Fatty acid alpha-hydroxylation and its relation to myelination

FA2H controls cool temperature sensing through modifying membrane sphingolipids in Drosophila

Animals have evolved the ability to detect ambient temperatures, allowing them to search for optimal living environments. In search of the molecules responsible for cold-sensing, we examined a Gal4 insertion line in the larvae of Drosophila melanogaster from previous screening work, which has a specific expression pattern in the cooling cells (CCs). We identified that the targeted gene, fa2h, which encodes a fatty acid 2-hydroxylase, plays an important role in cool temperature sensing. We found that fa2h mutants exhibit defects in cool avoidance behavior and that this phenotype could be rescued by genetically re-introducing the wild-type version of FA2H in CCs but not the enzymatically disabled point mutation version. Calcium imaging data showed that CCs require fa2h to respond to cool temperature. Lipidomic analysis revealed that the 2-hydroxy sphingolipids content in the cell membranes diminished in fa2h mutants, resulting in increased fluidity of CC neuron membranes. Furthermore, in mammalian systems, we showed that FA2H strongly regulates the function of the TRPV4 channel in response to its agonist treatment and warming. Taken together, our study has uncovered a novel role of FA2H in temperature sensing and has provided new insights into the link between membrane lipid composition and the function of temperature-sensing ion channels.

Proteomic signature of muscle fibre hyperplasia in response to faba bean intake in grass carp

Fish muscle growth is important for the rapidly developing global aquaculture industry, particularly with respect to production and quality. Changes in muscle fibre size are accomplished by altering the balance between protein synthesis and proteolysis. However, our understanding regarding the effects of different protein sources on fish muscle proteins is still limited. Here we report on the proteomic profile of muscle fibre hyperplasia in grass carp fed only with whole faba bean. From the results, a total of 99 significantly changed proteins after muscle hyperplasia increase were identified (p < 0.05, ratio <0.5 or >2). Protein–protein interaction analysis demonstrated the presence of a network containing 56 differentially expressed proteins, and muscle fibre hyperplasia was closely related to a protein–protein network of 12 muscle component proteins. Muscle fibre hyperplasia was also accompanied by decreased abundance in the fatty acid degradation and calcium signalling pathways. In addition, metabolism via the pentose phosphate pathway decreased in grass carp after ingestion of faba bean, leading to haemolysis. These findings could provide a reference for the prevention and treatment of human glucose-6-phosphate dehydrogenase deficiency (“favism”).

Role of thiamine pyrophosphate in oligomerisation, functioning and import of peroxisomal 2-hydroxyacyl-CoA lyase

During peroxisomal α-oxidation, the CoA-esters of phytanic acid and 2-hydroxylated straight chain fatty acids are cleaved into a (n-1) fatty aldehyde and formyl-CoA by 2-hydroxyacyl-CoA lyase (HACL1). HACL1 is imported into peroxisomes via the PEX5/PTS1 pathway, and so far, it is the only known peroxisomal TPP-dependent enzyme in mammals. In this study, the effect of mutations in the TPP-binding domain of HACL1 on enzyme activity, subcellular localisation and oligomerisation was investigated. Mutations of the aspartate 455 and serine 456 residues within the TPP binding domain of the human HACL1 did not affect the targeting upon expression in transfected CHO cells, although enzyme activity was abolished. Gel filtration of native and mutated N-His(6)-fusions, expressed in yeast, revealed that the mutations did not influence the oligomerisation of the (apo)enzyme. Subcellular fractionation of yeast cells expressing HACL1 showed that the lyase activity sedimented at high density in a Nycodenz gradient. In these fractions TPP could be measured, but not when mutated HACL1 was expressed, although the recombinant enzyme was still targeted to peroxisomes. These findings indicate that the binding of TPP is not required for peroxisomal targeting and correct folding of HACL1, in contrast to other TPP-dependent enzymes, and suggest that transport of TPP into peroxisomes is dependent on HACL1 import, without requirement of a specific solute transporter.

