Membrane lipid biosynthesis in Acholeplasma laidlawii B: incorporation of exogenous fatty acids into membrane glyco- and phospholipids by growing cells

Nutritional effects of culture media on mycoplasma cell size and removal by filtration

Careful media filtration prior to use is an important part of a mycoplasma contamination prevention program. This study was conducted to increase our knowledge of factors that influence efficient filtration of mycoplasma. The cell size of Acholeplasma laidlawii was measured after culture in various nutritional conditions using scanning electron microscopy. The maximum cell size changed, but the minimum cell size remained virtually unchanged and all tested nutritional conditions resulted in a population of cells smaller than 0.2 microm. Culture in Tryptic Soy Broth (TSB) resulted in an apparent increase in the percentage of very small cells which was not reflected in increased penetration of non-retentive 0.2 microm rated filters. A. laidlawii cultured in selected media formulations was used to challenge 0.2 microm rated filters using mycoplasma broth base as the carrier fluid. We used 0.2 microm rated filters as an analytical tool because A. laidlawii is known to penetrate 0.2 microm filters and the degrees of penetration can be compared. Culture of A. laidlawii in TSB resulted in cells that did not penetrate 0.2 microm rated filters to the same degree as cells cultured in other media such as mycoplasma broth or in TSB supplemented with 10% horse serum.

Observation of CO(2) in Fourier transform infrared spectral measurements of living Acholeplasma laidlawii cells

In monitoring the time course of conformational disorder by Fourier transform infrared spectroscopy for intact Acholeplasma laidlawii cells grown at 37 degrees C on binary fatty acid mixtures containing oleic acid and for cells grown on pure palmitic acid, an absorption band at 2343 cm(-1) was observed. The band intensity was found to increase with time. This band was not observed in the spectra for isolated membranes. It is suggested that the 2343 cm(-1) band is due to CO(2) dissolved in water, most likely produced at the final point of fermentation of amino acid by this microorganism.

The comparative metabolism of the mollicutes (Mycoplasmas): the utility for taxonomic classification and the relationship of putative gene annotation and phylogeny to enzymatic function in the smallest free-living cells

Mollicutes or mycoplasmas are a class of wall-less bacteria descended from low G + C% Gram-positive bacteria. Some are exceedingly small, about 0.2 micron in diameter, and are examples of the smallest free-living cells known. Their genomes are equally small; the smallest in Mycoplasma genitalium is sequenced and is 0.58 mb with 475 ORFs, compared with 4.639 mb and 4288 ORFs for Escherichia coli. Because of their size and apparently limited metabolic potential, Mollicutes are models for describing the minimal metabolism necessary to sustain independent life. Mollicutes have no cytochromes or the TCA cycle except for malate dehydrogenase activity. Some uniquely require cholesterol for growth, some require urea and some are anaerobic. They fix CO2 in anaplerotic or replenishing reactions. Some require pyrophosphate not ATP as an energy source for reactions, including the rate-limiting step of glycolysis: 6-phosphofructokinase. They scavenge for nucleic acid precursors and apparently do not synthesize pyrimidines or purines de novo. Some genera uniquely lack dUTPase activity and some species also lack uracil-DNA glycosylase. The absence of the latter two reactions that limit the incorporation of uracil or remove it from DNA may be related to the marked mutability of the Mollicutes and their tachytelic or rapid evolution. Approximately 150 cytoplasmic activities have been identified in these organisms, 225 to 250 are presumed to be present. About 100 of the core reactions are graphically linked in a metabolic map, including glycolysis, pentose phosphate pathway, arginine dihydrolase pathway, transamination, and purine, pyrimidine, and lipid metabolism. Reaction sequences or loci of particular importance are also described: phosphofructokinases, NADH oxidase, thioredoxin complex, deoxyribose-5-phosphate aldolase, and lactate, malate, and glutamate dehydrogenases. Enzymatic activities of the Mollicutes are grouped according to metabolic similarities that are taxonomically discriminating. The arrangements attempt to follow phylogenetic relationships. The relationships of putative gene assignments and enzymatic function in My. genitalium, My. pneumoniae, and My. capricolum subsp. capricolum are specially analyzed. The data are arranged in four tables. One associates gene annotations with congruent reports of the enzymatic activity in these same Mollicutes, and hence confirms the annotations. Another associates putative annotations with reports of the enzyme activity but from different Mollicutes. A third identifies the discrepancies represented by those enzymatic activities found in Mollicutes with sequenced genomes but without any similarly annotated ORF. This suggests that the gene sequence is significantly different from those already deposited in the databanks and putatively annotated with the same function. Another comparison lists those enzymatic activities that are both undetected in Mollicutes and not associated with any ORF. Evidence is presented supporting the theory that there are relatively small gene sequences that code for functional centers of multiple enzymatic activity. This property is seemingly advantageous for an organism with a small genome and perhaps under some coding restraint. The data suggest that a concept of "remnant" or "useless genes" or "useless enzymes" should be considered when examining the relationship of gene annotation and enzymatic function. It also suggests that genes in addition to representing what cells are doing or what they may do, may also identify what they once might have done and may never do again.

