Incorporation in vivo of [32P]orthophosphate and [Me-3H]choline into rough microsomal, Golgi, and plasma membranes of rat liver

Compartmentalization and differential labeling of phospholipids of rat liver subcellular membranes

The in vivo, interorganelle movement of phospholipids synthesized by different biosynthesis routes has been investigated in rat liver. Rats were injected with [methyl-3H]choline, [1-3H]ethanolamine or [3-3H]serine into the portal vein. Subcellular membranes (endoplasmic reticulum, Golgi apparatus, plasma membrane and mitochondria) were isolated, and the specific radioactivites of the phospholipids in each membrane were determined. There was a very rapid distribution of phospholipids from their sites of synthesis to the other organelles. In the plasma membrane, for example, the specific radioactivity of phosphatidylcholine derived from choline, ethanolamine or serine was as high as, or higher than, in the endoplasmic reticulum at all times examined. In addition, the specific radioactivity of phosphatidylserine (derived from serine) in the plasma membrane was approximately double that in the endoplasmic reticulum, even though the latter is the major site of phosphatidylserine synthesis. There was no evidence for the sequential flow of phospholipid from the endoplasmic reticulum, via the Golgi apparatus, to the plasma membrane. The experiments also demonstrated that the various subcellular membranes were labeled to different extents with phospholipids synthesized from different biosynthetic routes. It is unlikely that there is sufficient phospholipid biosynthetic enzyme activity in subcellular organelles other than the endoplasmic reticulum (Vance, J.E. and Vance, D.E. (1988) J. Biol. Chem. 263, 5898-5909) and the mitochondria for phosphatidylserine decarboxylase, to account for the efficient labeling of phospholipids of the plasma membrane, mitochondria and Golgi apparatus. The data suggest that although phospholipids can move very rapidly from one organelle to another, and within the plane of the lipid bilayer, there is neither a rapid mixing of newly synthesized phospholipids with the endogenous phospholipid pool, nor a rapid mixing of phospholipids derived from different biosynthetic origins.

Heterogeneity of lipoprotein particles in hepatic Golgi fractions

Newly synthesized phospholipids, labeled with either [14C]choline, [3H]myo-inositol, or [33P]phosphate, partioned preferentially (greater than 80% of total incorporated radioactivity) in a Golgi membrane subfraction, although the cognate content subfraction contained a relatively large amount of secretory lipoproteins. The labeling pattern was the same for all phospholipids tested in the two subfractions. An active exchange process of polar lipids between Golgi membranes and Golgi secretory lipoproteins is postulated as a plausible explanation for these findings. Less than half of all Golgi lipoprotein particles have the density of serum VLDLs and a similar, but not identical, biochemical composition. The remaining lipoprotein particles are characterized by a continuous spectrum of sizes, and (to the extent tested) by a lipid and protein composition different from that of serum VLDLs and HDLs. Results obtained in control experiments rule out the possibility that the heterogeneous population of Golgi lipoprotein particles is an artefact caused by our preparation procedures. It is assumed that these heterogeneous particles are immature precursors of both VLDLs and HDLs.