Proapolipoprotein-converting enzymes and high-density lipoprotein early events in biogenesis.

The early events in high-density lipoprotein biogenesis involve the extracellular action of two converting enzymes affecting the cleavage of the prosegment of either proapolipoprotein A-I or proapolipoprotein A-II and the generation of mature apolipoprotein (apo) A-I and apo A-II, the main apolipoprotein of high-density lipoproteins. These two converting enzymes differ from each other in mechanism of action and specificity. The observation that they can be secreted by human hepatocarcinoma G2 cells in culture provides an experimental basis for examining the possible coordination between the synthesis and secretion of these two converting enzymes and the events attending the production and cellular export of apo A-I and apo A-II.