The use of phenobarbital and carbon tetrachloride to examine liver phospholipid exchange in intact rats

Evidence for the activity of a phospholipid exchange protein in vivo

Liposomes containing a self quenching concentration of a fluorescent phosphatidylethanolamine analog, were microinjected into Chinese hamster ovary cells. Immediately after microinjection, little intracellular fluorescence was observed. 10 min post-injection, labeling of the nuclear envelope and mitochondria became evident. Combined with control studies, our results suggest that phospholipid exchange protein(s) facilitates phosphatidylethanolamine movement in vivo.

Intracellular transport of phosphatidylcholine to the plasma membrane

We have used pulse-chase labeling of Chinese hamster ovary cells with choline followed by plasma membrane isolation on cationic beads to study the transport of phosphatidylcholine from the endoplasmic reticulum to the plasma membrane. We have found that the process is rapid (t1/2 [25 degrees C] = 2 min) and not affected by energy poisons or by cytochalasin B, colchicine, monensin, or carbonyl cyanide p-chlorophenylhydrazone. Cooling cells to 0 degree C effectively stops the transport process. The intracellular transport of phosphatidylcholine is distinct in several ways from the intracellular transport of cholesterol (Kaplan, M. R., and R. D. Simoni, 1985, J. Cell. Biol., 101:446-453).

Extensive exchange of rat liver microsomal phospholipids

Liver microsomal fractions were prepared from rats injected with a single dose of choline [14C]methylchloride or with single or multiple doses of 32Pi. Exchangeability of microsomal phospholipids was determined by incubation with an excess of mitochondria and phospholipid exchange proteins derived from beef heart, beef liver or rat liver. Labeled phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol were found to act as a single pool and were 85--95% exchangeable in 1--2h. High latencies of mannose-6-phosphate phosphohydrolase activities and impermeability of microsomes to EDTA proved that phospholipid exchange proteins did not have access to the intracisternal space. If microsomal membranes are largely composed of phospholipid bilayers, the experiments suggest that one or more of the phospholipid classes in microsomal membranes undergo rapid translocation between the inner and outer portions of the bilayer.

Mechanism of the enrichment of phosphatidylcholine in liver accompanying enzyme induction by phenobarbital

The mechanism of the increase of phosphatidylcholine in liver, accompanying enzyme induction by phenobarbital, has been studied in rats. Using radioactively labeled precursors, the two main pathways of phosphatidylcholine biosynthesis--the CDP-choline pathway and the methylation of phosphatidylethanolamine--were analyzed after pretreatment with 4 doses of phenobarbital (80 mg/kg) on 3 consecutive days. After i.v. injection of choline [Me-3H], choline [Me-14C] or NaH2[32P]O4 the specific radioactivity (sp. act.) of phosphatidylcholine (dpm/nmol) was decreased by 60%, and after methionine [Me-3H] or ethanolamine [1.2-14C] by 40% compared to control rats. These changes are partly due to the increased concentration of phosphatidylcholine and phosphatidylethanolamine, causing the incorporated precursors to dilute, and partly to a secondary effect which leads to a reduction of the sp. act. of free choline in pretreated animals. The concentration of glycerylphosphorylcholine, one of the metabolites of phosphatidylcholine catabolism, was also diminished by almost 50%. From these results it may be concluded that the increase of phosphatidylcholine is due to a retardation of its breakdown rather than to an increase of its synthesis.

Abnormal membrane phospholipid content in subcellular fractions from the Morris 7777 hepatoma

1. Mitochondrial and microsomal fractions were prepared from normal rat liver and the Morris 7777 hepatoma and characterized by the use of the marker enzymes, succinate dehydrogenase and rotenone-insensitive NADPH-cytochrome c reductase. 2. The phospholipid content per mg membrane protein of Morris 7777 hepatoma mitochondria was increased by 75% as compared with mitochondria from normal rat liver. Microsomes from this poorly-differentiated tumor were found to have a 45% decrease in the content of phospholipid. These abnormalities were independent of tumor size or age. 3. The percent phospholipid content of the subcellular fractions was determined, and revealed an increase in the percent sphingomyelin in both the microsomal and mitochondrial fractions of the tumor. Decreases in the percent phosphatidylcholine and phosphatidylethanolamine were noted in tumor microsomes as compared with normal liver. Diphosphatidylglycerol was not found in significant quantities in the microsomal fraction of this hepatoma line. 4. The content of the various phospholipid classes per mg protein in the respective mitochondrial and microsomal fractions was determined. Large increases in nearly all the major phospholipid classes were found in tumor mitochondria; tumor microsomes were characterized by an increased content of sphingomyelin but the content of nearly all other phospholipids was significantly decreased. These findings suggest the presence of disturbances in the regulation of phospholipid metabolism in subcellular organelle membranes of the Morris 7777 hepatoma.

Renewal of fatty acids in the membranes of visual cell outer segments

The renewal of fatty acids in the visual cells and pigment epithelium of the frog retina was studied by autoradiographic analysis of animals injected with tritiated palmitic, stearic, or arachidonic acids. Most of the radioactive material could be extracted from the retina with chloroform-methanol, indicating that the fatty acids had been esterified in lipids. Analysis of the extracts, after injection of [(3)H]palmitic acid, revealed that the radioactivity was predominantly in phospholipid. Palmitic acid was initially concentrated in the pigment epithelium, particularly in oil droplets which are storage sites for vitamin A esterified with fatty acid. The cytoplasm, but not the nucleus of these cells, was also heavily labeled. Radioactive fatty acid was bound immediately to the visual cell outer segment membranes, including detached rod membranes which had been phagocytized by the pigment epithelium. This is believed to be due to fatty acid exchange in phospholipid molecules already situated in the membranes. Gradually, the concentration of radioactive material in the visual cell outer segment membranes increased, apparently as a result of the addition of new phospholipid molecules, possibly augmented by the transfer from the pigment epithelium of esterified vitamin A. Injected fatty acid became particularly concentrated in new membranes which are continually assembled at the base of rod outer segments. This localized concentration was short-lived, apparently due to the rapid renewal of fatty acid. The results support the conclusion that rods renew the lipids of their outer segments by membrane replacement, whereas both rods and cones renew the membrane lipids by molecular replacement, including fatty acid exchange and replacement of phospholipid molecules in existing membranes.

Exchange of phospholipids between brain membranes in vitro

1. When unlabelled mitochondria from guinea-pig brain were incubated with a (32)P-labelled microsomal fraction from brain there was a transfer of phospholipid to the mitochondria, which could not be accounted for by an aggregation of microsomes and mitochondria or an exchange with microsomes contaminating the mitochondria. Under similar circumstances there was a transfer of phospholipid from (32)P-labelled mitochondria to microsomes, indicating that the process was one of exchange. 2. The transfer from microsomes was greatly stimulated by a non-dialysable heat-labile macromolecular component in the brain supernatant fraction but not by the concentration of the particulate fractions. 3. Phospholipid-exchange processes occurred most readily between pH7 and 7.5 and were inhibited by the presence of myelin and on the addition of lysophosphatidylcholine. 4. The rates of transfer of individual phospholipids from brain microsomes to mitochondria were similar. 5. (32)P-labelled microsomes could slowly donate phospholipid to the isolated synaptosomal (nerve-ending) fraction but the phospholipids of the myelin fraction did not exchange. 6. Subfractionation of the synaptosomal fraction after [(32)P]phospholipid transfer showed that the mitochondria were most actively labelled during the incubation. All of the isolated individual synaptosomal membranes were capable of acquiring phospholipid on incubation with a (32)P-labelled brain supernatant fraction although a greater percentage was again exchanged by the mitochondrial fraction.