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.