Distribution of apolipoproteins A-I and A-II in serum, secreted proteins by cultured hepatocytes, and the plasma membrane of hepatocytes in eel

Apolipoprotein A-I (apoA-I) of high-density lipoprotein (HDL) plays a key r ole in cholesterol transport as a ligand of HDL binding proteins or HDL rec eptors and an acceptor of intracellular cholesterol. It has been known that the latter role of apoA-I is carried out by free apoA-I dissociated from H DL. We investigated the distribution of free apoA-I and apoA-II in eel seru m and secreted proteins from cultured eel hepatocytes using monoclonal anti bodies specific to apoA-I or apoA-II of eel serum HDL. Free apoA-I was foun d in eel serum and the secreted proteins, but free apoA-II was not. ApoA-I and apoA-II were also found in serum very low-density lipoprotein and chylo micron-like lipoprotein secreted by cultured eel hepatocytes. We observed t he existence of apoA-I and apoA-II in the plasma membrane of the hepatocyte s by immunological method after fixation of the cells. Furthermore, when th e isolated hepatocytes treated with the antibody to apoA-II were incubated with HDL stained by a fluorescent lipophilic dye, the binding of the HDL to the hepatocytes was inhibited by 30%. However, we did not observe the inhi bition by the hepatocytes treated with the antibody to apoA-I.