Phosphate-containing dialysis solution prevents hypophosphatemia during continuous renal replacement therapy

BACKGROUND

hypophosphatemia occurs in up to 80% of the patients during continuous renal replacement therapy (CRRT). Phosphate supplementation is time-consuming and the phosphate level might be dangerously low before normophosphatemia is re-established. This study evaluated the possibility to prevent hypophosphatemia during CRRT treatment by using a new commercially available phosphate-containing dialysis fluid.

METHODS

forty-two heterogeneous intensive care unit patients, admitted between January 2007 and July 2008, undergoing hemodiafiltration, were treated with a new Gambro dialysis solution with 1.2 mM phosphate (Phoxilium) or with standard medical treatment (Hemosol B0). The patients were divided into three groups: group 1 (n=14) receiving standard medical treatment and intravenous phosphate supplementation as required, group 2 (n=14) receiving the phosphate solution as dialysate solution and Hemosol B0 as replacement solution and group 3 (n=14) receiving the phosphate-containing solution as both dialysate and replacement solutions.

RESULTS

standard medical treatment resulted in hypophosphatemia in 11 of 14 of the patients (group 1) compared with five of 14 in the patients receiving phosphate solution as the dialysate solution and Hemosol B0 as the replacement solution (group 2). Patients treated with the phosphate-containing dialysis solution (group 3) experienced stable serum phosphate levels throughout the study. Potassium, ionized calcium, magnesium, pH, pCO(2) and bicarbonate remained unchanged throughout the study.

CONCLUSION

the new phosphate-containing replacement and dialysis solution reduces the variability of serum phosphate levels during CRRT and eliminates the incidence of hypophosphatemia.

Renal toxicity mediated by glucose degradation products in a rat model of advanced renal failure

BACKGROUND

In peritoneal dialysis (PD) residual renal function contributes to improved patient survival and quality of life. Glucose degradation products (GDP) generated by heat sterilization of PD fluids do not only impair the peritoneal membrane, but also appear in the systemic circulation with the potential for organ toxicity. Here we show that in a rat model of advanced renal failure, GDP affect the structure and function of the remnant kidney.

MATERIALS AND METHODS

Sprague-Dawley rats were randomly assigned to a two stage subtotal nephrectomy (SNX) or sham operation and were left untreated for 3 weeks. The SNX + GDP group continuously received chemically defined GDP intravenously for 4 weeks; the SNX and the sham-operated rats remained without GDP. The complete follow-up for all groups was 7 weeks postoperatively. We analysed renal damage using urinary albumin excretion as well as a semiquantitative score for glomerulosclerosis and tubulointerstitial damage, as well as for immunohistochemical analyses.

RESULTS

The SNX + GDP rats developed significantly more albuminuria and showed a significantly higher score of glomerulosclerosis index (GSI) and tubulointerstitial damage index (TII) as compared to SNX or control rats. In the SNX + GDP group the expression of carboxymethyllysine and methylglyoxal was significantly higher in the tubulointerstitium and the glomeruli compared to the SNX rats. Caspase 3 staining and TUNEL assay were more pronounced in the tubulointerstitium and the glomeruli of the SNX + GDP group. In SNX + GDP animals, the expression of the slit diaphragm protein nephrin, was significantly lower compared to SNX or control animals.

CONCLUSION

In summary, our data suggests that GDP can significantly advance chronic kidney disease and argues that PD solutions containing high GDP might deteriorate residual renal function in PD.

Differential expression of receptors for advanced glycation end-products in peritoneal mesothelial cells exposed to glucose degradation products

Autoclaving peritoneal dialysate fluid (PDF) degrades glucose into glucose degradation products (GDPs) that impair peritoneal mesothelial cell functions. While glycation processes leading to formation of advanced glycation end-products (AGE) were viewed commonly as being mediated by glucose present in the PDF, recent evidence indicates that certain GDPs are even more powerful inducers of AGE formation than glucose per se. In the present study, we examined the expression and modulation of AGE receptors on human peritoneal mesothelial cells (HPMC) cultured with GDPs, conventional PDF or PDF with low GDP content. HPMC cultured with GDPs differentially modulated AGE receptors (including RAGE, AGE-R1, AGE-R2 and AGE-R3) expression in a dose-dependent manner. At subtoxic concentrations, GDPs increased RAGE mRNA expression in HPMC. 2-furaldehyde (FurA), methylglyoxal (M-Glx) and 3,4-dideoxy-glucosone-3-Ene (3,4-DGE) increased the expression of AGE-R1 and RAGE, the receptors that are associated with toxic effects. These three GDPs up-regulated the AGE synthesis by cultured HPMC. In parallel, these GDPs also increased the expression of vascular endothelial growth factor (VEGF) in HPMC. PDF with lower GDP content exerted less cytotoxic effect than traditional heat-sterilized PDF. Both PDF preparations up-regulated the protein expression of RAGE and VEGF. However, the up-regulation of VEGF in HPMC following 24-h culture with conventional PDF was higher than values from HPMC cultured with PDF containing low GDP. We have demonstrated, for the first time, that in addition to RAGE, other AGE receptors including AGE-R1, AGE-R2 and AGE-R3 are expressed on HPMC. Different GDPs exert differential regulation on the expression of these receptors on HPMC. The interactions between GDPs and AGE receptors may bear biological relevance to the intraperitoneal homeostasis and membrane integrity.

Peritoneal leukocyte survival and respiratory burst responses in patients treated with a low glucose degradation and high pH peritoneal dialysis fluid

The discovery of toxicity related to glucose degradation products (GDP) has initiated the development of new PD fluids with low GDP concentrations and higher, more physiological, pH levels. Cell numbers, differential counts and the respiratory burst responses of peritoneal leukocytes were compared between patients treated with the low GDP, high pH fluid Gambrosol-trio (n=10) and a conventional fluid (n=12). Effluents from over-night dwells were collected and leukocytes were evaluated morphologically and by luminol-amplified chemiluminescence (CL) after stimulation with opsonized zymosan. The frequency of necrosis and early apoptosis was quantified by means of annexin V binding and propidium iodide uptake. The Gambrosol-trio group produced significantly higher (p<5%) macrophage counts and stronger CL responses (p<10%) than did the conventional fluid group. The cell compositions did not differ significantly between the groups. Necrosis was significantly more common among the cells in the conventional fluid group. The occurrence of apoptosis did not differ between the fluids.

Glucose degradation products in peritoneal dialysis fluids: how they can be avoided

OBJECTIVES

A patient on peritoneal dialysis (PD) uses 3-7 tons of PD fluid every year. The result is considerable stress on the peritoneal tissue. Aspects of PD fluids that have been considered responsible for bioincompatibility are low pH, high osmolality, high glucose and lactate concentrations, and the presence of glucose degradation products (GDPs). However, the relative importance of each factor in PD fluid has so far not been investigated. Discovering their relative importance was the aim of the present study.

METHODS

Two main methods for investigating biocompatibility were used in this study: cytotoxicity measured as in vitro inhibition of cell growth, and in vitro AGE formation measured as albumin-linked fluorescence.

RESULTS

The two most important factors for determining in vitro bioincompatibility of PD fluids were the presence of GDPs, which caused both severe cytotoxicity and strong AGE promotion, and low pH, which induced severe cytotoxicity.

CONCLUSIONS

The biocompatibility of PD fluids can be monitored through fairly simple in vitro methods such as cell proliferation and AGE formation. Bioincompatibility of PD fluids is caused mainly by the presence of GDPs and low pH. These findings correlate well with known clinical bioincompatibility.

Protection by glutathione of neutrophils against the toxic effects of peritoneal dialysis fluid

The possibility of reducing the cytotoxic effect of heat-sterilized peritoneal dialysis (PD) fluid by addition of antioxidants/scavengers during incubation of titanium-adhering cells was investigated. Capillary blood from healthy donors was placed in drops on commercially available titanium pieces and incubated in a humidified chamber at 37 degrees C for 60min. After incubation the adherent polymorphonuclear leukocytes were immersed for 1-4h in PD-fluid, pH 7.4, containing 2.5% glucose with glutathione (GSH), superoxide dismutase, catalase or dithiothreitol (DTT). Luminol- or isoluminol-amplified chemiluminescence was used to measure the zymosan- and phorbol myristate acetate (PMA)-stimulated respiratory burst activity, as an indicator of the cytotoxicity of the PD-fluids. Heat sterilized PD-fluid had inhibitory effect on zymosan-induced respiratory burst and impaired both the extracellular and intracellular PMA-induced respiratory burst. Addition of GSH to the PD-fluid resulted in reduction of cytotoxical effects on the zymosan-induced and extracellular PMA-induced respiratory burst. The intracellular respiratory burst was not affected. The present results show that GSH and DTT have the ability to protect polymorphonuclear leukocytes against the cytotoxic effects of the PD-fluid by keeping the cell membrane in a reduced state.

