Journal of Lipid Research, Vol 28, 207-215, Copyright © 1987 by Lipid Research, Inc.
Differential labeling of rat hepatic Golgi and serum very low density lipoprotein apoprotein B variants
LL Swift, RJ Padley and GS Getz
The synthesis of apoB-100 and apoB-48 by rat liver was investigated by
studying the apoB complement of very low density lipoproteins (VLDL) from
hepatic perfusates and Golgi fractions. The relative amounts of apoB-100
and apoB-48 in perfusate and Golgi VLDL as determined by sodium dodecyl
sulfate (SDS) polyacrylamide gel electrophoresis were similar to those in
serum VLDL. To investigate the relative rates of synthesis of the VLDL B
proteins, rats were injected intraportally with tritiated amino acid, and
hepatic Golgi and serum VLDL were isolated from 7.5 to 120 min later. In
hepatic Golgi VLDL, apoB-100 and apoE were maximally labeled at 15 min
after the tritiated amino acid pulse. In contrast, VLDL apoB-48 attained
maximum radioactivity at 30 min after isotope injection. In serum VLDL,
apoB-100 and apoE were maximally labeled at 30 min post-isotope injection,
while activity in apoB-48 peaked at 60 min. The data suggest that the
synthesis of the B proteins and incorporation into rat liver nascent VLDL
are independently regulated. The differential labeling patterns of the VLDL
B proteins may be explained by an intracellular pool of apoB-48 that is
larger than that of apoB-100. An alternative explanation of the results is
that apoB-100 is a precursor to apoB-48.