Biosynthesis of lecithin by skeletal muscle

Biodisposition and metabolism of [(18)F]fluorocholine in 9L glioma cells and 9L glioma-bearing fisher rats

PURPOSE

[(18)F]Fluorocholine ([(18)F]FCH) was developed as an analog of [(11)C]choline for tumor imaging; however, its metabolic handling remains ill defined. In this study, the metabolism of [(18)F]FCH is evaluated in cultured 9L glioma cells and Fisher 344 rats bearing 9L glioma tumors.

METHODS

9L glioma cells were incubated with [(18)F]FCH and [(14)C]choline under normoxic and hypoxic (1% O(2)) conditions and analyzed for metabolic fate. [(18)F]FCH and [(14)C]choline kinetics and metabolism were studied in Fisher 344 rats bearing subcutaneous 9L tumors.

RESULTS

[(18)F]FCH and [(14)C]choline were similarly metabolized in 9L cells in both normoxic and hypoxic conditions over a 2-h incubation period. In normoxia, radioactivity was predominantly in phosphorylated form for both tracers after 5-min incubation. In hypoxia, the tracers remained mainly in nonmetabolized form at early timepoints (<20 min). Slow dephosphorylation of intracellular [(18)F]phosphofluorocholine (0.043-0.060 min(-1)) and [(14)C]phosphocholine (0.072-0.088 min(-1)) was evidenced via efflux measurements. In rat, both [(18)F]FCH and [(14)C]choline showed high renal and hepatic uptake. Blood clearance of both tracers was rapid with oxidative metabolites, [(18)F]fluorobetaine and [(14)C]betaine, representing the majority of radiolabel in plasma after 5 min postinjection. Oxidation (in liver) and lipid incorporation (in lung) were somewhat slower for [(18)F]FCH relative to [(14)C]choline. The majority of radiolabel in hypoxic subcutaneous tumor, as in hypoxic cultured 9L cells, was found as nonmetabolized [(18)F]FCH and [(14)C]choline.

CONCLUSIONS

[(18)F]FCH mimics choline uptake and metabolism by 9L glioma cells and tumors. However, subtle changes in biodistribution, oxidative metabolism, dephosphorylation, lipid incorporation, and renal excretion show moderate effects of the presence of the radiofluorine atom in [(18)F]FCH. The decrease in phosphorylation of exogenous choline by cancer cells should be considered in interpretation of positron emission tomography images in characteristically hypoxic tumors.

Defective synthesis of polyunsaturated phosphatidylcholines as the primary lesion in Duchenne and murine dy muscular dystrophies

An impaired synthesis of highly unsaturated phosphatidylcholines of the sarcoplasmic reticulum is suggested to be the primary defect in Duchenne and murine dy muscular dystrophies. The lesion is proposed to occur at the level of de novo glycerophosphorylcholine synthesis. These phosphatidylcholine species are postulated to be required for the Ca2+ transport ATPase of this organelle.

Solubilization and reconstitution of cholinephosphotransferase from sarcoplasmic reticulum: stabilization of solubilized enzyme by diacylglycerol and glycerol

Cholinephosphotransferase (CDPcholine: 1,2-diacylglycerol cholinephosphotransferase, EC 2.7.8.2), which catalyzes the terminal step in phosphatidylcholine synthesis via the CDPcholine pathway, is present in sarcoplasmic reticulum from rabbit skeletal muscle (Cornell, R. and MacLennan, D.H. (1985) Biochim. Biophys. Acta 835, 567-576). The conditions for solubilization and reconstitution of this enzyme were investigated as a preliminary step towards its eventual purification. The activity was not released by treatment of membranes with 1 M KCl, but was solubilized after dissolution of membranes with detergents. Cholinephosphotransferase was inactivated by cholate, deoxycholate, Triton X-100, octylglucoside, Tween-20 or SDS at concentrations which solubilize the membrane. However, the activity could be fully recovered after reconstituting the membrane by adding excess lipid (soybean) and removing detergent by gel filtration, dialysis or by absorption to Bio-Beads. When the membrane was solubilized with octylglucoside or cholate at weight ratios of detergent: membrane protein of at least 10, the activity was irreversibly lost unless stabilizers were added with detergent. The substrate diacylglycerol and glycerol were effective stabilizers.

Impaired biosynthesis of highly unsaturated phosphatidylcholines: a hypothesis on the molecular etiology of some muscular dystrophies

A brief review of the literature concerning the synthesis of phosphatidylcholine and phosphatidylethanolamine in muscle suggests that the cytidine pathways are replaced by the recently proposed acyl-specific de novo and salvage glycerolphosphodiester pathways (Infante, 1984) in fully differentiated muscle. An analysis of published data suggests an impaired synthesis of 4,7,10,13,16,19-docosahexaenoic phosphatidylcholine, at the level of de novo sn-3-glycerolphosphorylcholine synthesis, as the primary defect in Duchenne and (dy) murine muscular dystrophies. This phosphatidylcholine species is postulated to be required for optimum sarcoplasmic Ca2+ transport activity. It is proposed that this impairment initiates the secondary series of events which lead to the observed pathology of these diseases. Based on some predictions of the hypothesis, potential diagnosis and treatments are suggested.