Sphingomyelin analogs with branched N-acyl chains: the position of branching dramatically affects acyl chain order and sterol interactions in bilayer membranes

Sphingolipids have been found to have single methyl branchings both in their long-chain base and in their N-linked acyl chains. In this study we determined how methyl-branching in the N-linked acyl chain of sphingomyelin (SM) affected their membrane properties. SM analogs with a single methyl-branching at carbon 15 (of a 17:0 acyl chain; anteiso) had a lower gel-liquid transition temperature as compared to an iso-branched SM analog. Phytanoyl SM (methyls at carbons 3, 7, 11 and 15) as well as a SM analog with a methyl on carbon 10 in a hexadecanoyl chain failed to show a gel-liquid transition above 10 degrees C. Only the two distally branched SM analogs (iso and anteiso) formed ordered domains with cholesterol in a 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer. However, domains formed by the branched SM analogs appeared to contain less sterol when compared to palmitoyl SM (PSM) as the saturated phospholipid. Sterol-enriched domains formed by the anteiso SM analog were also less stable against temperature than domains formed by PSM. Both the 10-methyl and phytanoyl SM analogs failed to form sterol-enriched domains in the POPC bilayer. Acyl chain branching weakened SM/sterol interactions markedly when compared to PSM, as also evidenced from the decreased affinity of cholestatrienol to bilayers containing branched SM analogs. Our results show that methyl-branching weakened intermolecular interactions in a position-dependent manner.

Biochemistry and genetics of inherited disorders of peroxisomal fatty acid metabolism

In humans, peroxisomes harbor a complex set of enzymes acting on various lipophilic carboxylic acids, organized in two basic pathways, alpha-oxidation and beta-oxidation; the latter pathway can also handle omega-oxidized compounds. Some oxidation products are crucial to human health (primary bile acids and polyunsaturated FAs), whereas other substrates have to be degraded in order to avoid neuropathology at a later age (very long-chain FAs and xenobiotic phytanic acid and pristanic acid). Whereas total absence of peroxisomes is lethal, single peroxisomal protein deficiencies can present with a mild or severe phenotype and are more informative to understand the pathogenic factors. The currently known single protein deficiencies equal about one-fourth of the number of proteins involved in peroxisomal FA metabolism. The biochemical properties of these proteins are highlighted, followed by an overview of the known diseases.

Fatty acid 2-Hydroxylation in mammalian sphingolipid biology

2-Hydroxy fatty acids (hFA) are important components of a subset of mammalian sphingolipids. The presence of hFA in sphingolipids is best described in the nervous system, epidermis, and kidney. However, the literature also indicates that various hFA-sphingolipids are present in additional tissues and cell types, as well as in tumors. Biosynthesis of hFA-sphingolipids requires fatty acid 2-hydroyxlase, and degradation of hFA-sphingolipids depends, at least in part, on lysosomal acid ceramidase and the peroxisomal fatty acid alpha-oxidation pathway. Mutations in the fatty acid 2-hydroxylase gene, FA2H, have been associated with leukodystrophy and spastic paraparesis in humans, underscoring the importance of hFA-sphingolipids in the nervous system. In the epidermis, hFA-ceramides are essential for the permeability barrier function. Physiological function of hFA-sphingolipids in other organs remains largely unknown. Recent evidence indicates that hFA-sphingolipids have specific roles in cell signaling.

The role of 2-hydroxyacyl-CoA lyase, a thiamin pyrophosphate-dependent enzyme, in the peroxisomal metabolism of 3-methyl-branched fatty acids and 2-hydroxy straight-chain fatty acids

2-Hydroxyphytanoyl-CoA lyase (abbreviated as 2-HPCL), renamed to 2-hydroxyacyl-CoA lyase (abbreviated as HACL1), is the first peroxisomal enzyme in mammals that has been found to be dependent on TPP (thiamin pyrophosphate). It was discovered in 1999, when studying alpha-oxidation of phytanic acid. HACL1 has an important role in at least two pathways: (i) the degradation of 3-methyl-branched fatty acids like phytanic acid and (ii) the shortening of 2-hydroxy long-chain fatty acids. In both cases, HACL1 catalyses the cleavage step, which involves the splitting of a carbon-carbon bond between the first and second carbon atom in a 2-hydroxyacyl-CoA intermediate leading to the production of an (n-1) aldehyde and formyl-CoA. The latter is rapidly converted into formate and subsequently to CO(2). HACL1 is a homotetramer and has a PTS (peroxisomal targeting signal) at the C-terminal side (PTS1). No deficiency of HACL1 has been described yet in human, but thiamin deficiency might affect its activity.