Glucose degradation products in peritoneal dialysis fluids may have both local and systemic effects: a study of residual fluid and mesothelial cells

OBJECTIVE

When peritoneal dialysis (PD) fluids are heat sterilized, glucose is degraded to carbonyl compounds. These compounds are known to interfere with many cellular functions and to promote the formation of advanced glycation end-products. However, little is known about what actually happens with glucose degradation products (GDPs) after infusion into the peritoneal cavity. The aim of the present study was to investigate possible targets for GDPs in the peritoneal cavity.

DESIGN

In vitro reactions between residual fluid and GDPs were studied by incubating unused PD fluid with overnight dialysate. Confluent monolayer cultures of human mesothelial cells were used as a model to study the reactions of GDPs with the cells lining the peritoneal cavity.

METHODS

Samples were analyzed, using high pressure liquid chromatography, for the presence of formaldehyde, acetaldehyde, 5-hydroxymethyl-2-furaldehyde (5-HMF), methylglyoxal, and 3-deoxyglucosone (3-DG). Cytotoxicity was determined as inhibition of proliferation of cultured fibroblasts.

RESULTS

None of the analyzed GDPs reacted with overnight dialysate. Formaldehyde and methylglyoxal, in contrast to 3-DG and 5-HMF, reacted with the cultured mesothelial cells.

CONCLUSIONS

Low molecular weight carbonyls such as formaldehyde and methylglyoxal most probably react with the mesothelial cells lining the peritoneal cavity, and could be responsible for the disappearance of these cells during long-term treatment. 3-Deoxyglucosone showed remarkably low reactivity and was most probably transported within the patient.

Degradation in peritoneal dialysis fluids may be avoided by using low pH and high glucose concentration

OBJECTIVE

When glucose is present in a medical fluid, the heat applied during sterilization leads to degradation. The glucose degradation products (GDPs) give rise to bioincompatible reactions in peritoneal dialysis patients. The extent of the degradation depends on a number of factors, such as heating time, temperature, pH, glucose concentration, and catalyzing substances. In the present work, we investigated the influence of pH and concentration in order to determine how to decrease the amounts of GDPs produced.

DESIGN

Glucose solutions (1%-60% glucose; pH 1-8) were heat sterilized at 121 degrees C. Ultraviolet (UV) absorption, aldehydes, pH, and inhibition of cell growth (ICG) were used as measures of degradation.

RESULTS

Glucose degradation was minimum at an initial pH (prior to sterilization) of around 3.5 and at a high concentration of glucose. There was considerable development of acid degradation products during the sterilization process when the initial pH was high. Two different patterns of development of UV-absorbing degradation products were seen: one below pH 3.5, dominated by the formation of 5-hydroxy-methyl-2-furaldehyde (5-HMF); and one above, dominated by degradation products absorbing at 228 nm. 3-Deoxyglucosone (3-DG) concentration and the portion of 228 nm UV absorbance not caused by 5-HMF were found to relate to the in vitro bioincompatibility measured as ICG; there was no relation between 5-HMF or absorbance at 284 nm and bioincompatibility.

CONCLUSION

In order to minimize the development of bioincompatible GDPs in peritoneal dialysis fluids during heat sterilization, pH should be kept around 3.2 and the concentration of glucose should be high. 5-HMF and 284 nm UV absorbance are not reliable as quality measures. 3-DG and the portion of UV absorbance at 228 nm caused by degradation products other than 5-HMF seem to be reliable indicators of bioincompatibility.

Sequence properties of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii membranes. Recognition of a large group of lipid glycosyltransferases in eubacteria and archaea

Synthesis of the nonbilayer-prone alpha-monoglucosyldiacylglycerol (MGlcDAG) is crucial for bilayer packing properties and the lipid surface charge density in the membrane of Acholeplasma laidlawii. The gene for the responsible, membrane-bound glucosyltransferase (alMGS) (EC ) was sequenced and functionally cloned in Escherichia coli, yielding MGlcDAG in the recombinants. Similar amino acid sequences were encoded in the genomes of several Gram-positive bacteria (especially pathogens), thermophiles, archaea, and a few eukaryotes. All of these contained the typical EX(7)E catalytic motif of the CAZy family 4 of alpha-glycosyltransferases. The synthesis of MGlcDAG by a close sequence analog from Streptococcus pneumoniae (spMGS) was verified by polymerase chain reaction cloning, corroborating a connection between sequence and functional similarity for these proteins. However, alMGS and spMGS varied in dependence on anionic phospholipid activators phosphatidylglycerol and cardiolipin, suggesting certain regulatory differences. Fold predictions strongly indicated a similarity for alMGS (and spMGS) with the two-domain structure of the E. coli MurG cell envelope glycosyltransferase and several amphipathic membrane-binding segments in various proteins. On the basis of this structure, the alMGS sequence charge distribution, and anionic phospholipid dependence, a model for the bilayer surface binding and activity is proposed for this regulatory enzyme.

Very high daily intraperitoneal doses of carbonyl compounds affect the morphology, but not the exchange characteristics, of rat peritoneum

Glucose degradation products (GDP) are carbonyl compounds, that are formed by heat sterilization of conventional peritoneal dialysis (PD) fluids. Carbonyl compounds are known to be toxic in vitro and potentially toxic also in vivo. The aim of this study was to evaluate the effects of daily, short-term exposure of the peritoneum to very high concentrations of GDP in vivo on peritoneal transport parameters and on peritoneal morphology in a well-established rat model of PD. Rats were exposed to three daily intraperitoneal (IP) injections (10 ml) for 9 days of a largely neutral (pH 7.2) PD fluid containing 1.5% glucose and sterilized by filtration, with (n = 8) or without (n = 8) the presence of different carbonyl compounds in concentrations 100 times higher than those reported in commercial PD fluids. Seven rats, not subjected to any exposure, served as controls. After the exposure, the rats were subjected to acute PD in 4-hour dwells. Twenty milliliters of 4% glucose dialysis fluid were instilled into the rat peritoneal cavity. Blood and dialysate samples were taken during the dwell for measurements of dialysate sodium, and for assessments of the mass transfer area coefficient (PS) for glucose and 51Cr-EDTA and of transperitoneal clearance (Cl) or radiolabelled albumin (RISA). At the end of the dwell, parts of the liver, diaphragm and peritoneum were removed for measurements of tissue cell density and thickness of the submesothelial peritoneal tissue. The exposure of the peritoneum to very high doses of carbonyl compounds did not affect the peritoneal transport of fluid and small solutes significantly, but seemed to slightly reduce lymph flow and albumin clearance out of the peritoneal cavity. Assessed after a hypertonic dwell, and compared to the situation in nontreated rats after the same kind of dwell, there was a significant thinning of the submesothelial tissue, but no difference in tissue cell density. It is concluded that short-term exposure of the peritoneum in vivo to very high doses of GDP resulted in almost no signs of acute toxicity.

Biological significance of reducing glucose degradation products in peritoneal dialysis fluids

Carbohydrates are not stable when exposed to energy; they degrade into new molecules. In peritoneal dialysis (PD) fluids, degradation of glucose occurs during the heat sterilization procedure. The biological consequences of this degradation are side effects such as impaired proliferation and impaired host defense mechanisms, demonstrated in vitro for a great variety of cells. Several highly toxic compounds--such as formaldehyde and 3-deoxyglucosone--have been identified in PD fluids. Carbonyl compounds, apart from being cytotoxic, are also well-known promoters of irreversible advanced glycation end-products (AGEs), which might participate in the long-term remodeling of the peritoneal membrane. Various approaches can be used to reduce the formation of glucose degradation products (GDPs) during heat sterilization. Some examples are shortening the sterilization time, lowering the pH, removing catalyzing substances, and increasing glucose concentration. The latter three factors are employed in the multi-compartment bag with a separate chamber containing pure glucose at high concentration and low pH. Gambrosol trio, a PD fluid produced in this way, shows reduced cytotoxicity, normalized host defense reactions, less AGE formation, and reduced concentrations of formaldehyde and 3-deoxyglucosone. Moreover, in the clinical situation, the fluid turns out to be more biocompatible for the patient, causing less mesothelial cell damage, which in the long term could lead to a more intact peritoneal membrane.

CONCLUSION

Glucose degradation products in heat-sterilized fluids for peritoneal dialysis are cytotoxic, promote AGE formation, and cause negative side effects for the patient. Using improved and well-controlled manufacturing processes, it is possible to produce sterile PD fluids with glucose as the osmotic agent but without the negative side effects related to GDPs.

