Synthesis of HDL apolipoproteins by rat hepatocytes

The authors have used 2D-PAGE to study the synthesis, intracellular modification, and secretion of rat HDL apolipoproteins by primary cultures of rat hepatocytes. ApoA-IV, apoA-II and apoE synthesized after a 10 min pulse with 35S-methionine coincided on 2D-gels with their corresponding plasma forms and they were not modified further intracellularly or following secretion. A fraction (< 10%) of apoE was modified intracellularly to minor isoprotein forms that were insensitive to neuraminidase treatment. These later forms also constituted a minor component of the secreted and plasma rat apoE. The intracellular and newly secreted apoA-I differed from its plasma counterpart by -1 charge as described previously. The intracellular forms of rat apoA-I, apoA-IV and unmodified apoE differed from the products of cell free translation of rat liver mRNA by +1 charge. Their findings (a) establish the charge relationship between nascent and plasma rat apolipoproteins, (b) indicate that rat apoA-I, apoA-II and apoA-IV are not modified intracellularly, (c) suggest that there is a difference in the post-translational modification patterns between the rat and human hepatic apoE