IMPAIRED BIOSYNTHESIS OF PHOSPHATIDYLCHOLINE CAUSES A DECREASE IN THENUMBER OF VERY-LOW-DENSITY LIPOPROTEIN PARTICLES IN THE GOLGI BUT NOTIN THE ENDOPLASMIC-RETICULUM OF RAT-LIVER

We have investigated the mechanism by which inhibition of phosphatidyl choline biosynthesis in rat hepatocytes by choline deprivation causes a reduction in the secretion of very low density lipoprotein (VLDL) (Y ao, Z., and Vance, D. E. (1988) J. Biol. Chem. 263, 2998-3004). Rats i ngested a choline-deficient or control diet for 3 days, and subcellula r fractions of liver were prepared. No change in the amount of apolipo protein 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 hepatocy tes, with trypsin showed similar degradation of apolipoprotein B, indi cating similar quantities of this protein are present on the surface a nd within the lumina. The VLDL particles in the Golgi of liver cells a nd in plasma, on average, were larger in samples derived from choline- deficient compared with choline-supplemented animals. Incubation of pl asma VLDL with proteases demonstrated that the apolipoprotein B of pla sma VLDL particles from choline-deficient animals had a different susc eptibility to digestion than did VLDL from choline-supplemented animal s. 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 G olgi from choline-deficient animals.