FA2H is responsible for the formation of 2-hydroxy galactolipids in peripheral nervous system myelin

Myelin in the mammalian nervous system has a high concentration of galactolipids [galactosylceramide (GalCer) and sulfatide] with 2-hydroxy fatty acids. We recently reported that fatty acid 2-hydroxylase (FA2H), encoded by the FA2H gene, is the major fatty acid 2-hydroxylase in the mouse brain. In this report, we show that FA2H also plays a major role in the formation of 2-hydroxy galactolipids in the peripheral nervous system. FA2H mRNA and FA2H activity in the neonatal rat sciatic nerve increased rapidly during developmental myelination. The contents of 2-hydroxy fatty acids were approximately 5% of total galactolipid fatty acids at 4 days of age and increased to 60% in GalCer and to 35% in sulfatides at 60 days of age. The chain length of galactolipid fatty acids also increased significantly during myelination. FA2H expression in cultured rat Schwann cells was highly increased in response to dibutyryl cyclic AMP, which stimulates Schwann cell differentiation and upregulates myelin genes, such as UDP-galactose:ceramide galactosyltransferase and protein zero. These observations indicate that FA2H is a myelination-associated gene. FA2H-directed RNA interference (RNAi) by short-hairpin RNA expression resulted in a reduction of cellular 2-hydroxy fatty acids and 2-hydroxy GalCer in D6P2T Schwannoma cells, providing direct evidence that FA2H-dependent fatty acid 2-hydroxylation is required for the formation of 2-hydroxy galactolipids in peripheral nerve myelin. Interestingly, FA2H-directed RNAi enhanced the migration of D6P2T cells, suggesting that, in addition to their structural role in myelin, 2-hydroxy lipids may greatly influence the migratory properties of Schwann cells.

FA2H-dependent fatty acid 2-hydroxylation in postnatal mouse brain

2-Hydroxy fatty acids are relatively minor species of membrane lipids found almost exclusively as N-acyl chains of sphingolipids. In mammals, 2-hydroxy sphingolipids are uniquely abundant in myelin galactosylceramide and sulfatide. Despite the well-documented abundance of 2-hydroxy galactolipids in the nervous system, the enzymatic process of the 2-hydroxylation is not fully understood. To fill this gap, we have identified a human fatty acid 2-hydroxylase gene (FA2H) that is highly expressed in brain. In this report, we test the hypothesis that FA2H is the major fatty acid 2-hydroxylase in mouse brain and that free 2-hydroxy fatty acids are formed as precursors of myelin 2-hydroxy galactolipids. The fatty acid compositions of galactolipids in neonatal mouse brain gradually changed during the course of myelination. The relative ratio of 2-hydroxy versus nonhydroxy galactolipids was very low at 2 days of age ( approximately 8% of total galactolipids) and increased 6- to 8-fold by 30 days of age. During this period, free 2-hydroxy fatty acid levels in mouse brain increased 5- to 9-fold, and their composition was reflected in the fatty acids in galactolipids, consistent with a precursor-product relationship. The changes in free 2-hydroxy fatty acid levels coincided with fatty acid 2-hydroxylase activity and with the upregulation of FA2H expression. Furthermore, mouse brain fatty acid 2-hydroxylase activity was inhibited by anti-FA2H antibodies. Together, these data provide evidence that FA2H is the major fatty acid 2-hydroxylase in brain and that 2-hydroxylation of free fatty acids is the first step in the synthesis of 2-hydroxy galactolipids.