Defective synthesis of glycerophosphorylcholine in murine muscular dystrophy; the primary molecular lesion?

Activities of the rate-limiting enzymes of the cytidine pathway for the synthesis of phosphatidylcholine and phosphatidylethanolamine are negligible in differentiated mouse gastrocnemius, whereas that of the respective proposed de novo glycerophosphodiester pathways is high in this muscle. Rates of de novo glycerophosphorylcholine synthesis in dystrophic mouse gastrocnemius are about half that of the wild-type homozygotes, whereas that of the heterozygotes is near the mean of the two homozygous groups. These results suggest that defective de novo synthesis of glycerophosphorylcholine is the primary lesion in murine dy muscular dystrophy.

Inside-outside distribution and diffusion of phosphatidylcholine in rat sarcoplasmic reticulum as determined by 13C NMR and phosphatidylcholine exchange protein

1. The transverse distribution of phosphatidylcholine in rat sarcoplasmic reticulum was investigated employing 13C NMR in conjunction with the shift reagent DyCl3. 2. Sarcoplasmic reticulum phosphatidylcholine was enriched with 13C by feeding rats a diet containing [N-Me3-13C]choline. Up to 32% of the sarcoplasmic reticulum phosphatidyl-[N-Me3-12C]choline was replaced by phosphatidyl-[N-Me3-13C]choline. 3. Titration of 13C-enriched sarcoplasmic reticulum with Dy3+ indicates that 40% of the phosphatidyl-[N-Me3-13C]choline is exposed to the external medium, whereas 60% is shielded from interaction with Dy3+. 4. Incubation of 32P-labelled sarcoplasmic reticulum with excess mitochondria and phosphatidylcholine exchange protein results in a fast transport of approx. 80% of [32P]phosphatydylcholine to the mitochondria indicating that part of the phosphatidylcholine pool is involved in a rapid transbilayer movement.

Phospholipid biosynthesis in sarcoplasmic reticulum membrane during development

Biosynthesis of phosphatidic acid, phosphatidylcholine and phosphatidylethanolamine in the sarcoplasmic reticulum membrane has been investigated. The results show that sarcoplasmic reticulum, in addition to its main function, i.e. transport and accumulation of Ca2+, is able to synthetize phospholipids by the same pathways as endoplasmic reticulum of other tissues. The changes of activity of enzymes involved in phospholipid biosynthesis during muscle development have been analysed. The extent of sn-glycero-3-phosphate and lysophosphatidylcholine acylation by acyl-CoA or free fatty acids in the presence of ATP and CoA is the same at every stage of development. The specific activity of glycerolphosphate acyltransferase(s) increases progressively during development up to about the 10th day of postnatal life and then decreases to the adult level. Linoleate esterifies sn-glycero-3-phosphate to a higher extent than palmitate, especially during postnatal period. The main product of sn-glycero-3-phosphate acylation is phosphatidic acid. The specific activity of lysolecithin acyltransferase increases from the embryonic period to a maximum between the 4th and the 9th day of postnatal life followed by a decrease to the adult value. the low embryonic value to a maximum at about the 3rd day of postnatal life, followed by a decrease to the adult value. The activity of cholinephosphotransferase decreases from a high value observed during the earliest embryonic period studied until the 3rd day before birth, and then begins to increase again from about the 5th day of postnatal life. The activity of ethanolaminephosphotransferase decreases continuously with age. The main product of phosphatidylethanolamine methylation is phosphatidylmonomethylethanolamine. The specific activity of phosphatidylethanolamine methyltransferase increases from

Incorporation of ortho[32P]phosphate into phosphatidylcholines and phosphatidylethanolamines in rat skeletal muscle

1. The specific radioactivities of individual molecular species of muscle phosphatidylcholine and phosphatidylethanolamine have been measured by a combination of argentation thin-layer chromatography and countercurrent distribution. 2. The specific radioactivities of individual molecular species of muscle phosphoglycerides have been determined 3 h after intraperitoneal injection of ortho[32 P] phosphate. Under these conditions the specific radioactivities of the species present in rat muscle were found to be measures of the relative turnover times of these molecules. 3. The specificity radioactivity of phosphatidylcholine was approx. three times that of phosphatidylethanolamine. The 1-palmitoyl-2-oleoyl and 1-oleoyl-2-linoleoyl phosphatidylcholines had the fastest turnover and the 1-palmitoyl-2-arachidonoyl the slowest. Of the phosphatidylethanolamines, the linoleoyl and the docosahexaenoyl species showed the fastest turnover and 1-stearoyl-2-arachidonoyl the slowest. 4. The results indicate that phosphoglycerides in muscle turn over more slowly and more evenly than do liver phosphoglycerides.