Turnover of rat liver plasma membrane phospholipids comparison with microsomal membranes

The purpose and ubiquity of turnover

Turnover-constant component production and destruction-is ubiquitous in biology. Turnover occurs across organisms and scales, including for RNAs, proteins, membranes, macromolecular structures, organelles, cells, hair, feathers, nails, antlers, and teeth. For many systems, turnover might seem wasteful when degraded components are often fully functional. Some components turn over with shockingly high rates and others do not turn over at all, further making this process enigmatic. However, turnover can address fundamental problems by yielding powerful properties, including regeneration, rapid repair onset, clearance of unpredictable damage and errors, maintenance of low constitutive levels of disrepair, prevention of stable hazards, and transitions. I argue that trade-offs between turnover benefits and metabolic costs, combined with constraints on turnover, determine its presence and rates across distinct contexts. I suggest that the limits of turnover help explain aging and that turnover properties and the basis for its levels underlie this fundamental component of life.

Catalytic modulation of human cytochromes P450 17A1 and P450 11B2 by phospholipid

Unlike most of the drug-metabolizing cytochrome P450s, microsomal P450 17A1 and mitochondrial P450 11B2 catalyze sequential multi-step reactions in steroid biosynthesis. The membrane phospholipid composition might be one parameter that modulates the efficiency and processivity of specific pathways. Here we systematically examined the effects of physiologically relevant phospholipids on the catalysis of purified P450 17A1, P450 11B2, and P450 11B1 in reconstituted assay systems. Both dioleoylphosphatidylcholine (DOPC, 18:1) and dilauroylphosphatidylcholine (DLPC, 12:0) were found to be very efficient in reconstituting 17-hydroxylase and 1720-lyase reactions of P450 17A1. Phosphatidylethanolamine (PE) specifically enhanced 1720-lyase activity up to 2.4-fold in the presence of phosphatidylcholine. On the other hand, P450 11B2-catalyzed production of aldosterone from 11-deoxycorticosterone was very low and from 18-hydroxycorticosterone nil, implying low processivity. DOPC or cardiolipin, which is exclusively located in the inner mitochondrial membrane, maximized aldosterone yield. In sharp contrast, reconstitution of homologous P450 11B1 with DOPC significantly decreased corticosterone formation without affecting the synthesis of 18-hydroxycorticosterone. The intrinsic fluorescence of P450 17A1 and 11B2 increased in the presence of DOPC, DLPC and PE. Acrylamide quenching studies showed that PE decreased solvent accessibility for tryptophan in P450 17A1, as did 20:4 PC or 18:2 PC for P450 11B2. A moderately positive correlation between the proportion of high-spin substrate-bound species and catalytic activity was only observed in the presence of phosphatidylcholines with low-temperature phase transition. These results demonstrate the potential for phospholipids to regulate the activity of steroidogenic P450 activities and thereby steroid hormone biosynthetic pathways.

Nonspecific lipid transfer proteins as probes of membrane structure and function

A protein that accelerates transfer of phospholipids of varying head group and fatty acid composition has been purified from bovine liver. As previously found for other phospholipid transfer proteins, "nonspecific lipid transfer protein" stimulates a kinetically biphasic transfer of radioactively labeled phospholipid from small unilamellar vesicles to unlabeled multilamellar vesicles. The kinetics are consistent with rapid transfer of phospholipid from the outer monalyer and slow transfer of that localized in the inner monolayer (half-times greater than 3 days for phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol). Protein catalyzed transfer is inhibited by high ionic strength and has an activation energy of 35 kJ/mol. The broad lipid specificity and ease of large-scale purification make these proteins candidates for membrane phospholipid compositional modification. The compositions of rat liver mitochondrial and microsomal membranes and Morris hepatoma 7288c mitochondrial membranes were altered by incubation with lipid vesicles and nonspecific lipid transfer protein. Incubation with phosphatidylcholine vesicles led to increased levels of phosphatidylcholine and decreased levels of other transferrable lipids (phosphatidylethanolamine, phosphatidylinositol, and cholesterol) unless the latter were included in the vesicles. When vesicles containing dipalmitoylphosphatidylcholine were incubated with microsomal membranes, a large increase in disaturated phosphatidylcholine was also observed. These changes in composition were correlated with activities of membrane enzymes. It appears that microsomal glucose-6-phosphatase is inhibited by increased phosphatidylcholine saturation. Moreover, this enzyme is also inhibited by decreases in the phosphatidylethanolamine/phosphatidylcholine ratio whereas NADPH cytochrome c reductase is not. Likewise, decreased cholesterol to phospholipid ratios did not greatly affect the abnormally low levels of hepatoma succinate cytochrome c reductase activity.