A novel method for the measurement of in vitro fatty acid 2-hydroxylase activity by gas chromatography-mass spectrometry

Fatty acid 2-hydroxylase (FA2H), encoded by the FA2H gene, is an enzyme responsible for the de novo synthesis of sphingolipids containing 2-hydroxy fatty acids. 2-Hydroxy sphingolipids are highly abundant in the brain, as major myelin galactolipids (galactosylceramide and sulfatide) contain a uniquely high proportion ( approximately 50%) of 2-hydroxy fatty acids. Other tissues, such as epidermis, epithelia of the digestive tract, and certain cancers, also contain 2-hydroxy sphingolipids. The physiological significance of the 2-hydroxylation on N-acyl chains of subsets of sphingolipids is poorly understood. To study the roles of FA2H and 2-hydroxy sphingolipids in various tissues, we developed a highly sensitive in vitro FA2H assay. FA2H-dependent fatty acid 2-hydroxylation requires an electron transfer system, which was reconstituted in vitro with an NADPH regeneration system and purified NADPH:cytochrome P-450 reductase. A substrate [3,3,5,5-D(4)]tetracosanoic acid was solubilized in alpha-cyclodextrin solution, and the 2-hydroxylated product was quantified by gas chromatography-mass spectrometry after conversion to a trimethylsilyl ether derivative. When the microsomes of FA2H-transfected COS7 cells were incubated with the electron transfer system and deuterated tetracosanoic acid, deuterated 2-hydroxy tetracosanoic acid was formed in a time- and protein-dependent manner. With this method, FA2H activities were reproducibly measured in murine brains and tissue culture cell lines.

Identification of the pathway of alpha-oxidation of cerebronic acid in peroxisomes

Cerebronic acid (2-hydroxytetracosanoic acid), an alpha-hydroxy very long-chain fatty acid (VLCFA) and a component of cerebrosides and sulfatides, is unique to nervous tissues. Studies were carried out to identify the pathway and the subcellular site involved in the oxidation of cerebronic acid. The results from these studies revealed that cerebronic acid was catabolized by alpha-oxidation to CO2 and tricosanoic acid (23:0). Studies with subcellular fractions indicated that cerebronic acid was alpha-oxidized in fractions having particulate bound catalase and enzyme systems for the beta-oxidation of VLCFA (e.g., lignoceric acid), suggesting peroxisomes as the subcellular organelle responsible for alpha-oxidation of cerebronic acid. Etomoxir, an inhibitor of mitochondrial fatty acid oxidation, had no effect on cerebronic acid alpha-oxidation. Further, cerebronic acid oxidation was found to be dependent on the presence of NAD+ but not FAD, NADPH, ATP, Mg2+, or CoASH. Intraorganellar localization studies indicated that the enzyme system for the alpha-oxidation of cerebronic acid was associated with the peroxisomal limiting membranes. Studies on cultured fibroblasts from normal subjects and patients with peroxisomal disorders indicated an impairment of alpha-oxidation of cerebronic acid in cell lines that lack peroxisomes [e.g., Zellweger syndrome (ZS)]. On the other hand, alpha-oxidation of cerebronic acid was found to be normal in cell lines from X-linked adrenoleukodystrophy, adult Refsum disease, and rhizomelic chondrodysplasia punctata. Our results clearly demonstrate that alpha-oxidation of alpha-hydroxy VLCFA (cerebronic acid) is a peroxisomal function and that this oxidation is impaired in ZS. Furthermore, this alpha-oxidation enzyme system is distinct from the one for the alpha-oxidation of beta-carbon branched-chain fatty acids (e.g., phytanic acid).