Fungitoxicity of fatty acids against dermatophytes

Ten saturated and three unsaturated fatty acids over a wide spectrum of their carbon chain length were tested for their fungitoxicity against five dermatophytes. The experiments showed that short chain saturated fatty acids (C 7:0-C 11:0) are more toxic to dermatophytes than long chain ones (> C 12:0). The antifungal activity decreases with increasing carbon chain length in the even-numbered carbon chain series. Odd-numbered carbon fatty acids show irregularities in these respects. Undecanoic acid (C 11:0) is outstandingly most toxic in the C 7:0-C 18:0 series. Polyunsaturated fatty acids are more toxic than their corresponding saturated acids. The per cent growth inhibition decreased from day 4 to day 12 at all concentrations suggesting the degradation and/or utilization of fatty acids by the fungi during the observation period. The results are discussed on the background of the biochemistry of fatty acids.

The effect of variations in growth temperature, fatty acid composition and cholesterol content on the lipid polar head-group composition of Acholeplasma laidlawii B membranes

We have systematically investigated the effect of variations in growth temperature, fatty acid composition and cholesterol content on the membrane lipid polar headgroup composition of Acholeplasma laidlawii B. Two important lipid compositional parameters have been determined from such an analysis. The first parameter studied was the ratio of the two major neutral glycolipids of this organism, monoglucosyldiacylglycerol (MGDG) and diglucosyldiacylglycerol (DGDG). As the former lipid prefers to exist in a reversed hexagonal phase at higher temperatures, with unsaturated fatty acyl chains or in the presence of cholesterol, the ratio of these two lipids reflects the phase state preference of the total A. laidlawii membrane lipids. Although we find that the MGDG/DGDG ratio is reduced in response to an increase in fatty acid unsaturation, increases in growth temperature or cholesterol content reduce this ratio only in cells enriched in a saturated but not an unsaturated fatty acid. The second parameter studied was the ratio of these neutral glycolipids to the only phosphatide in the A. laidlawii membrane, phosphatidylglycerol (PG); this parameter reflects the relative balance of uncharged and charged lipids in the membrane of this organism. We find that the MGDG + DGDG/PG ratio is lowest in cells enriched in the saturated fatty acid even though these cells already have the highest lipid bilayer surface charge density. Moreover, this ratio is not consistently related to growth temperature or changes in cholesterol levels, as expected. We therefore conclude that A. laidlawii strain B, apparently unlike strain A, does not possess coherent regulatory mechanisms for maintaining either the phase preference or the surface charge density of its membrane lipid constant in response to variations in growth temperature, fatty acid composition or cholesterol content.

Method for exchange of the lipid environment of the membrane-bound (Mg2+ + Na+)-ATPase of Acholeplasma laidlawii B

A method that exchanges the endogenous lipids in the environment of the membrane-bound (Mg2+ + Na+)-ATPase of Acholeplasma laidlawii B with defined exogenous lipids has been devised. Results demonstrate that 99.9% of the original membrane lipids were replaced with phosphatidylcholine and phosphatidic acid. ATPase enzyme activity was maintained throughout the substitution procedure.

Thermotropic properties of bilayers containing branched-chain phospholipids. Calorimetric, Raman, and 31P NMR studies

Diisopalmitoylphosphatidylcholine (DIPPC), -phosphatidylethanolamine (DIPPE), and -phosphatidylglycerol (DIPPG) have been synthesized, and the structures of aqueous dispersions of these lipids have been examined by high-sensitivity differential scanning calorimetry, 31P nuclear magnetic resonance, and Raman spectroscopy. DIPPC at temperatures below 23.1 degrees C readily forms a gel phase with the acyl chains packed in an orthorhombic subcell. Above this temperature, this "orthorhombic" phase converts directly to the liquid-crystalline phase. The phase diagram for the system DIPPC--dipalmitoyl-PC (DIPPC-DPPC) shows that the gel phases formed by either lipid can accommodate only limited amounts of the other species and suggests that the low-temperature orthorhombic phase of DIPPC is distinct in its structure from the "subgel" phase of DPPC. DIPPE forms a well-ordered gel phase only in samples that are equilibrated at low temperatures for long times (approximately days to weeks) or at very high lipid concentrations. However, this lipid readily forms an "intermediate" phase with a very disordered acyl chain packing upon cooling from the liquid-crystalline state. Mixtures of DIPPE with DIPPG exhibit similar thermotropic properties. Hydrated DIPPE appears to be stable in the lamellar phase up to at least 98 degrees C, while di-cis- and di-trans-9-hexadecenoyl-PE convert to the hexagonal II phase at 43.5 and 92.5 degrees C, respectively. We discuss the relevance of these results to the structure and stability of bacterial membranes containing branched-chain acyl lipids.

Autoxidation of Acholeplasma laidlawii membranes

Autoxidation of Acholeplasma laidlawii membranes (with equimolar ratio of palmitic and linoleic acid) lacks an obvious induction period, and the overall rate of disappearance of substrate does not follow closely that of typical autocatalytic kinetics. Throughout the course of autoxidation, the major oxygenated products isolated were hydroperoxides (as hydroxy esters) and compounds that gave rise to trihydroxy esters. The yield of trihydroxy esters was appreciable even at the early stage of the oxidation and eventually grew to surpass that of hydroperoxides. The positions of the three hydroxyl groups in the trihydroxy esters were determined to be mostly of the 1,2,5-type rather than 1,2,3-type arrangement. To a lesser extent, some degraded products, including dimethyl nonanedioate, methyl myristate, methyl pentadecanoate, methyl hexadecadienoate and methyl heptadecadienoate also were obtained. Dimethyl nonanedioate was a previously known degradation product from 9-hydroperoxide. The shorter chain esters presumably arise from the cleavage of alpha-hydroperoxides of palmitate and linoleate moieties.

Overproduction of fumarate reductase in Escherichia coli induces a novel intracellular lipid-protein organelle

The expression of fumarate reductase in Escherichia coli has been amplified over 30-fold by utilizing a recombinant plasmid, pFR63 , carrying the fumarate reductase operon. More than 50% of the inner-membrane protein could be accounted for by the enzyme, whereas the total amount of protein associated with the membrane fraction doubled. The membrane accommodated this excess fumarate reductase without reducing the levels of other membrane-associated enzymes. At the same time, the amount of membrane lipid increased such that the lipid/protein ratio remained constant, indicating that the total amount of membrane had doubled. Small alterations in fatty acid composition as well as a large increase in cardiolipin were detected in the fumarate reductase-enriched membranes. The excess membrane was localized in novel tubular structures which were observed in thin-section and negatively stained electron-microscopic preparations. The tubules only appeared after the cytoplasmic membrane became highly enriched in fumarate reductase. They branched from the cytoplasmic membrane and were fumarate reductase. They branched from the cytoplasmic membrane and were composed of an aggregate of fumarate reductase and lipid.