Turnover of phospholipid fatty acyl chains in cultured neuroblastoma cells: involvement of deacylation-reacylation and de novo synthesis in plasma membranes

Cultured neuroblastoma cells (NIE-115) rapidly incorporated the essential fatty acid, linoleic acid (18:2 (n = 6), into membrane phospholipids. Fatty acid label appeared rapidly (2-10 min) in plasma membrane phospholipids without evidence of an initial lag. Specific activity (nmol fatty acid/mumol phospholipid) was 1.5-2-fold higher in microsomes than in plasma membrane. In these membrane fractions phosphatidylcholine had at least 2-fold higher specific activity than other phospholipids. With 32P as radioactive precursor, the specific activity of phosphatidylinositol was 2-fold higher compared to other phospholipids in both plasma membrane and microsomes. Thus a differential turnover of fatty acyl and head group moieties of both phospholipids was suggested. This was confirmed in dual-label (3H fatty acid and 32P), pulse-chase studies that showed a relatively rapid loss of fatty acyl chains compared to the head group of phosphatidylcholine; the opposite occurred with phosphatidylinositol. A high loss of fatty acyl chain relative to phosphorus indicated involvement of deacylation-reacylation in fatty acyl chain turnover. The patterns of label loss in pulse-chase experiments at 37 and 10 degrees C indicated some independent synthesis and modification of plasma membrane phospholipids at the plasma membrane. Lysophosphatidylcholine acyltransferase and choline phosphotransferase activities were demonstrated in isolated plasma membrane in vitro. Thus, studies with intact cells and with isolated membrane fractions suggested that neuroblastoma plasma membranes possess enzyme activities capable of altering phospholipid fatty acyl chain composition by deacylation-reacylation and de novo synthesis at the plasma membrane itself.

The rate of formation of surface membrane ether lipids in Ehrlich ascites tumor cells: kinetic considerations

A previous investigation has shown that O-alkyl phospholipids are present in the surface membrane of Ehrlich ascites tumor cells. In the present investigation it was shown that 90% or more of [1-3H]hexadecanol injected intraperitoneally into mice bearing Ehrlich ascites tumors is taken up by the neoplastic cells in less than 15 min. Near maximum formation of surface membrane O-alkyl phospholipids requires approximately 8 h. The rate of accumulation of O-alkyl phospholipids is very similar both for the whole cell and for the surface membrane. Further examination of the data revealed that the conversion of hexadecanol into O-alkyl glycerophospholipids can be described by a simple model in which O-alkyl lipids appear at a single rate constant of 0.25 to 0.35 per hour and disappear at a rate of 0.02 per hour or less. These rate constants were obtained initially by stochastic analysis and validated both by deterministic methods and by compartmental analysis using the SAAM computer program. The method of kinetic analysis described may find broader application in providing comparative rate constants for the in vivo turnover of O-alkyl lipids in both normal and neoplastic tissues. The advantage of a stochastic approach is that kinetic data may be obtained with fewer assumptions relating to pool structure or specific models.

Renal cortical brush-border and basolateral membranes: cholesterol and phospholipid composition and relative turnover