Identification and characterization of HAOX1, HAOX2, and HAOX3, three human peroxisomal 2-hydroxy acid oxidases

Computer-based approaches identified three distinct human 2-hydroxy acid oxidase genes, HAOX1, HAOX2, and HAOX3, that encode proteins with significant sequence similarity to plant glycolate oxidase, a prototypical 2-hydroxy acid oxidase. The products of these genes are targeted to peroxisomes and have 2-hydroxy acid oxidase activities. Each gene displays a distinct tissue-specific pattern of expression, and each enzyme exhibits distinct substrate preferences. HAOX1 is expressed primarily in liver and pancreas and is most active on the two-carbon substrate, glycolate, but is also active on 2-hydroxy fatty acids. HAOX2 is expressed predominantly in liver and kidney and displays highest activity toward 2-hydroxypalmitate. HAOX3 expression was detected only in pancreas, and this enzyme displayed a preference for the medium chain substrate 2-hydroxyoctanoate. These results indicate that all three human 2-hydroxy acid oxidases are involved in the oxidation of 2-hydroxy fatty acids and may also contribute to the general pathway of fatty acid alpha-oxidation. Primary hyperoxaluria type 1 (PH1) is caused by defects in peroxisomal alanine-glyoxylate aminotransferase, the enzyme that normally eliminates intraperoxisomal glyoxylate. The presence of HAOX1 in liver and kidney peroxisomes and the ability of HAOX1 to oxidize glyoxylate to oxalate implicate HAOX1 as a mediator of PH1 pathophysiology.

5’-Nucleotidase Activity and Galactolipid Accumulation in Triethyllead-exposed Glial Cell Cultures

The developing nervous system is vulnerable to heavy metal exposure, which can cause alterations in neuronal and glial cells in the brain. Thus, heavy metals such as mercury and lead reduce myelin galactocerebroside (GalC) synthesis, and increase the ratio of non-hydroxylated fatty acids to hydroxylated fatty acids (GalC-N:GalC-OH) in the GalC molecules in newborn rats. This study investigated the effect of lead on the expression of myelin components by the myelin-forming oligodendroglial cells (OG) in vitro. Primary cultures of mixed glial cells from brains of newborn rats were continuously exposed to triethyllead (TEL; 1nM, 10nM, 50nM and 100nM) for 3 weeks, one week after seeding. The first morphological alteration observed was an increased proliferation of OG in cultures exposed to 10nM TEL. Biochemical analyses showed up-regulation of the enzymes, 2'3’-cyclic nucleotide 3’-phosphodiesterase and 5’-nucleotidase. GalC synthesis was also stimulated, and the ratio GalC-N:GalC-OH was reduced. The results indicate that TEL stimulates the differentiation and maturation of OG in cultures, which suggests that the alterations induced by heavy metals in newborn rats are not due to interference with the OG maturation.

Synthesis of non-hydroxy-galactosylceramides and galactosyldiglycerides by hydroxy-ceramide galactosyltransferase

Galactosylceramide (GalCer) is the major glycolipid in brain. In order to characterize the activity of brain UDPgalactose: ceramide galactosyltransferase (CGalT), it has been stably expressed in CGalT-negative Chinese hamster ovary (CHO) cells. After fractionation of transfected cells, CHO-CGT, on sucrose gradients, the activity resides at the density of endoplasmic reticulum and not of Golgi. A lipid chromatogram from CHO-CGT cells revealed two new iodine-staining spots identified as GalCer, since they comigrate with GalCer standards, can be metabolically labelled with [3H]galactose, are recognized by anti-GalCer antibodies, and are resistant to alkaline hydrolysis. A third [3H]galactose lipid was identified as galactosyldiglyceride. In the homogenate CGalT displays a 25-fold preference for hydroxy fatty acid-containing ceramides. Remarkably, endogenous GalCer of transfected cells contains exclusively non-hydroxy fatty acids: fast atom bombardment and collision-induced dissociation mass spectrometric analysis revealed mainly C16:0 in the lower GalCer band on TLC and mainly C22:0 and C24:0 in the upper band. Our results suggest that CGalT galactosylates both hydroxy- and non-hydroxy fatty acid-containing ceramides and diglycerides, depending on their local availability. Thus, CGalT alone may be responsible for the synthesis of hydroxy- and non-hydroxy-GalCer, and galactosyldiglyceride in myelin.