Radiation studies of Acholeplasma laidlawii: the role of membrane composition

Acholeplasma laidlawii, a mycoplasma, is unable to synthesize unsaturated fatty acids but it will incorporate them into its plasma membrane if they are supplied exogeneously. Thus the fatty acid composition of the cell membrane can be defined by growing the organism in media containing specific fatty acids. We obtained cells with predominantly one type of unsaturated fatty acid (either oleic, linoleic or linolenic acid) or cells with only saturated fatty acid in the cell membrane. The cells were irradiated with 7 MeV electrons and the effect of membrane fatty acid composition on cell survival was examined. At 200 Gy/min and 0.5 degrees C (melting ice) there was little difference in the radiation sensitivities of the cells grown in unsaturated fatty acids either in aerated or anoxic radiation conditions. However, the cells containing saturated fatty acids irradiated in anoxic conditions were markedly more sensitive than the cells containing unsaturated fatty acids. At 200 Gy/min and 37 degrees C the two types of cells were of similar sensitivity both in aerated and anoxic radiation conditions. At 5 Gy/min at 0.5 degrees C the cells containing linolenic acid (18:3) were less sensitive than those containing solely saturated fatty acids. However, at 5 Gy/min at 37 degrees C there was no difference in sensitivity between these two types of cell. Our results strongly argue against the involvement of lipid peroxidation as a molecular change leading to cell death.

Occurrence of dialkyl ether phospholipids in Stigmatella aurantiaca DW4

We investigated the lipid composition of vegetative cells of Stigmatella aurantiaca. Four phospholipids were isolated and identified: phosphatidylethanolamine as the main component, phosphatidylglycerol, lysophosphatidylethanolamine in an exceptionally large amount (17%), and phosphatidylinositol (18 to 25%), rare in procaryotic cells. This composition did not change significantly during growth. The fatty acids of total lipids were found to be rather similar to those of other strains of myxobacteria; the main fatty acids found were unsaturated and branched. We noted a different fatty acid pattern for each phospholipid. The presence of unusual alkyl ether linkages, established by chemical hydrolysis and infrared spectroscopy, was unexpected in these bacteria. Diacyl ester, dialkyl ether, and monoacyl-monoalkyl structures were shown in phosphatidylethanolamine and phosphatidylglycerol. Lysophosphatidylethanolamine was essentially a monoacyl form, whereas phosphatidylinositol was a unique dialkyl ether phospholipid.

Effect of fatty acid supplementation on the lipid composition of Mycobacterium smegmatis ATCC 607, grown at 27 degrees and 37 degrees C

Mycobacterium smegmatis ATCC 607 was grown at 27 and 37 degrees C, with and without exogenous unsaturated fatty acids, viz. elaidic, oleic and palmitoleic acids, added to the growth medium. The total lipid content of M. smegmatis ATCC 607 was lower at 27 degrees C, and with added oleic acid, when compared with the controls, but higher in presence of palmitoleic acid. At 37 degrees C no significant differences were noted in the total lipid content. In general, the total lipid content was lower with all of the fatty acid supplementations at both 27 and 37 degrees C. The phosphatidylethanolamine content was slightly higher at 27 degrees C in the presence of elaidic or palmitoleic acid, but was markedly lower with oleic acid supplementation at 37 degrees C. The cardiolipin content was lower in the presence of any of the fatty acids at 27 degrees C, and higher in the medium supplemented with elaidic or oleic acid at 37 degrees C. The unsaturated to saturated fatty acids ratio was higher with palmitoleic acid supplementation at 27 degrees C, but remained unchanged in cells grown at 37 degrees C. The modifications in mycobacterial lipids are a reflection of the organism's ability to adapt to changing growth conditions.

The biosynthetic incorporation of diacetylenic fatty acids into the biomembranes of Acholeplasma laidlawii A cells and polymerisation of the biomembranes by irradiation with ultraviolet light

Acholeplasma laidlawii A has been grown in media containing synthetic, long chain C20- and C23-fatty acids possessing a diacetylene group in their acyl chains. Growth on the C23 diacetylenic acid was poor but was good on the C20 acid. Biosynthetic incorporation of the fatty acids occurs; as much as 90% of the membrane lipid fatty acyl chains consisting of the C20-diacetylenic fatty acid, the remainder being shorter chain, saturated fatty acids. The thermal phase transition of this biomembrane has been studied and a differential scanning calorimetry heating curve shows the presence of an endotherm corresponding to a membrane lipid phase transition occurring at about 26 degrees C. The lipid class composition of membranes containing the C20-diacetylene lipids was examined and found to be similar to membranes from cells grown on oleic acid-containing medium. (The ratio of monoglucosyl- to diglucosyldiacylglycerols was the same but the ratio of glycolipid to phosphatidylglycerol was higher in the cells grown with diacetylene fatty acids). Upon irradiation with ultraviolet light the cells and isolated biomembranes become coloured, either red or yellow depending upon their thermal history. The colour change indicates that extensive cross-linking of the lipids of the biomembranes of A. laidlawii has occurred and that a conjugated polymeric structure has been formed. Analysis of the extracted lipids from the biomembranes by GLC indicates that extensive cross-linking of the lipid chains within the biomembrane of a natural cell system has been achieved. The monoglucosyldiacylglycerols cross-link more readily that do the phosphatidylglycerol lipids. The effect of such lipid cross-linking or polymerisation on the activity at 35 degrees C of an intrinsic membrane-bound enzyme, NADH oxidase, and ribonuclease, an extrinsic membrane-bound enzyme, was studied. The NADH oxidase activity decreased rapidly upon cross-linking of the lipid environment whereas ribonuclease activity was unaffected. The potential for future studies of polymerised model and natural biomembranes is discussed.

Lipid and protein composition and thermotropic lipid phase transitions in fatty acid-homogeneous membranes of Acholeplasma laidlawii B

The membrane composition and lipid physical properties have been systematically investigated as a function of fatty acid composition for a series of Acholeplasma laidlawii B membrane preparations made homogeneous in various fatty acids by growing cells on single fatty acids and avidin, a potent fatty acid synthetic inhibitor. The membrane protein molecular weight distribution is essentially constant as a function of fatty acid composition, but the lipid/protein ratio varies over a 2-fold range when different fatty acid growth supplements are used. The membrane lipid head-group composition varies somewhat under these conditions, particularly in the ratio of the two major neutral glycolipids. Differential thermal analytical investigations of the thermotropic phase transitions of various combinations of membrane components suggest that these compositional changes are unlikely to result in qualitative changes in the nature of lipid-protein or lipid-lipid interactions, although lesser changes of a quantitative nature probably do occur. The total lipids of membranes made homogeneous in their lipid fatty acyl chain composition exhibit sharper than normal gel-to-liquid-crystalline phase transitions of which midpoint temperatures correlate very well with the phase transition temperatures of synthetic hydrated phosphatidylcholines with like acyl chains. Our results indicate that using avidin and suitable fatty acids to grow A. laidlawii B, it is possible to manipulate the position and the sharpness of the membrane lipid phase transition widely and independently without causing major modifications in other aspects of the membrane composition. This fact makes the fatty acid-homogeneous A. laidlawii B membrane a very useful biological membrane preparation in which to study lipid physical properties and their functional consequences.

Membrane lipid physical state and modulation of the Na+,Mg2+-ATPase activity in Acholeplasma laidlawii B

Careful analysis of the Arrhenius plot of the Na+,Mg2+-ATPase (ATP pyrophosphohydrolase, EC 3.6.1.8) activity in Acholeplasma laidlawii B membranes of varying fatty acid composition has been combined with differential thermal analysis of the membrane lipid phase transitions to evaluate the effects of membrane lipid properties on the enzyme activity. Our results indicate that the enzyme is active only in association with liquid-crystalline lipids, exhibiting a significant heat capacity of activation, delta Cp++, for the ATP hydrolytic reaction in this case. Quantitative analyses of Arrhenius plots for the enzyme activity in membranes whose lipids exhibit a gel-to-liquid-crystalline phase transition in the physiological temperature range suggest that the ATPase is inactivated when its boundary lipids undergo a phase transition that is driven by the bulk lipid phase transition but is less cooperative than the latter. Our results suggest that the familiar "biphasic linear" Arrhenius plots obtained for many membrane enzymes may in fact have a more complex shape, analysis of which can furnish useful information regarding the behavior of the enzyme molecule.