Long-term clinical effects of a peritoneal dialysis fluid with less glucose degradation products

BACKGROUND

Glucose degradation products (GDPs) are cytotoxic in vitro and potentially toxic in vivo during peritoneal dialysis (PD). We are presenting the results of a two-year randomized clinical trial of a new PD fluid, produced in a two-compartment bag and designed to minimize heat-induced glucose degradation while producing a near neutral pH. The effects of the new fluid over two years of treatment on membrane transport characteristics, ultrafiltration (UF) capacity, and effluent markers of peritoneal membrane integrity were investigated and compared with those obtained during treatment with a standard solution.

DESIGN

A two-group parallel design with 80 continuous ambulatory peritoneal dialysis patients was used. The patients were randomly assigned to either the new fluid (N = 40) or to a conventional one (N = 40), and were stratified with respect to age, diabetes, and time on PD. Peritoneal transport characteristics were assessed by the Personal Dialysis Capacity (PDCtrade mark) test at 1, 6, 12, 18, and 24 months after inclusion and by weighing the overnight bag daily. Infusion pain and handling were evaluated using a questionnaire. Peritoneal mesothelial and interstitial integrity were evaluated by analyzing overnight effluent dialysate concentrations of CA 125, hyaluronan (HA), procollagen-1-C-terminal peptide (PICP), and procollagen-3-N-terminal peptide (PIIINP) at 1, 6, 12, 18, and 24 months.

RESULTS

The handling of the new two-compartment bag was considered easy, and there were no indications of increased discomfort with the new system. Furthermore, no changes in peritoneal fluid or solute transport characteristics were observed during the study period for either fluid, and neither were there any differences with regard to peritonitis incidence. However, significantly higher dialysate CA 125 (73 +/- 41 vs. 25 +/- 18 U/mL), PICP (387 +/- 163 vs. 244 +/- 81 ng/mL), and PIIINP (50 +/- 24 vs. 29 +/- 13 ng/mL) and significantly lower concentrations of HA (395 +/- 185 vs. 530 +/- 298 ng/mL) were observed in the overnight effluent during treatment with the new fluid.

CONCLUSIONS

We conclude that the new fluid with a higher pH and less GDPs is safe and easy to use and has no negative effects on either the frequency of peritonitis or peritoneal transport characteristics as compared with conventional ones. Our results indicate that the new solution causes less mesothelial and interstitial damage than conventional ones; that is, it may be considered more biocompatible than a number of conventional PD solutions currently in use.

The nonbilayer/bilayer lipid balance in membranes. Regulatory enzyme in Acholeplasma laidlawii is stimulated by metabolic phosphates, activator phospholipids, and double-stranded DNA

In membranes of Acholeplasma laidlawii a single glucosyltransferase step between the major, nonbilayer-prone monoglucosyl-diacylglycerol (MGlcDAG) and the bilayer-forming diglucosyl-diacylglycerol (DGlcDAG) is important for maintenance of lipid phase equilibria and curvature packing stress. This DGlcDAG synthase is activated in a cooperative fashion by phosphatidylglycerol (PG), but in vivo PG amounts are not enough for efficient DGlcDAG synthesis. In vitro, phospholipids with an sn-glycero-3-phosphate backbone, and no positive head group charge, functioned as activators. Different metabolic, soluble phosphates could supplement PG for activation, depending on type, amount, and valency. Especially efficient were the glycolytic intermediates fructose 1,6-bisphosphate and ATP, active at cellular concentrations on the DGlcDAG but not on the preceding MGlcDAG synthase. Potencies of different phosphatidylinositol (foreign lipid) derivatives differed with numbers and positions of their phosphate moieties. A selective stimulation of the DGlcDAG, but not the MGlcDAG synthase, by minor amounts of double-stranded DNA was additive to the best phospholipid activators. These results support two types of activator sites on the enzyme: (i) lipid-phosphate ones close to the membrane interphase, and (ii) soluble (or particulate)-phosphate ones further out from the surface. Thereby, the nonbilayer (MGlcDAG) to bilayer (DGlcDAG) lipid balance may be integrated with the metabolic status of the cell and potentially also to membrane and cell division.

Molecular analysis of the cytolytic protein ClyA (SheA) from Escherichia coli

Escherichia coli K-12 carries a gene for a protein denoted ClyA or SheA that can mediate a cytolytic phenotype. The ClyA protein is not expressed at detectable levels in most strains of E. coli, but overproduction suitable for purification was accomplished by cloning the structural gene in an hns mutant strain. Highly purified ClyA protein was cytotoxic to macrophage cells in culture and caused detachment and lysis of the mammalian cells. Results from osmotic protection assays were consistent with the suggestion that the protein formed pores with a diameter of up to 3 nm. Using Acholeplasma laidlawii cells and multilamellar liposomes, we studied the effect of ClyA on membranes with varying compositions and in the presence of different ions. ClyA induced cytolytic release of the fluorescent tracer from carboxyfluorescein-loaded liposomes, and the release was stimulated if cholesterol was present in the membranes whereas addition of calcium had no effect. Pretreatment of the ClyA protein with cholesterol inhibited the pore formation, suggesting that ClyA could bind to cholesterol. Efficient coprecipitation of ClyA with either cholesterol or 1,2,3-trioctadecanoylglycerol in aqueous solutions showed that ClyA directly interacted with the hydrophobic molecular aggregates. We tested the possible functional importance of selected ClyA protein regions by site-directed mutagenesis. Defined mutants of ClyA were obtained with alterations in postulated transmembrane structures in the central part and in a postulated membrane-targeting domain in the C-terminal part. Our results were consistent with the suggestion that particular amphiphilic segments are required for ClyA activity. We propose that these domains are necessary for ClyA to form pores.

Different sequence patterns in signal peptides from mycoplasmas, other gram-positive bacteria, and Escherichia coli: a multivariate data analysis

Signal peptides are essential N-terminal extensions in export proteins, and have a positively charged N-terminus, a hydrophobic central core, and a C-terminal cleavage region. They interact in a consecutive manner with different accessory proteins during the secretion process. Potential patterns or periodicity in the amino acid (aa) sequence were searched, using multivariate techniques, for a large number of signal peptides from mollicutes (mycoplasmas), other Gram-positive bacteria, and Escherichia coli. Mollicutes signal peptides were significantly different from the E. coli and Gram-positive ones by their N-terminal charge, peptide length, and especially, unique periodicities of side chain hydrophobicity and volumes. Their lipoprotein signal peptides were longer than for any other bacteria. Significant differences were also recorded between the other bacterial peptide groups. Specific aa patterns were more related within the signal peptides from several groups of secreted bacillus enzymes, than for all signal peptides from one bacillus species. In E. coli, signal peptides from proteins routed for the various destinations revealed significant and compartment-specific sequence patterns not evident by other methods. This was substantiated from a large number of signal peptide secretion mutants for the E. coli periplasmic space. It is proposed that the differences in aa patterns and side-chain properties are related to the secondary structure sidedness and topology of the signal peptides, and important for specific interactions during the secretion process.

Key role of the diglucosyldiacylglycerol synthase for the nonbilayer-bilayer lipid balance of Acholeplasma laidlawii membranes

In the single membrane of Acholeplasma laidlawii, a specific glucosyltransferase (DGlcDAG synthase) synthesizes the major, bilayer-forming lipid diglucosyldiacylglycerol (DGlcDAG) from the preceding major, nonbilayer-prone monoglucosyldiacylglycerol (MGlcDAG). This is crucial for the maintenance of phase equilibria close to a potential bilayer-nonbilayer transition and a nearly constant spontaneous curvature for the membrane bilayer lipid mixture. The glucolipid pathway is also balanced against the phosphatidylglycerol (PG) pathway to maintain a certain lipid surface charge density. The DGlcDAG synthase was purified approximately 5000-fold by three chromatographic techniques and identified as a minor 40 kDa membrane protein. In CHAPS mixed micelles, a cooperative dependence on anionic lipid activators was confirmed, with PG as the best. The dependence of the enzyme on the soluble UDP-glucose substrate followed Michaelis-Menten kinetics, while the kinetics for the other (lipid) substrate MGlcDAG exhibited cooperativity, with Hill coefficients in the range of 3-5. Vmax and the Hill coefficient, but not Km, for the MGlcDAG substrate were increased by increased PG concentrations, but above 3 mol % MGlcDAG, the rate of synthesis was constant. Hence, the DGlcDAG synthase is more affected by the lipid activator than by the lipid substrate at physiological lipid concentrations. The enzyme was shown to be sensitive to curvature "stress" changes, i.e., was stimulated by various nonbilayer lipids but inhibited by certain others. Certain phosphates were also stimulatory. With the two purified MGlcDAG and DGlcDAG synthases reconstituted together in the presence of a potent nonbilayer lipid, the strong responses in the amounts of MGlcDAG and DGlcDAG synthesized mimicked the responses in vivo. This supports the important regulatory functions of these enzymes.