Hydroxy- and non-hydroxy-galactolipids in developing rat CNS

Rat spinal cord (1-24 weeks postnatal) was analysed by HPLC for various species of galactolipids that accumulate in mammalian myelin during development. Cerebral tissue of the same animals was taken as reference. The levels of the major galactolipids, galactosylceramide (GalCer) and its sulfated analog (SGalCer), increased linearly during the first 2 months after birth. At 3 months, constant levels were reached that were approx. 4-fold (GalCer) and 2.5-fold (SGalCer) higher than in cerebral tissue of corresponding age. The accumulation of galactoglycerolipids slightly preceded that of galactosphingolipids. Levels of galacto-glycerolipids were much lower (4% of galactosphingolipids in 3-and 2.5% in 6-month-old spinal cord on weight basis) and decreased upon CNS maturation. During the first postnatal month, the ratio of non-hydroxy- over hydroxy-species (NFA/HFA) of cerebral GalCer declined from 2.2 to 0.5 whereas the NFA/HFA ratio for cerebral SGalCer increased from 1.0 to 1.8 in the same period. Through development the hydroxy-species contributed 56-60% to GalCer and 28-41% to SGalCer in spinal cord, whereas in cerebrum of 24-week-old rats 73% of GalCer and 48% of SGalCer was alpha-hydroxylated in the ceramide moiety. These data point to different developmental programs with respect to galactolipid metabolism of oligodendrocytes in high- (spinal cord) as compared to low-myelinated (cerebral) areas of rat CNS.

Metamorphic changes in glycolipids and myelin proteins and 2',3'-cyclic nucleotide 3'-phosphohydrolase in bullfrog and axolotl brains

The metamorphic changes in levels of glycolipids and myelin proteins and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) in the brains of bullfrog tadpoles, adult frogs, and axolotls were investigated, with particular emphasis on myelin maturation. The concentrations of cerebroside, sulfatide, and galactosyldiacylglycerol gradually increased from the onset of prometamorphosis throughout the active metamorphic period and then greatly increased after metamorphosis was completed. The ratio of glucocerebroside to galactocerebroside increased greatly in the prometamorphic period and then rapidly decreased to the frog level during the climax period. The fatty acid compositions of cerebroside and sulfatide showed a developmental change, with 24:1 being more predominant in the later metamorphic stage. The proportion of hydroxy fatty acids increased up to the onset of the prometamorphic stage and thereafter remained constant at approximately 50% of the total. The CNP activity remained unchanged throughout metamorphosis at 60% that in frog myelin and increased in the adult frog. The composition of tadpole myelin proteins remained constant during metamorphosis, with large basic protein being the most abundant, and in the frog, proteolipid protein and large basic protein were present in comparable amounts. The two adult forms of axolotl, i.e., the neotenous and metamorphosed forms, exhibited almost identical myelin constituents, and CNP activity in the neotenous form amounted to one-fifth that in the bullfrog. These results indicate that active biosynthesis of myelin marker components occurs as metamorphosis proceeds, but more pronounced changes of myelin components occur after metamorphosis is completed.

Alpha-hydroxylation of lignoceroyl-CoA in rat brain microsomes: involvement of NADPH-cytochrome c reductase and topical distribution

1. The enzymatic mechanism of the alpha-hydroxylation of lignoceroyl-CoA, an intermediate in the synthesis of hydroxyceramide, was studied. In the presence of NADPH, sphingosine and microsomes from 20-day-old rat brain, 14C from [1-14C]lignoceroyl-CoA was incorporated into hydroxyceramide. 2. The alpha-hydroxylation of lignoceroyl-CoA in rat brain microsomes was strongly inhibited by a rabbit anti-immunoglobulin G which was prepared against rat liver microsomal NADPH-cytochrome c reductase. However, anti-immunoglobulin G against cytochrome b5 did not inhibit the alpha-hydroxylase activity. 3. The alpha-hydroxylation activity was more sensitive to trypsin treatment than was NADPH-cytochrome c reductase in rat brain microsomes. This indicates that either alpha-hydroxylase itself or an unknown factor essential in alpha-hydroxylation is highly exposed to the surface of brain microsomes.

Acyl-CoA ligases from rat brain microsomes: an immunochemical study

Acyl-CoA ligase activities, solubilized from rat brain microsomes, were fractionated into three different peaks by hydroxyapatite chromatography. