Factors controlling the activities of phosphatidate phosphohydrolase and phosphatidate cytidylyltransferase. The effects of chlorpromazine, demethylimipramine, cinchocaine, norfenfluramine, mepyramine and magnesium ions.

1. Microsomal membranes from rat liver were incubated with ATP, CoA, Mg2+, [14C]palmitate, F- and sn-glycerol 3-phosphate in order to label them with [14C]phosphatidate. These membranes were isolated and used in a second incubation in which [3H]CTP was present, and the simultaneous synthesis of [14C]diacylglycerol and [3H]CDP-diacylglycerol was measured. 2. The addition of phosphatidate phosphohydrolase, which had been partially purified from the particle-free supernatant, supplemented the activity of the endogenous phosphohydrolase, but it did not alter the rate of CDP-diacylglycerol formation. 3. Adding EDTA inhibited phosphatidate cytidylyl-transferase activity and stimulated the activity of the phosphohydrolases by removing excess of Mg2+. 4. Increasing the concentration of Mg2+, norfenfluramine or chlorpromazine in the assay system stimulated cytidylyltransferase activity, but decreased the activities of both phosphohydrolases. 5. The mechanism for the stimulation of cytidylyl=transferase activity by the cationic drugs and Mg2+ was investigated with emulsions of phosphatidate and the microsomal fraction of rat liver. 6. There was a threshold concentration of about 5mM-MgCl2 below which no cytidylyltransferase activity was detected in the presence or absence of norfenfluramine. Just above this threshold concentration norfenfluramine stimulated cytidylyltransferase activity, but this stimulation disappeared as the Mg2+ concentration was raised to its optimum of 20mM. Norfenfluramine therefore partially replaced the bivalent-cation requirement. 7. At 30 mM-MgCl2 amphiphilic cationic drugs inhibited cytidylyltransferase activity at relatively high concentrations in a non-competitive manner with respect to phosphatidate. 8. The implications of these results are discussed with respect to the regulation of the synthesis of the acidic phospholipids compared with the synthesis of phosphatidylcholine, phosphatidylethanolamine and triacylglycerol.     

The effect of starvation on the incorporation of palmitate into glycerides and phospholipids of rat liver homogenates

1. Glyceride biosynthesis from glycerol phosphate and [1-(14)C]palmitate was studied in liver homogenates of rats that were fed ad libitum or starved for 36-40hr. The changes in enzyme activity were related to total DNA content or total liver homogenate as these were found to be equivalent and to be the most meaningful parameters. 2. In liver homogenates from fed rats, labelled palmitate was incorporated mainly into phosphatidate (58% of the total incorporation into lipids), diglycerides (25%) and triglycerides (16%), whereas monoglycerides, cholesterol esters and phospholipids other than phosphatidate were labelled only to a small extent. Addition of particle-free supernatant to full homogenates increased the total incorporation of palmitate by 45% and the pattern of incorporation altered to 53% incorporated into triglycerides, 24% into diglycerides and 17% into phosphatidate. This result suggested that, in liver homogenates, phosphatidate phosphohydrolase (EC 3.1.3.4) may be rate-limiting in the biosynthesis of glycerides via the glycerol phosphate pathway. 3. Upon starvation, the amount of palmitate incorporated per liver into total phospholipids plus glycerides was decreased to between 68% and 75% of that observed with fed animals. In homogenates from fed animals 41-44% of the labelled phospholipids plus glycerides was in glycerides; this value increased to between 63% and 75% with starved rats. Of the palmitate incorporated into total phospholipids, between 85% and 86% was found in phosphatidate, independent of the nutritional state of the animal. The ratio of palmitate incorporated into triglycerides/diglycerides rose from 0.7, obtained with fed rats, to 1.0 with starved animals. 4. These results indicate that starvation caused a decrease in the activity (per total liver) of acyl-CoA-glycerol phosphate acyltransferase(s) (EC 2.3.1.15) and an increase in the activity of acyl-CoA-diglyceride acyltransferase (EC 2.3.1.20). The largest change, however, seemed to be related to the increased activity of the phosphatidate phosphohydrolase in the particle-free supernatant. 5. The latter enzyme was assayed in the particle-free supernatant with membrane-bound phosphatidate as substrate. In starvation, the activity per total liver was increased to between 130% and 190% and the specific activity to between 180% and 320% of the values for fed rats.     

Acetylcholine synthesis in rat brain: dissimilar effects of clozapine and chlorpromazine.

Experiments have been conducted in which glucagon-induced stimulation of α-aminoisobutyric acid (AIB) transport in rat liver, presumably mediated by cyclic AMP, was markedly inhibited by actinomycin D, cycloheximide or puromycin. Inhibition of transport occurred despite the presence of high concentrations of cyclic AMP, suggesting that the nucleotide may act prior to, or at the same point as, the antibiotics. Kinetic studies showed that 1) the half-life of the glucagon-stimulated system(s) was less than 1 hour, and 2) glucagon treatment doubled V_max without having any effect on the apparent K_m for hepatic AIB transport. The results suggest that glucagon stimulates hepatic AIB transport by increasing the synthesis of some critical protein having a rapid turnover rate; this effect may be due to a prior increase in RNA synthesis.     

Effect of clofibrate on the CoA thioester profile in rat liver.

Acute and chronic treatment with clofibrate increased the total CoA content of rat liver and altered the profile of the various CoA thioesters. There resulted a 2–3 fold increase in the contents of long chain acyl CoA, acetyl CoA and free CoA, contrasting with significant decreases found in succinyl CoA, malonyl CoA and acetoacetyl CoA contents. It is postulated that the known increase in fatty acid binding protein and/or the increased extramitochondrial β-oxidation of fat by an increased peroxisomal population may direct the compartmentation and metabolic fate of fatty acids and their CoA derivatives following clofibrate treatment.     

The rat retina is a useful in vivo model to study membrane lipid synthesis: Rates of biosynthesis of neutral glycerides and phospholipids

The phospholipid composition was studied in the whole rat retina, as well as in its subcellular fractions. A relative enrichment of phosphatidic acid, phosphatidylethanolamine, and phosphatidylserine was observed in rod outer segments (ROS) in comparison with entire retina: nuclear-photoreceptor inner segmentssynaptic bodies (P₁) and synaptosomal-mitochondrial (P₂) fractions. Phosphatidylcholine was the predominant phospholipid class found in all subcellular fractions analyzed. The microsomal fraction was relatively enriched in phosphatidic acid and in phosphatidylinositol. In addition, the rat eye has been used as an in vivo system to study membrane lipid synthesis. After intravitreal injections of [2-³H]glycerol a rapid labeling of retinal glycerolipids took place. Up to 120 min after injection only the glycerol backbone of lipids was labeled. Phosphatidic acid and diacylglycerol displayed rapid rates of synthesis and breakdown. Fastest rates of labeling were attained by phosphatidylcholine followed by phosphatidylinositol. Differences were found when in vitro labeling by [2-³H]glycerol was compared with intravitreal injections. Labeling of phospholipids of subcellular fractions by intravitreally injected [2-³H]glycerol showed that most of the label accumulated in microsomal phosphatidylcholine and phosphatidylinositol. Diacylglycerols and phosphatidylethanolamine also took up 10 and 20% respectively of the precursor. It is concluded that the rat eye is a useful experimental model to study synthesis and metabolism of membrane lipids in the retina.     

Effect of carbon tetrachloride administration on the synthesis of triglycerides and phospholipids in rat liver

In vivo incorporation of choline-methyl-(14)C into liver lecithin and its biosynthetic precursors was studied in CCl(4)-treated rats. Radioactivity in cytidine diphosphoryl (CDP-)choline and lecithin was reduced to one-third of control levels, whereas that of phosphorylcholine was increased to 4.7 times control levels. Incorporation of phosphorylcholine-(32)P into lecithin by homogenates prepared from livers of CCl(4)-treated animals was reduced, but conversion of CDP-choline-(32)P to lecithin by the isolated microsomal fraction did not show any significant depression. A block in the synthesis of CDP-choline is indicated. The in vivo utilization of methionine for lecithin synthesis was not affected. After intravenous injection of palmitic acid-1-(14)C, radioactivity of triglycerides from microsomal and mitochondrial fractions was markedly lower than the controls, whereas radioactivity of triglycerides in the soluble fraction was greatly increased. Radioactivity of diglycerides changed from 0.5% of total lipids in the control to 10% of total lipids in CCl(4)-treated animals. Incorporation of palmitic acid into phospholipids was also suppressed. The results demonstrate that synthesis of both phospholipids and triglycerides is inhibited in rats 4-5 hr after CCl(4) administration.     

The effects L-leucine on the synthesis of urea, glutamate and glutamine by isolated rat liver cells.

With either alanine or a mixture of 15 different amino acids as nitrogen source, the addition of L-leucine inhibited the synthesis of urea by isolated rat liver cells. With alanine present leucine promoted the production of glutamate and glutamine. Comparison of effects of leucine on soluble glutamate dehydrogenase, mitochondria and isolated cells supports the postulate that leucine exerts its effect through activation of glutamate dehydrogenase. It is suggested that this latter enzyme may not be as important for the production of NH3 for carbamoyl phosphate synthesis as has been considered hitherto.     

Changes of fatty acid composition of phospholipids and lipid structural order in rat liver mitochondrial membrane subsequent to galactosamine intoxication. Effect of clofibrate

Since it has been earlier reported that D-galactosamine induces an inhibition of palmitoylcarnitine transferase I and a depletion of mitochondrial phospholipids which were both prevented by clofibrate, an evaluation of the effects of these drugs on mitochondrial fatty acid composition was made. Galactosamine does not alter the fatty acid pattern of these fatty acids whereas clofibrate induces a 2-fold increase in monounsaturated/saturated fatty acids ratio and a 10-fold decrease of the 20:4 (n - 6)/20:3 (n - 6) ratio in phosphatidylcholine. These alterations suggest an increase of delta 9-desaturation and a decrease of delta 5-desaturation. To determine whether the drug-induced changes in mitochondrial phospholipids has an effect on the physical properties of the membrane, the lipid structural order of mitochondrial preparations was studied using the lipophilic probes DPH and TMA-DPH. Mitochondrial isolated either from galactosamine- or clofibrate-treated rats showed a decrease in fluorescence polarization, indicating an overall decrease in lipid structural order. This alteration is more drastic when both drugs are administered. This phenomenon suggests drastic changes in the bulk phase of inner mitochondrial membrane lipids after treatments and could explain the altered kinetic properties of palmitoylcarnitine transferase I.