3-Deoxyglucosone, a promoter of advanced glycation end products in fluids for peritoneal dialysis

OBJECTIVE

The accumulation of irreversible formed advanced glycosylation end products (AGE) in the peritoneal cavity might play an important role in the development of ultrafiltration failure and peritoneal membrane destruction. 3-Deoxyglucosone (3-DG), more formally named 3-deoxy-D-erythro-hexos-2-ulose or 3-deoxy-D-erythro-hexosulos is known to be a potent cross-linker responsible for the polymerization of proteins and a precursor of AGE. The purpose of the present study was to determine if the dicarbonyl compound 3-DG, is formed as a glucose degradation product during heat sterilization of fluids for peritoneal dialysis (PD).

DESIGN

Four fluids were examined: a commercially available PD fluid Gambrosol (Gambro, Lund, Sweden); Gambrosol-Bio (Gambro), a new PD-fluid produced under conditions that minimize the generation of toxic glucose degradation products; a fluid prepared in the laboratory by sterile-filtration; and a fluid prepared in the laboratory by heat sterilization.

METHODS

The concentration of 3-DG was analyzed by measuring the concentration of its diaminonaphthalene derivative by HPLC using a Waters Symmetry C18 column.

RESULTS

The 3-DG concentrations in the commercially- and laboratory-prepared heat-sterilized fluids were 118 and 154 micromol/L, respectively. Gambrosol-Bio and the sterile-filtered fluid produced in the laboratory contained 3-DG in concentrations of 12.3 and less than 1.2 micromol/L, respectively.

CONCLUSION

Our results demonstrate that during the heat sterilization of conventional PD-fluids, 3-DG is produced as a degradation product of glucose. It was also demonstrated that, through an alteration of the manufacturing condition, the production of 3-DG could be considerably reduced. We speculate that the presence of 3-DG in unused conventional PD-fluid could act as a local promoter, and increase local AGE formation within the peritoneal cavity.

Effects of acidity, glucose degradation products, and dialysis fluid buffer choice on peritoneal solute and fluid transport in rats

OBJECTIVE

To evaluate the effects of acidity, glucose degradation products (GDP), and different solution buffer systems on solute and fluid transport during acute peritoneal dialysis (PD) in rats.

DESIGN

Dialysis fluid (16 mL) containing 2.5% glucose as the osmotic agent was instilled intraperitoneally in Wistar rats (280 g) via a thin catheter in dwells lasting 4 hours. Blood and dialysis fluid samples (25 microL) were taken for measurement of glucose, sodium, and radioactive markers. The mass transfer area coefficient (MTAC or PS) for glucose and for 51Cr-EDTA (given as an intravenous infusion) and the peritoneal clearance (Cl) of 125I albumin (RISA), as well as the clearance of RISA to plasma (Cl --> P) were assessed for a commercial, heat-sterilized, acidic PD solution (2.5% glucose, pH 5.5; Gambrosol, Gambro, Lund, Sweden), containing GDP, and for four filter-sterilized solutions containing either lactate (40 mmol/L, pH 5.5 or 7.2), bicarbonate (38 mmol/L, pH 7.2), or pyruvate (40 mmol/L, pH 7.2) as buffers and being devoid of GDP.

RESULTS

The initial pH of the acidic solutions increased rapidly, and attained physiological levels within 40 minutes. The initial drop of sodium, which is expected during the first part of the dwell, was significantly more pronounced with neutral than with acidic lactate. The PS for glucose and 51Cr-EDTA were slightly, but significantly, higher with the acidic and heat-sterilized solution (Gambrosol) than with the neutral, sterile-filtered lactate-buffered solution (p < 0.01), especially early during the dwell. Such an increase may be due to initial vasodilatation, and hence, recruitment of capillaries by the combination of acidity and GDP. However, there were no significant differences with respect to small solute PS values among sterile-filtered solutions, regardless of the presence of acidity or of buffer choice.

CONCLUSION

There were no major differences in fluid and solute transport among sterile-filtered PD solutions having differing buffer systems and pH. Neither were there any effects of GDP alone. However, the combination of a low pH and the presence of GDP in the PD solutions seemed to cause significant increases in peritoneal small solute transport.

Purification of a phosphatase which hydrolyzes phosphatidic acid, a key intermediate in glucolipid synthesis in Acholeplasma laidlawii A membranes

A phosphatidic acid phosphatase (PAP; EC 3.1.3.4.), dephosphorylating phosphatidic acid (PA) to diacylglycerol (DAG), was identified and purified from the plasma membrane of Acholeplasma laidlawii A. After four purification steps, including membrane preparation, Tween 20 solubilization, preparative gel electrophoresis and electro-elution, PAP was purified about 400 times to near homogeneity. The molecular weight of PAP was according to SDS-polyacrylamide gel electrophoresis approximately 25 kDa and the enzyme was a stable and integral membrane protein. It is proposed to catalyze the first enzymatic step in the important glucolipid pathway of A. laidlawii. No essential cofactors or activator lipids were found. However, some divalent cations and phosphate analogues were potent inhibitors. Beside the in vivo substrate (PA), PAP was found to dephosphorylate p-nitrophenylphosphate. This less stringent specificity makes alternative in vivo functions for PAP plausible, the importance which is discussed.

Activating amphiphiles cause a conformational change of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii membranes according to proteolytic digestion

1,2-Diacylglycerol 3-glucosyltransferase synthesizes the major nonbilayer-prone lipid monoglucosyldiacylglycerol (MGlcDAG) in the membrane of Acholeplasma laidlawii, which is important for the spontaneous curvature, and is a regulatory site for the lipid surface charge density. A potential connection between activity and a conformational change of this enzyme, governed by essential lipid activators, was studied with purified MGlcDAG synthase in different lipid aggregates. Critical fractions of anionic phospholipids 1, 2-dioleoyl-phosphatidylglycerol (DOPG) and 1,2-dioleoyl-phosphatidylserine (DOPS) were essential for the restoration of enzyme activity, while the zwitterionic 1,2-dioleoyl-phosphatidylcholine (DOPC) and the uncharged diglucosyldiacylglycerol (DGlcDAG) were not. Proteolytic resistance had a very good correlation with the enzyme activity in various lipid-CHAPS mixed micelles. Anionic lipids DOPG and DOPS could protect the exposed MGlcDAG synthase from digestion, whereas DOPC and DGlcDAG could not. Similar features were observed in liposome bilayers. Likewise, the detergent dodecylphosphoglycerol (PGD), with a phosphatidylglycerol-like headgroup, could also stimulate the MGlcDAG synthase activity efficiently with a concomitant protection toward proteolytic digestion. Neither proteolytic resistance nor restored enzyme activity was observed using soluble glycerol 3-phosphate. It is concluded that in addition to critical amounts, both the negatively charged headgroup and hydrophobic chains of the activator amphiphiles, but not a certain aggregate curvature, seem necessary for a proper conformation and the resulting active state of the MGlcDAG synthase.

Lipid dependence and basic kinetics of the purified 1,2-diacylglycerol 3-glucosyltransferase from membranes of Acholeplasma laidlawii

UDP-glucose: 1,2-diacylglycerol 3-glucosyltransferase (EC 2.4.1.157), catalyzes the transfer of glucose from UDP-glucose to diacylglycerol (DAG) to yield monoglucosyldiacylglycerol (MGlcDAG) and UDP. MGlcDAG is the first glucolipid along the glucolipid pathway, and a major (nonbilayer-prone) lipid in the single membrane of Acholeplasma laidlawii. MGlcDAG is further glucosylated to give the major diglucosyldiacylglycerol (DGlc-DAG). The bilayer fractions of these lipids are crucial for the metabolic maintenance of phase equilibria close to a potential bilayer-nonbilayer transition and a nearly constant spontaneous curvature. The glucolipid syntheses are also balanced against the phosphatidylglycerol pathway, competing for the common minor precursor phosphatidic acid, to retain a constant lipid surface charge density. The 1,2-diacylglycerol 3-glucosyltransferase was purified to homogeneity from detergent-solubilized A. laidlawii cells by three column chromatography methods (enrichment approximately 9000 x), and identified as a minor 40-kDa protein by using SDS-polyacrylamide gel electrophoresis. In CHAPS detergent, mixed micelles, a cooperative dependence on anionic lipids for activity was confirmed. Dependence of the enzyme on UDP-glucose followed Michaelis-Menten kinetics while the other hydrophobic substrate dioleoylglycerol stimulated the enzyme by an activating, potentially cooperative mechanism. Physiological concentrations of the activator lipid dioleoyl-phosphatidylglycerol influenced the turnover number of the enzyme but not the interaction with UDP-glucose, as inferred from variable and constant values of the apparent Vmax and Km, respectively. Dipalmitoylglycerol was a better substrate than the oleoyl species, supporting earlier in vivo and crude enzyme data. The responses of the purified 1,2-diacylglycerol 3-glucosyltransferase indicated that (i) the regulatory features of the MGlcDAG synthesis is held by the catalytic enzyme itself, and (ii) this strongly corroborates the "homeostasis" model for lipid bilayer properties in A. laidlawii proposed earlier.

