We have investigated the mechanism by which inhibition of phosphatidylcholine biosynthesis in rat hepatocytes by choline deprivation causes a reduction in the secretion of very low density lipoprotein (VLDL) (Yao, Z., and Vance, D. E. (1988) J. Biol. Chem. 263, 2998-3004). Rats ingested a choline-deficient or control diet for 3 days, and subcellular fractions of liver were prepared. No change in the amount of apolipoprotein B in the lumina of the endoplasmic reticulum was observed, but there was a 40-50% decrease of apolipoprotein B in the lumina of the Golgi from choline-deficient compared with control rats. Incubation of microsomes, derived from choline-deficient and -supplemented hepatocytes, with trypsin showed similar degradation of apolipoprotein B, indicating similar quantities of this protein are present on the surface and within the lumina. The VLDL particles in the Golgi of liver cells and in plasma, on average, were larger in samples derived from choline-deficient compared with choline-supplemented animals. Incubation of plasma VLDL with proteases demonstrated that the apolipoprotein B of plasma VLDL particles from choline-deficient animals had a different susceptibility to digestion than did VLDL from choline-supplemented animals. These data indicate that the number of VLDL particles assembled in the endoplasmic reticulum of liver is similar in choline-deficient and -supplemented rats, but the number of particles is decreased in the Golgi from choline-deficient animals.
Animals Apolipoproteins B