Control of membrane polar lipid composition in Acholeplasma laidlawii a by the extent of saturated fatty acid synthesis

The low level of endogenous fatty acid synthesis in Acholeplasma laidlawii A strain EF22 was found to be caused by a deficiency of pantetheine in the lipid-depleted growth medium. By supplementing the oleic acid-containing medium with increasing concentrations of pantethein, saturated fatty acid synthesis was stimulated (having an apparent Km of 5 microM for pantetheine) and the incorporation of endogenously synthesized fatty acids in membrane lipids increased markedly. Furthermore, carotenoid biosynthesis was stimulated. Exogenous palmitic acid was found to inhibit partially the endogenous fatty acid synthesis. A gradual stimulation of fatty acid synthesis was accompanied by a linear increase in the molar proportion between the two dominating membrane glucolipids, monoglucosyldiacylglycerol and diglucosyldiacylglycerol. The total amount of charged membrane lipids decreased upon increasing the degree of fatty acid saturation. These regulations are discussed in terms of membrane stability, and influence of membrane molecular ordering and surface charge density on lipid polar head group synthesis.

Membrane lipids in bromodeoxyuridine-differentiated astroglial cells in culture

Embryonic hamster astroblasts (NN strain) grown in continuous line were cultivated in the presence of bromodeoxyuridine (BrdU). A decrease in the growth rate of the cells and striking changes in their morphology were observed, the morphology of the cells resembling that of mature astrocytes. Membrane lipids of BrdU-differentiated and standard cells were compared. No modification of the lipid/protein ratio was observed. Phospholipids and cholesterol were increased in the same proportions in the cells, and no modification of the phospholipid distribution was observed. Ganglioside sialic acid remained at the same level, but the ganglioside distribution was highly modified. Complex gangliosides appeared (GM1 and GD1a), while the proportion of simple gangliosides (GM3 and GD3) decreased. However, neither GT1 nor GQ1 were detected in differentiated cells. The distribution of phosphoglyceride acyl groups was highly modified, the proportion of arachidonic and docosapentaenoic acids being 2 to 3 times higher in BrdU-treated cells than in proliferating ones. These results were compared to those obtained with another clonal line of glial cells (C6) which exhibited no morphological differentiation in the presence of BrdU; the lipids of these cells were not modified by such a treatment.

Lecithin changes in murine Mycoplasma pulmonis respiratory infection

We examined the lipid content of bronchoalveolar (BA) washes from both mice and rats infected with Mycoplasma pulmonis, an etiological agent of murine pneumonia. During a 30-day period after intranasal inoculation, the total lipid content from infected and control rats (in milligrams per animal) remained relatively equal and unchanged. The saturated, unsaturated, and total lecithin contents in infected rats (in milligrams per animal) all increased. The maximum lecithin values were detected at 7 to 10 days after infection; later, the levels fell to control values. There was essentially no change in any lecithin value from uninfected animals. Although in BA washes from infected animals the mass of disaturated lecithins increased, the percentage of this fraction in the total lecithin pool decreased. The fatty acids of the lecithins from BA washes of infected mice had significantly less palmitic and significantly more oleic and linoleic acids than the lecithins isolated from the BA washes of control animals. Both the relative decrease in the mass of disaturated lecithins in the BA washes and the increase in the percentage of esterified unsaturated fatty acids in the lecithins may be directly related to the reduced lung function reported to occur during the course of murine M. pulmonis pneumonia.

Positional specificity in the incorporation of isomeric cis- and trans-octadecenoic acids into glycerolipids of cultured soya cells

Heterotrophically grown cell suspension cultures of soya (Glycine max L.) were incubated with two different mixed substrates consisting of positional isomers of either cis-[1-(14)C]octadecenoic acids (Δ8 to δ15) or trans-[1-(14)C]octadecenoic acids (Δ8 to Δ16), each with known composition. With both substrates, about one-fourth of the radioactivity supplied was incorporated into the diacylglycerophosphocholines, while another one-fourth of the radioactivity was almost equally distributed between diacylglycerophos-phoethanolamines and triacylglycerols. All the positional isomers of cis-and trans-octadecenoic acids supplied to the cells were readily incorporated into various classes of glycerolipids. None of the octadecenoic acids was isomerized, elongated or desaturated during incubation. From the cis-octadecenoic acids, only the naturally occurring Δ9-isomer (oleic acid) was preferentially incorporated into position 2 of diacylglycerophosphocholines, diacylglycerophospho-ethanolamines, and triacyglycerols; all the other isomers exhibited a strong affinity for position 1 of the glycerophospholipids and positions 1 and 3 of the triacylglycerols. From the trans-octadecenoic acids, only the Δ9-isomer (elaidic acid) was preferentially incorporated into position 2 of diacylglycerophospho-cholines and triacylglycerols; all the other isomers preferred position 1 and positions 1 and 3, respectively, of these lipids. In diacylglycerophospho-ethanolamines, however, each of the trans-octadecenoic acids, including the Δ9-isomer, exhibited a strong affinity for position 1. Apparently, the enzymes involved in the incorporation of exogenous monounsaturated fatty acids into membrane lipids of plant cells can recognize the preferred substrate in a mixture of closely related isomers.

The mycoplasmas

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Membrane lipid biosynthesis in Acholeplasma laidlawii b: elongation of medium- and long-chain exogenous fatty acids in growing cells

The chain elongation of a wide variety of exogenous fatty acids and the subsequent incorporation of the chain elongation products into the total membrane lipids of Acholeplasma laidlawii B were systematically studied. Within each chemical class of fatty acids examined, the extent of chain elongation increased with increases in chain length, reached a maximum value, and then declined with further increases in chain length. Depending on chemical structure, exogenous fatty acids containing less than 6 to 9 carbon atoms or more than 15 to 18 carbon atoms were not substrates for the chain elongation system. The substrate specificity of this fatty acid elongation system was strikingly broad, and straight-chain, methyl isobranched, and methyl anteisobranched saturated fatty acids, as well as cis- and trans-monounsaturated, cis-cyclopropane, and cis-polyunsaturated fatty acids, underwent chain elongation in vivo. The extent of chain elongation and the average chain length of the primary elongation products correlated well with the physical properties (melting temperatures) of the exogenous fatty acid substrates. The specificity of fatty acid chain elongation in A. laidlawii B maintained the fluidity and physical state of the membrane lipids within a rather wide but definitely limited range. The fatty acid chain elongation system of this organism could be markedly influenced by the presence of a second exogenous fatty acid that was not itself a substrate for the chain elongation system but was incorporated directly into the membrane lipids. The presence of a relatively low-melting exogenous fatty acid increased both the extent of chain elongation and the average chain length of the elongation products generated, whereas the presence of a relatively high-melting fatty acid had the opposite effect. The extent of chain elongation and nature of the elongation products formed were not, however, dependent on the fluidity and physical state of the membrane lipids per se. The second exogenous fatty acid appeared instead to exert its characteristic effect by competing with the chain elongation substrate and elongation products for the stereospecific acylation of positions 1 and 2 of sn-glycerol-3-phosphate. The similar effects of alterations in environmental temperature, cholesterol content, and exposure to the antibiotic cerulenin on the fatty acid chain elongation and de novo biosynthetic activities suggested that the chain elongation system of this organism may be a component of the de novo biosynthetic system.

Membrane lipid biosynthesis in Acholeplasma laidlawii B: de novo biosynthesis of saturated fatty acids by growing cells

The de novo biosynthesis of fatty acids of 12 to 18 carbons from precursors of 5 carbons or fewer has been demonstrated in Acholeplasma laidlawii B. Radiolabeling experiments indicated that the normal primers for the synthesis of the even- and odd-chain fatty acids are acetate and propionate or valerate, respectively. Saturated straight-chain monomethyl-branched fatty acids of up to five carbons were readily utilized as primers, wheras more highly branched species and those possessing halogen substituents or unsaturation were not utilized. At primer concentrations of 1 to 3 mM, up to 80% of the total cellular lipid fatty acids were derived from exogenous primer. The mean chain length of the exogenous primer-derived fatty acids rose with increasing primer incorporation for methyl-branched short-chain fatty acids but was invariant for propionate. The products of de novo biosynthesis varied only slightly with temperature or cholesterol supplementation, suggesting that de novo biosynthesis is not directly influenced by membrane fluidity. Cerulenin inhibited de novo biosynthesis in a fashion that suggests the presence of two beta-ketoacyl thioester synthetases, which differ in substrate chain length specificity and in susceptibility to inhibition by the antibiotic.