Toxicity of 20 Chemicals from the MEIC Programme Determined by Growth Inhibition of L-929 Fibroblast-like Cells

The Multicentre Evaluation of In vitro Cytotoxicity (MEIC) programme is an international project aimed at evaluating the relevance of in vitro tests in predicting human toxicity. We have screened 20 chemicals (MEIC codes 31-50) from the programme, by using a cytotoxicity test based on growth inhibition of the mouse fibroblast-like L-929 cell line. Inhibition of cell growth was determined by the neutral red uptake method, which is well established and is used for screening the cytotoxicity of chemicals and plastics for pharmaceuticals and medical devices. The concentrations causing 50% inhibition of cell growth after a 72-hour exposure period varied from 3.1μM for hexachlorophene, to 1.4mM for caffeine. This is within the same range as results recently obtained with five other cell models. However, with some chemicals (chloroform, carbon tetrachloride and dichloromethane), no reliable results were obtained. These substances could not be dissolved in a reproducible way in any of the solvents used and, furthermore, they were highly volatile, which led to difficulties in maintaining the concentrations.

Clinical and physiological effects of a new, less toxic and less acidic fluid for peritoneal dialysis

OBJECTIVE

To report our first clinical experience with a new continuous ambulatory peritoneal dialysis (CAPD) fluid (PD-Bio), which is nearly devoid of glucose degradation products and has a higher pH (6.3) than conventional peritoneal dialysis (PD) solutions, and to discuss in general terms some acute and long-term effects of conventional acidic solutions containing glucose degradation products.

DESIGN

1) Pilot study on 4 patients investigated using a modified peritoneal equilibration test (PET) and cytobiology parameters. 2) Computer simulation study, assuming that conventional acidic solutions cause vasodilatation and recruitment of capillary surface area initially (during 0-60 minutes) in a PD dwell.

PATIENTS

Four stable CAPD patients were chosen in an open cross-over study. After a period of three months using conventional PD fluid, the patients were switched to three months on the new PD fluid.

RESULTS

Cancer antigen 125 increased significantly, and patients with discomfort/infusion pain during the control period improved during the period with the new fluid. No significant changes were observed in mass-transfer coefficients or drained volumes with the new solution. PH in the effluent dialysis was, however, higher for PD-Bio at all times during a two-hour dwell. In the computer simulation study, a less acidic solution caused an initially lower rate of glucose dissipation and improved ultrafiltration (UF) after a four-hour dwell, as compared to a conventional PD solution.

CONCLUSIONS

A new, differently produced, less toxic and less acidic PD fluid (PD-Bio) seems to be better tolerated than a conventional acidic solution with respect to discomfort/infusion pain. Theoretically, neutralized solutions should show slightly improved UF profiles over conventional acidic solutions, according to the computer simulation analysis. Furthermore, it is speculated that a neutral, less acidic, less toxic fluid would cause less interstitial-mesothelial alterations and less impairment of UF capacity than conventional solutions during long-term CAPD.

Correlation between bilayer lipid dynamics and activity of the diglucosyldiacylglycerol synthase from Acholeplasma laidlawii membranes

In the single membrane of Acholeplasma laidlawii a specific glucosyltransferase synthesize the major, lamellar-forming lipid diglucosyldiacylglycerol (DGlcDAG) from the major, nonlamellar-prone monoglucosyldiacylglycerol (MGlcDAG). This is crucial for the maintenance of phase equilibria close to a bilayer-nonbilayer transition and a nearly constant spontaneous curvature in the membrane lipid bilayer. Acyl chain order is also affected, but not kept constant. Phosphatidylglycerol (PG) is an essential activator, needed in substantial amounts by the DGlcDAG synthase, and likely to affect bilayer properties. A potential connection was investigated between the (i) lateral diffusion, (ii) domain formation of the PG activator and (iii) bilayer chain ordering (i.e., the hydrocarbon free volume), revealed in unilamellar liposomes by lipid probes containing one or two (fluorescent) pyrene acyl chains, and (iv) activity of the DGlcDAG synthase. Different activator, nonbilayer perturbant, and bilayer matrix conditions were employed. Diffusion of PG was substantially slower in a DGlcDAG compared to a phosphatidylcholine (PC) matrix with 18:1c chains but increased with the PG content in both. No obvious correlation between diffusion and enzyme activity, and no local concentration of PG as a function of chain ordering or curvature, was detected. However, an enrichment of PG activator into domains could be induced by a chain length mismatch between 18:1c-PG and 14:1c-PC (but not 22:1c-PC), even at small PG fractions. Patching was sufficient to stimulate enzyme activity 4-fold in relation to the activities normally valid at low PG concentrations. Chain order was substantially lower (i.e., free volumes larger) in bilayers of DGlcDAG than in bilayers of PC and increased in an additive fashion in both by the content of especially the nonbilayer-prone 1,3-18:1c-DAG but also by PG. At physiological concentrations of PG in DGlcDAG bilayers (approximately 20%) a good correlation was evident between increased DAG content and chain ordering and strongly enhanced enzyme activities, with maxima close to a bilayer-nonbilayer transition. It is concluded that regulation of packing conditions in A. laidlawii membranes by the DGlcDAG synthase seems to be governed not by the absolute extent of chain order but more by the spontaneous curvature within a certain range of conditions. Domain formation of the essential PG activator due to bilayer conditions is a second mechanism, potentially overriding the curvature effects.

Efficient modulation of glucolipid enzyme activities in membranes of Acholeplasma laidlawii by the type of lipids in the bilayer matrix

It is generally anticipated, but so far not fully shown, that the physical properties of membrane lipid bilayers are governed by the concerted actions of the lipid-synthesizing enzymes. In the membrane of Acholeplasma laidlawii a constant surface charge density, similar phase equilibria, and a nearly constant spontaneous curvature are maintained for the polar lipids. Important for these properties are monoglucosyldiacylglycerol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG), forming mainly reversed nonlamellar and lamellar phases, respectively. The syntheses of these lipids (from 1,2-DAG and MGlcDAG) by two consecutively acting, membrane-bound glucosyltransferases have been analyzed in synthetic lipid bilayers of selected physical properties. Both enzymes demanded the presence of activator lipids; for MGlcDAG synthesis a critical fraction of anionic lipids was important, whereas for the DGlcDAG synthesis substantial amounts of a liquid-crystalline phosphatidylglycerol (PG) with a certain chain length were essential. The rates of the syntheses for the two glucolipids increased with decreasing chain length of the DAG and MGlcDAG substrates. The enzymatic formation of DGlcDAG (bilayer-forming) was influenced in a dose-dependent manner by the nonbilayer (curvature) propensities of several amphiphilic and hydrophobic lipids in two different bilayer matrixes. However, the preceding synthesis of the nonlamellar MGlcDAG was only affected to a minor extent by such additives. The mechanism for modulation involved an enhancement of the activating potencies of PG in a cooperative fashion at physiological concentrations for PG.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of foreign genes and selection of promoter sequences in Acholeplasma laidlawii

The stable maintenance and expression of foreign genes in mollicutes (mycoplasmas) have been difficult to achieve due to the lack of suitable vectors. In this paper we show for the first time that a replicating vector can been used to express foreign genes other than antibiotic resistance genes in Acholeplasma laidlawii. Plasmids derived from the lactococcal vector pNZ18 could introduce and maintain four different genes for many generations in A. laidlawii. One of these, encoding the dominant membrane lipoprotein spiralin from the mollicute Spiroplasma citri, was expressed; however, expression was weak, the signal peptide of spiralin was not cleaved and the protein was not covalently modified by fatty acids. This resulted in a hydrophilic character of spiralin and its cytoplasmic localization in A. laidlawii. To increase the expression of foreign genes, random A. laidlawii DNA fragments were cloned into a pNZ18-related plasmid and expression signals were selected using the Bacillus licheniformis alpha-amylase gene as a probe. Selection was done in Escherichia coli as well as directly in A. laidlawii. Active recombinants from E. coli were also able to express alpha-amylase activities and an enzyme of native size in A. laidlawii. The highest activity was obtained from a recombinant selected directly in A. laidlawii. This is the first example of a promoter sequence selected in a mollicute. Analysis of the putative promoters in seven clones revealed similar -10 and -35 regions, and similar spacer distances in A. laidlawii, Acholeplasma oculi, Lactococcus and E. coli. Vectors related to pNZ18 should be useful for the genetic analysis of specific A. laidlawii proteins and functions.