A new procedure for the rapid isolation of renal cortical brush-border and basolateral membranes from the same homogenate is described. Brush-border membranes isolated using Mg2+-EGTA precipitation were enriched 18-fold for leucine aminopeptidase and had a recovery of 32.5%. Basolateral membrane fractions were isolated using a discontinuous sucrose gradient and showed an enrichment of 10.7-fold and recovery of 12.8% using (Na+,K+)-ATPase as a marker enzyme. Lipid analysis using two-dimensional TLC separation of phospholipids and gas liquid chromatography for cholesterol showed marked differences in the lipid composition of the brush-border and basolateral membranes. The brush-border membrane had increased sphingomyelin, phosphatidylserine, ethanolamine plasmalogens, and an increased cholesterol-to-phospholipid and sphingomyelin-to-phosphatidylcholine ratio compared to the basolateral membrane. The relative turnover of total membrane and individual phospholipid species using a double isotope ratio method was carried out. Phospholipids were labeled with either phosphorus 32 and 33 or acetate (3H, 1-14C). The relative turnover of phospholipid species and cholesterol differed strikingly. Phosphatidylcholine showed a high turnover, phosphatidylethanolamine and phosphatidylinositol had intermediate values and sphingomyelin, phosphatidylserine and cholesterol had low relative turnover rates. The order of phospholipid class relative turnover was independent of the labeled precursor used. The brush-border membrane had a significantly reduced relative turnover rate for total membrane phospholipids, sphingomyelin and cholesterol compared to the basolateral membrane. These data show marked differences in the lipid composition and relative turnover rates of the phospholipid species of the brush-border and basolateral membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Filipin resistance in intermediate junction membranes of guinea pig ependyma: possible relationship to filamentous underlying

Plasma membranes in intermediate junctions of ependymal cells are found to show considerable resistance to the antibiotic filipin, suggesting low cholesterol in these membranes. Further, ependymal cells were treated with cytochalasin B (CB) infused into the cerebral ventricle in vivo, and then incubated with filipin. When treated with CB, intermediate junctions show a decrease in their underlying density, mainly composed of microfilaments, and their membranes are found to be more affected by filipin. This reduction of resistance to the antibiotic is clearly demonstrated by thin-section and freeze-fracture as well as quantitative analysis. Nonjunctional lateral membranes, however, show no significant difference in the degree of filipin effect whether treated with CB or not. Although biochemical data on lipid composition have not been available for the intermediate junction membranes, we bring forward a possibility that resistance to filipin in these membranes may come not from less cholesterol but from morphological membrane stability brought about by the filamentous underlying.

Intracellular phospholipid movement and the role of phospholipid transfer proteins in animal cells

The mechanism of the intracellular movement of phospholipids from their site of synthesis in the endoplasmic reticulum to mitochondria and other cell membranes is a major unsolved problem of cell biology. Phospholipid transfer proteins of varying specificity found in the soluble supernatant fractions of many tissues catalyze the transfer of phospholipids from microsomes to mitochondria in vitro. They are postulated to play a similar role in vivo, but evidence for their function in living cells is lacking. We have now used an analogue of choline, N-propyl-N,N-dimethylethanolamine [PDME, (2-hydroxyethyl)dimethylpropylammonium hydroxide], to devise a test for the function of the transfer proteins in living cells. The rates of translocation of newly synthesized phosphatidylcholine and the analogue phosphatidyl-PDME in living cells were compared with the rates of transfer in vitro catalyzed by soluble transfer proteins extracted from the same cells. Labeled PDME, choline, and ethanolamine were found to be rapidly incorporated into the lipids of isolated rat hepatocytes and of baby hamster kidney (BHK-21) cells in culture. The translocation of newly synthesized phosphatidylcholine and phosphatidyl-PDME was very rapid in both types of cells with a half-time for equilibration of a few minutes, while the translocation of phosphatidylethanolamine was much slower, with a half-time 20-80 fold longer than those of the other two phospholipids. We then compared these relative rates of movement with the activities of the phospholipid transfer proteins of the respective cells. Partially purified phosphatidylcholine transfer protein from rat liver transfers phosphatidylcholine and phosphatidyl-PDME at identical rates but transfers phosphatidylethanolamine at a rate too low to be detected. This result is consistent with an essential function of this transfer protein in vivo. In contrast, partially purified phosphatidylcholine phospholipid transfer protein from BHK cells transfers phosphatidylcholine rapidly, while no transfer of phosphatidyl-PDME and phosphatidylethanolamine was detected. We further found that the specific phosphatidylcholine transfer protein of BHK cells accounts for nearly all of the transfer activity detected in the crude soluble fraction. The rapid translocation of phosphatidyl-PDME in vivo in BHK cells is therefore inconsistent with the postulate that soluble phospholipid transfer proteins are responsible for the rapid movement of phospholipids from microsomes to mitochondria in living cells.

Plasma membrane: can its structure and function be modulated by dietary fat?

Compositional analysis of plasma membranes from rats fed nutritionally adequate diets different in fatty acid composition establishes that fundamentally different dietary fat intake results in alteration in structural lipid composition of plasma membranes in brain, liver and the intestinal mucosa. Dietary differences in fatty acid intake altered the fatty acyl tail composition of plasma membrane phospholipids in brain, liver and intestinal mucosa. Diet altered the phospholipid profile observed in brain synaptosomal and liver plasma membrane. Feeding high vs low polyunsaturated to saturated fat diets for 7 days altered the fatty acid composition of phosphatidylcholine, phosphatidylethanolamine, sphingomyelin and monoglucosylceramide isolated from plasma membrane of the intestinal mucosa.

Cellular and enzymic synthesis of sphingomyelin

The synthesis of sphingomyelin was studied in baby hamster kidney cells and in subcellular fractions derived from rat liver. During pulse-chase experiments with [3H]choline in tissue culture cells, the specific radioactivity of sphingomyelin continued to increase after the specific activities of phosphocholine and cytidine 5'-diphosphate choline (CDP-choline) had declined by a factor of 10. The addition of [3H]methionine to cells that were grown in 1 mM dimethylethanolamine efficiently radiolabeled phosphatidylcholine (by methylation of phosphatidyldimethylethanolamine) and sphingomyelin but not phosphocholine or CDP-choline. Thus, the proximal donor of the phosphocholine moiety of sphingomyelin was not CDP-choline but probably phosphatidylcholine. These in vivo results prompted investigation of the enzymic synthesis using phosphatidyl[3H]choline or [3H]ceramide as substrates. With both substrates the subcellular fraction with the highest specific enzyme activity was the plasma membrane. When phosphatidyl[3H]choline was used as the substrate, phospholipid exchange proteins were included in the reaction to effect the transfer of the labeled phospholipid from liposomes into the membrane bilayer in which the enzyme resided. Under these conditions the synthesis of sphingomyelin was almost completely dependent upon the addition of phospholipid exchange proteins. When [3H]ceramide was used as the substrate, the addition of detergents was necessary for sphingomyelin synthesis. The use of phospholipid exchange proteins to introduce lipid substrates to membrane-bound enzymes may have much broader applicability.

Studies in vitro on the biosynthesis of ceramide and sphingomyelin. A reevaluation of proposed pathways

The postulated biosynthetic-pathways of ceramide and sphingomyelin were reinvestigated in extensive investigations by means of synthetic stereo- and radio-chemically pure substrates of high specific radioactivity. As a result, the synthesis of ceramides requires the acyl-CoA-mediated acyltransfer to the long chain bases sphingenine and sphinganine. During the biosynthesis of sphingomyelins, phosphocholine is being transferred from the donor CDP-choline to the primary alcohol group of ceramides. Neither can the free long chain sphingosine bases act as acceptor molecule for the phosphocholine group from CDP-choline, nor has a transfer of [N-14CH3]phosphocholine from [N-14CH3]phosphatidyl choline to ceramide by rat liver enzyme preparations been observed. In agreement with previous studies in vivo, the acylation of sphingenylphosphocholine by acyl-CoA or free fatty acid, ATP and CoASH as an alternative pathway in sphingomyelin biosynthesis has been excluded. Other parameters of the CDP-choline:ceramide cholinephosphotransferase reaction (pH-optimum, ion requirement, competitive inhibition by diacyl glycerols, chain length of fatty acids) are reported. Sphingenine-containing ceramide species are preferred as acceptor molecules. Ceramide species with the L-threo (2S,3S)-enantiomeric long-chain bases are better acceptors than the corresponding D-erythro (2S,3R)-isomeric compounds. The meaning of the steric arrangement for the reaction is discussed.

The composition and biosynthesis of the glycoproteins and glycolipids of the rabbit small-intestinal brush border

1. The glycoprotein and glycolipid composition of isolated rabbit small-intestinal brush borders has been studied. 2. The total glycoprotein fraction contains an average 95 microgram carbohydrate per mg protein, composed of mannose, galactose, fucose, N-acetylglucosamine and N-acetylgalactosamine. Glucose is also present but sialic acid is absent. 3. The isolated glycolipids include ceramide lactoside, ceramide trihexoside and two N-acetylglucosamine-containing glycolipids. Sialic acid containing glycolipid (gangliosides) is present only in trace quantities. 4. The biosynthesis of the brush border-bound glycoproteins and glycolipids has been studied following intraperitoneal injection with D-[1-14C]glucosamine and isolation of the brush borders at intervals between 3 and 24 h. 5. The total glycoprotein fraction labels maximally 7.5 h after injection and subsequently exhibits an exponential loss of radioactivity with a half-life of 11.2 h. The labelling kinetics of one of the glucosamine-containing glycolipids is similar to that of the glycoproteins in that it labels maximally between 7.5 and 12 h, but the second glucosamine-containing glycolipid labels later at approximately 18 h. These results indicate that the glycoproteins and glycolipids are actively synthesized and degraded within the mature small intestinal enterocyte and that individual glycolipids turn over independently.

The immunoregulatory role of cholesterol and other lipids: a hypothesis

Membrane lipids play an important role in cellular responses to exogenous signals. In immunocompetent lymphocytes, marked changes in the concentrations of membrane lipids occur following cell-antigen interaction. These changes lead to an increase in membrane fluidity, thus facilitating the microaggregation of receptor-antigen complexes. This event constitutes the inductive signal for lymphocytes. Lipid profile alterations leading to increased concentration of membrane cholesterol, of polyunsaturated lipids, or of both, bring about a decrease in membrane fluidity. The latter interferes with receptor displacement preventing delivery of an inductive signal to the responding cell. Interference with microaggregation is readily brought about in interactions involving low affinity antigens, such as tumoral antigens. We postulate that in hyperlipidemic and hypercholesterolemic states there is decreased immune responsiveness to weak antigens due to the aformentioned lipid profile alterations in the membranes of immunocompetent cells. The manner in which an increase in the concentration of the lipids mentioned can lead to decreased immune responsiveness and hence to an increased incidence of malignancies in hyperlipidemic and hypercholesterolemic states is the hypothesis presented in this paper.

Turnover of lipid components in liver microsomes, mitochondria and plasma membrane of 6-, 12- and 24-month old rats

The turnover of lipid was examined in the livers of 6-, 12- and 24-month old rats. Heterogeneity of turnover was noted for each membrane fraction. The lipid turnover rate was highest in 12-month old rats and was the same in 6- and 24-month old rats. The higher rate of lipid turnover at 12 months was observed in both the neutral and polar lipid components of the liver membranes. In the polar lipid fractions isolated from the microsomal and mitochondrial membranes the increase in lipid turnover rate at 12 months was related to increase in the turnover of phosphatidylethanolamine.

Properties of microsomal phospholipases in rat liver and hepatoma

Phospholipase A1, A2 and lysophospholipase activities in microsomes of Novikoff hepatoma host rat liver and regenerating rat liver were compared using 1-[9', 10'-3H2]palmitoyl-2-[1'-14C] linoleoyl-sn-glycero-3-phosphoethanolamine, 1-[1' -3H-]hexadecyl-2-acyl-sn-glycero-3-phosphoethanolamine, and 1-[9', 10'-3H2]palmitoyl-sn-glycero-3-phosphoethanolamine as substrates. 1. Microsomes of all three tissues showed two pH dependent peaks of hydrolytic activity, one at pH 7.5 and another at pH 9.5. 2. Phospholipid hydrolytic activity in microsomes from host liver and regenerating liver require Ca2+ for hydrolysis at pH 9.5, but not at pH 7.5. Hepatoma microsomes require Ca2+ for activity at both pH values. 3. Phospholipase A1 activity, stimulated by addition of Triton X-100 to the incubation mixtures, was detected in both host liver and regenerating liver microsomes. There was no evidence of phospholipase A1 activity in hepatoma microsomes. 4. Phospholipase A2 was detected in microsomes of all three tissues using 1-[1'-3H] hexadecyl-2-acyl-sn-glycero-3-phosphoethanolamine as a substrate. The activity required calcium and was inhibited by Triton X-100. 5. Lysophospholipase activity was evident in the microsomes from all three tissues. The activity was inhibited by both Ca2+ and Triton X-100. 6. Differences were also detected between host liver and hepatoma microsomal phospholipid hydrolase activities with respect to the effect of increasing protein concentration, apparent Michaelis-Menten constants, and time course of the reaction.

Phospholipid acyl group stability in cultured fibroblasts. Differences between human cell lines of fetal and adult origin

Human fibroblasts of both fetal and adult origin incorporated [1-14C] acetate primarily into phospholipid acyl groups (70-80% of total radioactivity). When these labeled cells were replated in non-radioactive medium, there was continuous loss of 14C from steroids, triacylglycerols and non-lipid material. In contrast, after some initial loss, cell lines of fetal origin completely retained 14C in cellular phospholipids during continued cell division. Unlike cells of fetal origin, fibroblasts of adult origin continued to lose radioactivity from their phospholipid acyl groups during growth in unlabeled medium. Loss of radioactivity from [1-3H] acetate incorporated into phospholipids of adult cells cannot be attributed to cell death since it was not accompanied by any loss of previously incorporated [ME-14C] thymidine. If cellular phospholipids were labeled with [U-14C] glycerol, both fetal and adult fibroblasts continued to lose radioisotope from the cells during growth in nonradioactive medium. Thus, there is turnover of the phospholipid molecules themselves in fetal human fibroblasts grown in vitro, but their acyl groups are retained within cellular phospholipids. In this respect, fibroblasts of fetal origin resemble established cell lines such as the L fibroblast. Fibroblasts of adult origin do not exhibit this complete conservation of their phospholipid acyl groups.

Utilization of L-serine in the in vivo biosynthesis of glycerophospholipids by rat liver

The incorporation of L-serine-U-14C, L-serine-3-14C, and D,L-serine-1-14-C into the glycerophospholipids of rat liver in vivo was determined over a period of 3 min to 13 hr following intravenous injection. The radioactivity from these serines was transferred to variable extent into the glycerol, fatty acid, and nitrogenous base parts of all the glycerophospholipids and neutral lipids. The half-lives and turnover rates of phosphatidylserine calculated from the precursor-product specific activity curves obtained with L-serine-U-14C were 14 min and 0.28 mumol/min/liver, respectively. The half-lives and turnover rates of phosphatidylserine as measured from the decay data of lipid serine from all markers averaged, respectively, 8.2 hr and 0.0008 mumol/min/liver. The discrepancy between these turnover rates was attributed to an understimation of degradation of phosphatidylserine due to its continued biosynthesis and/or an extensive reutilization of L-serine. By monitoring the formation of radioactive lipid ethanolamine, it was found that phosphatidylserine was decarboxylated at one-half the rate of lipid serine biosynthesis. It is suggested that as much as one-half of total phosphatidylserine may be degraded by other mechanisms, such as base exchange with choline, ethanolamine, and serine, as already demonstrated in vitro by other workers. The time course and nature of labeling of phosphatidylcholine was consistent with an extensive conversion of radioactive L-serine to 1-carbon fragments and a rapid methylation of phosphatidylethanolamine to phosphatidylcholine.

Turnover of fatty acids in rat liver cardiolipin: comparison with other mitochondrial phospholipids

Following intraperitoneal administration of 1-14C-linoleic acid or 2-3H-acetate to rats, the specific radioactivities of both liver cardiolipin and other mitochondrial phospholipids after different time intervals were measured. Comparison of the data obtained with those from another stock of rats treated with 32P-phosphate or 2-3H-glycerol showed that the fatty acids of cardiolipin, like those of other phospholipids, exhibit an independent turnover with respect to the remaining parts of the molecule. The half-life of acyl moieties of cardiolipin is ca. 20% higher than that of the same components of other mitochondrial phospholipids. Moreover, it appears that, in both cardiolipin and other phospholipids, linoleyl residues turn over faster than nonessential fatty acids. Discussion is made as to whether this characteristic can be related to the role of phospholipids in the functioning of some enzymes bound to the inner mitochondrial membrane.