? For this study, we have developed competition and capture ELISA to measure apo-a either free or in complex with apo-B and applied these to human liver samples. The levels of free apo-a ranged from 25 to 440 micrograms/g liver in nine individual liver samples; those of apo B ranged from 90-700 micrograms/g liver. However, it was not possible to detect an apo-a/apo-B complex suggesting that apo-a is secreted in the free form and binds with apo-B/LDL in the extracellular fluid or plasma.