Lipid extracts from membranes of Acholeplasma laidlawii A grown with different fatty acids have a nearly constant spontaneous curvature

X-ray diffraction methods were used to explore the variation in the spontaneous curvature of lipid extracts from Acholeplasma laidlawii strain A-EF22 grown with different mixtures of palmitic acid and oleic acid. It was shown that the cells respond to the different growing conditions by altering the polar head group compositions in order to keep the phase transition between lamellar and nonlamellar structures within a narrow temperature range. This has been interpreted to mean that the membrane lipids are adjusted toward an optimal packing (Lindblom et al. (1986) Biochemistry 25, 7502). Here it is shown that for these extracts, the membrane curvature is kept within a narrow range (58-73 A), compared to the range in curvatures exhibited by pure lipids extracts from the membrane (17-123 A). These observations support the hypothesis (Gruner (1989) J. Phys. Chem. 93, 7562) that the spontaneous curvature is a functionally important membrane parameter which is regulated by the organism and is likely to be one of the constraints controlling the lipid composition of the bilayer.

Membrane lipid composition and cell size of Acholeplasma laidlawii strain A are strongly influenced by lipid acyl chain length

The small, cell-wall-less prokaryote Acholeplasma laidlawii strain A-EF22 could grow with membrane lipids having an average acyl chain length Cn varying over 14.5- almost 20 carbons by exogenous supplementation with selected fatty acids. For 16 < Cn < 18, the cells grew with lipids containing 100% (mol/100 mol) monounsaturated acyl chains, whereas for Cn < 16 and Cn > 18, cell growth only occurred with gradually lower fractions of unsaturated chains. Cn was actively increased and decreased by chain elongation or de novo fatty acid synthesis upon incorporation of short-chain and long-chain fatty acids, respectively. The membrane lipid composition was strongly affected by the acyl chain length and unsaturation, and the metabolic responses are readily explained as a regulation mechanism based on the established phase equilibria of the individual lipids in the A. laidlawii membrane. Monoglucosyldiacylglycerol (Glc-acyl2-Gro) was the dominating lipid with short chains but the fraction of this lipid decreased with increasing Cn, correlating with the decreasing lamellar to nonlamellar phase transition temperatures for this lipid. The fractions of diglucosyldiacylglycerol (Glc2-acyl2Gro) and phosphatidylglycerol (PtdGro), forming lamellar phases only, increased with increasing Cn over the entire chain-length interval. A weaker correlation was usually observed between the relative amount of a lipid and the extent of chain unsaturation; however, the fractions of Glc2-acyl2Gro and PtdGro increased clearly with an increasing degree of unsaturation. Moreover, the synthesis of the nonbilayer-forming lipids acyl2Gro and monoacyl-Glc-acyl2Gro was strongly stimulated by a high degree of chain saturation. Concomitantly, the phase equilibria of Glc-acyl2Gro are shifted towards lamellar phases at the growth temperature. The fraction of the three potentially nonbilayer-forming lipids varied over 10-80% (mol/100 mol) total lipids as a function of the acyl chain composition. The combined molar fractions of the three phospholipids increased strongly with chain unsaturation. However, the fraction of phosphate moieties in the different lipids was constant over the entire chain-length interval. It is concluded that the regulation of the membrane lipid composition aims at maintaining similar phase equilibria and surface charge densities of the lipid bilayer. The size of A. laidlawii cells was changed in a systematic manner and correlated qualitatively with the packing properties of the lipids. Cell diameters were increased by an increase in acyl chain length and saturation, and was affected by additives such an n-dodecane and acyl2Gro.

Membrane thickness and molecular ordering in Acholeplasma laidlawii strain A studied by 2H NMR spectroscopy

Since Acholeplasma laidlawii can be restricted to incorporating fatty acids from the growth medium into its membrane lipids, it is possible to study the effects of the length of the acyl chains on the properties of the membrane of the organism. A. laidlawii strain A-EF22 was grown with mixtures of one perdeuterated saturated fatty acid and one monounsaturated fatty acid. The average length (<Cn>) of the acyl chains in the membrane lipids varied from 14.6 to 19.9, and the degree of unsaturation ranged from 21 to 79 mol %. 2H nuclear magnetic resonance (NMR) spectra were recorded on whole cells, on intact membranes, and on lipids extracted from these membranes. It was found that the NMR spectra for all three cases were very similar, yielding deuterium quadrupolar splittings typical for the lamellar liquid-crystalline phase (L alpha) found in model membrane systems. The use of a perdeuterated acyl chain as a reporter molecule allowed for the calculation of order parameters averaged over the entire system. These measurements yielded a wide range of average order parameters varying from 0.136 to 0.186 for the membranes and from 0.137 to 0.181 for the extracted lipids. From the order parameters the average acyl chain length can be calculated, which is related to the average membrane thickness. This value ranged from 23.2 to 30.6 A. When either the order or the membrane thickness of the intact membranes was compared to that of the extracted lipids, only slight or even undetectable differences were found. This implies that the proteins associated with the membranes do not have any large effect on the overall packing of the membrane lipids, even though the membrane thickness varied by approximately 8 A over the series studied. A decrease in the ordering of the acyl chains was observed when the length of the acyl chains incorporated from the growth medium was increased in either the membranes or the extracted lipids. This decrease correlated with the decrease in the fraction of monoglucosyldiacylglycerol (MGlcDAG) found in the membrane. Since both the average order and the membrane thickness varied, it is proposed that by changing the mole fraction of MGlcDAG the organism regulates either the membrane curvature energy or the permeability, both of which are related to lipid packing in the bilayer.

Activation of the membrane glucolipid synthesis in Acholeplasma laidlawii by phosphatidylglycerol and other anionic lipids

In membrane lipids of the prokaryote Acholeplasma laidlawii similar phase equilibria and a nearly constant spontaneous curvature are maintained by an extensive metabolic regulation of especially the major polar lipids monoglucosyldiacylglycerol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG), forming nonlamellar and lamellar phases, respectively. A constant surface charge density is maintained by the anionic phospholipid fraction. These lipids are synthesized from phosphatidic acid in two competing pathways. The in vitro synthesis of MGlcDAG and DGlcDAG were totally lost upon delipidation of the membrane proteins by detergent solubilization or solvent extraction of lyophilized cells. Activities were restored by critical concentrations of anionic lipids, but not by bilayer or nonbilayer zwitterionic phospholipids or glucolipids. Phosphatidylglycerol (PG), and to a lesser extent certain other anionic lipids, could activate the synthesis of MGlcDAG in lipid bilayers, whereas the synthesis of DGlcDAG was similarly dependent upon PG only. Two endogenous phosphoglucolipids with no activating potency could partially replace the PG activator for the MGlcDAG synthesis but less so for DGlcDAG formation. A change of inert matrix from phosphatidylcholine to DGlcDAG lowered the apparent cooperativity, but enhanced the efficiency, of activation by PG for both glucolipid synthesizing enzymes, most strongly the synthesis of DGlcDAG. These results indicate that the enzymatic formation of MGlcDAG is regulated by the lipid surface charge density, whereas the consecutive step to DGlcDAG is more dependent upon the specific properties of PG. The modulating effect of the surrounding matrix on the activator efficiencies and cooperativity may constitute part of the bilayer-nonbilayer lipid regulation mechanism.

Similar regulatory mechanisms despite differences in membrane lipid composition in Acholeplasma laidlawii strains A-EF22 and B-PG9. A multivariate data analysis

Mycoplasmas are small, cell wall-deficient bacteria. The metabolic regulation of the lipid composition in the membrane of the species Acholeplasma laidlawii, strains A-EF22 and B-JU, is governed mainly by the balance between the potential formation of lamellar and nonlamellar phase structures. However, the regulatory features have not been consistently observed in the B-PG9 strain. A comparison has been performed between the membrane lipid composition for strains A-EF22 and B-PG9, simultaneously changing eight experimental conditions known to affect the regulation and packing properties of the A-EF22 lipids. Multiple regression and partial least-square discriminant analyses of many variables showed: (i) quantitative differences in membrane lipid and protein composition, and in membrane protein molecular masses of the two strains; (ii) different molar fractions of the major polar lipids monoglucosyldiacylglycerol (nonlamellar) and diglucosyldiacylglycerol (lamellar), which were caused by differences in lipid acyl chain length and unsaturation inherent in the strains and by the type of growth medium used; and (iii) similar regulatory mechanisms for changes in the lipid composition under most conditions, responding to the experimentally varied bilayer and nonbilayer properties of the lipid matrix. These regulatory principles are probably valid in other bacteria as well.

Membrane lipid regulation in Acholeplasma laidlawii grown with saturated fatty acids. Biosynthesis of a triacylglucolipid forming reversed micelles

The membrane lipid composition in several strains of Acholeplasma laidlawii is regulated upon a change in the growth conditions. Monoglucosyldiacylglycerol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG) are the most abundant lipids in the A. laidlawii membrane. A third glucolipid, 3-O-acyl-monoglucosyldiacylglycerol (MAMGlcDAG) is synthesized by strain A-EF22 when the membrane lipids contain large amounts of saturated acyl chains. The lipid regulation can be understood from a simple theoretical model, in which the cells strive to maintain a balance between the lipids constituting a bilayer and those forming reversed non-lamellar liquid crystalline phases. Thus, the physical chemistry of membrane lipids, in particular their ability to form different aggregate structures, constitutes the basis for the lipid regulation, and therefore an understanding of the phase equilibria of membrane lipids is crucial. MGlcDAG and MAMGlcDAG isolated from A. laidlawii strain A-EF22 membranes were studied mainly by 2H NMR, 1H NMR, and 1H NMR diffusion measurements. MAMGlcDAG, containing 96 mol % saturated acyl chains formed a gel/crystalline phase up to about 80 degrees C, where a transition occurred to a reversed micellar (L2) phase. This is an unexpected finding for a membrane lipid. However, this lipid homogeneously mixes with the other membrane lipids at physiological temperatures. Previous and new data on MGlcDAG show that the lamellar phase is stabilized when the length and the degree of unsaturation of the acyl chains are decreased. The physicochemical properties of MAMGlcDAG and MGlcDAG were compared and found to be of great significance for the physiological regulation of the lipids in the membrane. MAMGlcDAG is synthesized under conditions when the phase equilibria of MGlcDAG are shifted from a non-lamellar toward a lamellar phase. Apart from MAMGlcDAG, MGlcDAG is the major lipid in A. laidlawii strain A-EF22 which is able to form reversed aggregate structures. MAMGlcDAG probably assists MGlcDAG in maintaining an optimal molecular packing, or negative curvature, of the lipids in the membrane.

Toxicity of heat sterilized peritoneal dialysis fluids is derived from degradation of glucose

Heat sterilization makes peritoneal dialysis (PD) solutions cytotoxic. Two compounds in the solutions, lactate and glucose, can be degraded by heat. This study's goal was to discover which of the compounds was responsible for the cytotoxicity. The influence of sterilization temperature on degradation of the compounds was also subjected to investigation. Solutions of glucose and lactate and a mixture of lactate and glucose were prepared. These were sterilized in glass ampules in an oil bath at different temperatures for varying times. Toxicity was determined as inhibition of cell growth with a fibroblast cell line (L929), and ultraviolet (UV) absorbance was measured at 284 nm. Lactate solutions did not show cytotoxicity after heat sterilization. Glucose solutions that were heat sterilized showed an increase in UV absorbance at 284 nm and were cytotoxic. The mixture of lactate and glucose exhibited the same cytotoxicity as glucose alone. Lower sterilization temperatures lead to increased cytotoxicity and an increase in UV absorbance at 284 nm. Results indicate that the toxic products formed during heat sterilization of PD fluids are derived from glucose.

Isoprenoid modification of proteins distinct from membrane acyl proteins in the prokaryote Acholeplasma laidlawii

Isoprenylation is an important posttranslational modification that affects the activity, subunit interactions and membrane anchoring of different eukaryotic proteins. The small, cell-wall-less prokaryote Acholeplasma laidlawii has more than 20 membrane acyl-proteins enriched in myristoyl and palmitoyl chains. Radioactive mevalonate, a precursor to isoprenoids, was incorporated into several specific membrane proteins of 20 to 45 kDa and two soluble proteins of 23-25 kDa, respectively. No acyl proteins and none of the polar acyl lipids became labelled but these are all labelled by radioactive fatty acids. Mevalonate was incorporated mainly into a minor neutral, non-saponifiable lipid which migrated just above a C30-isoprenoid (squalene) on TLC-plates. The isoprenoid chains could not be released by mild alkaline hydrolysis from most of the isoprenylated proteins, although this procedure releases acyl chains from lipids and all acylated proteins. Isoprenylated proteins were enriched in the detergent phase upon partition with the non-ionic detergent Triton X-114. This behaviour is similar to the acyl proteins of this organism and indicates that the isoprenoid chains give the proteins a hydrophobic character.

The enzymatic synthesis of membrane glucolipids in Acholeplasma laidlawii

In membranes of the prokaryote Acholeplasma laidlawii, the physiological regulation of the two major membrane lipids, monoglucosyldiacylglycerol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG), is governed by factors affecting the equilibria between lamellar and non-lamellar phases of the membrane lipids. The synthesis of the glucolipids is considered to be a two-step glucosylation: (i) DAG+UDP-Glc----MGlcDAG+UDP; and (ii) MGlcDAG+UDP-Glc----DGlcDAG+UPD. This was corroborated by in vivo pulse labelling experiments showing turnover of MGlcDAG but not DGlcDAG. The enzymatic synthesis of MGlcDAG was localized to fresh or freeze-dried membranes in vitro. Synthesis of DGlcDAG was minor in such membranes but of substantial magnitude in intact cells. Synthesis of MGlcDAG was stimulated by small amounts of SDS but completely inhibited upon solubilization of the membranes by a variety of detergents. The inhibitory effect of several UDP-Glc analogs on glucolipid synthesis demonstrated the importance of UDP-Glc as the sugar donor. Synthesis of both glucolipids was lost in freeze-dried plus lipid-extracted cells but restored when lipids were transferred back to the extracted cell membrane. By selectively adding specific lipids, a strong dependence on the acceptor lipid DAG, as well as the need for general matrix lipids for enzyme activity, was established. In addition, the anionic phosphatidylglycerol (PG), but not the other phospholipids, had a strong stimulatory effect. The presence of different phosphorylating agents stimulated the synthesis of DGlcDAG and partially inhibited that of MGlcDAG. This, together with the lipid dependency, may constitute mechanisms for the regulation of the enzyme activities in vivo.

Membrane protein acylation. Preference for exogenous myristic acid or endogenous saturated chains in Acholeplasma laidlawii

Mycoplasmas are small bacteria without a cell wall, often found as surface parasites on eukaryotic cells. Of the more than 200 membrane proteins from Acholeplasma laidlawii resolved by two-dimensional PAGE, 23 were covalently modified with acyl chains. These acyl proteins had lower pI values than average and were all labelled by different exogenously supplied radioactive fatty acids attached by O-ester bonds. The fatty acids were selectively incorporated in the order myristic acid (14:0) greater than palmitic acid (16:0) greater than stearic acid (18:0) greater than oleic acid (18:1). However, endogenously synthesised saturated fatty acids, most of which were 16:0, were preferred over the supplied ones. A fraction of the exogenous 14:0 was elongated to 16:0. Absence of saturated fatty acids increased the incorporation of 18:1. The maximum extent of modification was one acyl chain for protein T2, on the exterior surface and two acyl chains for protein D12, spanning them membrane. Exogenously supplied fatty acids were incorporated into membrane lipids in proportion to their occurrence. However, the acylated proteins always contained 8-10 times more saturated chains than did the lipids. When exogenously supplied, all A. laidlawii polar membrane lipids could donate acyl chains to the acylated proteins but the neutral fraction (fatty acids and diacylglycerol) was most efficient. An incorporation into the acylated proteins of labelled cysteine, but not glucose or glycerol, was observed. Acylated proteins with different chains interacted similarly with a Triton X-114 detergent phase, and no full-size proteins (or acylated fragments) were released from cells by proteolytic enzymes. The results indicate an anchoring with peptide segments in addition to the acyl chains. Both 14:0 and 16:0 were attached at one end of both T2 and D12, but the N-terminal methionine of T2 was not acylated. The extent of modification and preference for saturated chains in the A. laidlawii membrane acylated proteins is more similar to eukaryotic than to eubacterial proteins.

Identification and analysis of the genes coding for the putative pyruvate dehydrogenase enzyme complex in Acholeplasma laidlawii

A monospecific antibody recognizing two membrane proteins in Acholeplasma laidlawii identified a plasmid clone from a genomic library. The nucleotide sequence of the 4.6-kbp insert contained four sequential genes coding for proteins of 39 kDa (E1 alpha, N terminus not cloned), 36 kDa (E1 beta), 57 kDa (E2), and 36 kDa (E3; C terminus not cloned). The N termini of the cloned E2, E1 beta, and native A. laidlawii E2 proteins were verified by amino acid sequencing. Computer-aided searches showed that the translated DNA sequences were homologous to the four subenzymes of the pyruvate dehydrogenase complexes from gram-positive bacteria and humans. The plasmid-encoded 57-kDa (E2) protein was recognized by antibodies against the E2 subenzymes of the pyruvate and oxoglutarate dehydrogenase complexes from Bacillus subtilis. A substantial fraction of the E2 protein as well as part of the pyruvate dehydrogenase enzymatic activity was associated with the cytoplasmic membrane in A. laidlawii. In vivo complementation with three different Escherichia coli pyruvate dehydrogenase-defective mutants showed that the four plasmid-encoded proteins were able to restore pyruvate dehydrogenase enzyme activity in E. coli. Since A. laidlawii lacks oxoglutarate dehydrogenase and most likely branched-chain dehydrogenase enzyme complex activities, these results strongly suggest that the sequenced genes code for the pyruvate dehydrogenase complex.

Order and dynamics in mixtures of membrane glucolipids from Acholeplasma laidlawii studied by 2H NMR

The two dominant glucolipids in Acholeplasma laidlawii, viz., 1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol (MGlcDG) and 1,2-diacyl-3-O-[alpha-D-glucopyranosyl-(1----2)-O-alpha-D-glucopyranosyl ]- sn-glycerol (DGlcDG), have markedly different phase behavior. MGlcDG has an ability to form nonlamellar phases, whereas DGlcDG only forms lamellar phases. For maintenance of a stable lipid bilayer, the polar headgroup composition in A. laidlawii is metabolically regulated in vivo, in response to changes in the growth conditions [Wieslander et al. (1980) Biochemistry 19, 3650; Lindblom et al. (1986) Biochemistry 25, 7502]. To investigate the mechanism behind the lipid regulation, we have here studied bilayers of mixtures of unsaturated MGlcDG and DGlcDG, containing a small fraction of biosynthetically incorporated perdeuterated palmitic acid, with 2H NMR. The order-parameter profile of the acyl chains and an apparent transverse spin relaxation rate (R2) were determined from dePaked quadrupole-echo spectra. The order of the acyl chains in DGlcDG-d31 increases upon addition of protonated MGlcDG, whereas the order of MGlcDG-d31 decreases when DGlcDG is added. The variation of order with lipid composition is rationalized from simple packing constraints. R2 increases linearly with the square of the order parameter (S2) up to S approximately 0.14; then, R2 goes through a maximum and decreases. The increase in R2 with S2, as well as the magnitude of R2, is largest for pure MGlcDG-d31, smallest for DGlcDG-d31, and similar for mixtures with the same molar ratio of MGlcDG/DGlcDG but with the deuterium label on different lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasmid transformation and replica filter plating of Acholeplasma laidlawii

The restriction deficient mutant 8195 of Acholeplasma laidlawii strain JA1 was transformed by the promiscuous streptococcal plasmid vector pNZ18 at a frequency of 4 x 10(-4)/cfu. The plasmid was maintained without structural rearrangements but was lost in the absence of a selection pressure, i.e. kanamycin or neomycin. Transformed primary colonies were easily recognized due to a different colony morphology. Replica filter plating, previously not obtained with mycoplasmas, was achieved using pNZ18 as a marker by incubating the replica filters with the cell side down on the new agar plates. These findings should greatly facilitate the genetic and functional analysis of A. laidlawii.

Amino acid stimulation of Na,K-ATPase activity in rat proximal tubule after high-protein diet

Ouabain-sensitive (OS) O2 consumption was determined in proximal tubular cells from weanling rats fed 21% (normal-protein, NP) or 50% (high-protein, HP) protein diet for 4 days. Butyric acid 10(-3)M was added as a substrate for mitochondrial respiration and the ionophore amphotericin B (10 micrograms ml-1) was used to sodium-load the cells. OS respiration was higher in HP than in NP cells in both DME and amino acid-free electrolyte solution (ES). Amphotericin B significantly increased OS respiration in both NP and HP cells, implying that the Na-K pump was activated by increased intracellular Na. In cells incubated in ES, addition of amino acids stimulated OS respiration significantly in HP cells (16.9 +/- 1.4 vs 21.2 +/- 1.1 nmol min-1 mg-1 protein) and in NP cells (13.9 +/- 0.3 vs 14.9 +/- 0.6 nmol min-1 mg-1 protein). Stimulation was significantly higher in HP cells (26 +/- 4%) than in NP cells (7 +/- 4%) (P less than 0.001). The amino acids did not stimulate ouabain-insensitive respiration. The results indicate that an HP diet to weanling rats will increase proximal tubule cell Na, K-ATPase-dependent respiration by enhancing Na entry via the Na-amino acid symports.

Use of cell culture to predict toxicity of solid materials in blood contact

A comparison was made, for in vitro assessment of toxicity, between six different procedures to extract biomaterials. A new method was developed, with a special insert inside the tissue culture well, that allows relevant contact between cells and material without disturbing physical contact. To predict the toxicity of materials in blood contact, our results suggest that two simple methods ought to be used, one based on water eluates and the other on contact between cells and material.

Topology and acylation of spiralin

Of the 51 polypeptides detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the plasma membrane of the helical mollicute Spiroplasma melliferum, 21 are acylated, predominantly with myristic (14:0) and palmitic (16:0) chains. This is notably the case for spiralin, the major membrane protein of this bacterium, which contains an average of 0.7 acyl chains per polypeptide, attached very probably by ester bonds to alcohol amino acids. The amphiphilicity of spiralin was demonstrated by the behavior of the protein in charge-shift electrophoresis, its incorporation into liposomes, and its ability to form in the absence of lipids and detergents, globular protein micelles (diameter, approximately 15 nm). The presence of epitopes on the two faces of the cell membrane, as probed by antibody adsorption and crossed immunoelectrophoresis, and the strong interaction between spiralin and the intracytoplasmic fibrils show that spiralin is a transmembrane protein. The mean hydropathy of the amino acid composition of spiralin (-0.30) is on the hydrophilic side of the scale. Surprisingly, the water-insoluble core of spiralin micelles, which is the putative membrane anchor, has a still more hydrophilic amino acid composition (mean hydropathy, -0.70) and is enriched in glycine and serine residues. Taking into account all these properties, we propose a topological model for spiralin featuring a transbilayer localization with hydrophilic domains protruding on the two faces of the membrane and connected by a small domain embedded within the apolar region of the lipid bilayer. In this model, the membrane anchoring of the protein is strengthened by a covalently bound acyl chain.

Cloning and expression of Acholeplasma laidlawii membrane acyl proteins in Escherichia coli

Many integral membrane proteins in Acholeplasma laidlawii are enriched in hydrophilic amino acid residues and covalently modified with fatty acids. In order to understand how these proteins are inserted and anchored in the bilayer, we have cloned several of the major A. laidlawii proteins in Escherichia coli: 900 recombinant clones containing 4-kbp DNA fragments, inserted into the BamHI site of the plasmid pAT 153, were screened with antibodies. With antimembrane antibodies, 26 positive clones were detected, and with a mixture of five different monospecific antibodies, another 7 clones were obtained. Immunological analysis of the colonies in situ verified that antigens for A. laidlawii membrane proteins D12, T2, T3, T4a, and unidentified proteins were produced in separate clones. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoblotting showed several fragments [25 to 94 kilodalton (kDa)] for each of these proteins, some of which were even visible on Coomassie Blue-stained gels. It is concluded that A. laidlawii membrane proteins can be efficiently expressed in E. coli.

Amphiphilic properties of spiralin, the major surface antigen of spiroplasmas. A preliminary report

Spiralin, a 28-kDa (kilodalton) polypeptide, is the major antigen of Spiroplasma citri and S. melliferum in which it usually represents more than 20% of total membrane protein. The amino acid compositions of the spiralins purified from both spiroplasma species unambiguously show that these proteins are homologous. In addition, several lines of evidence indicate that such a protein is present in the menbrane of S. apis. A 25-kDa polypeptide antigenically related to S. citri spiralin has also been purified from the membrane of the nonhelical variant ASP-1. The spiralin of S. melliferum B88 has been used as a model for extensive characterization. This antigen binds detergent under nondenaturing conditions, can be incorporated into liposomes, and forms protein micelles upon gentle removal of detergent. Digestion of the micelles with trypsin leads to the precipitation of an insoluble material containing a major polypeptide of 3.9 kDa. The amino acid composition of this fragment is different from that of intact spiralin. It is highly enriched in glycine and serine and, as an insoluble peptide, exhibits an unexpectedly high polarity index (PI = 51.4%). Screening for acyl proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunodetection in the membrane of S. melliferum indicates that spiralin is actually acylated. This set of properties is evidence that spiralin is an intrinsic membrane protein and strongly suggests that acylation triggers or facilitates integration of the molecule into the lipid bilayer of the spiroplasma membrane.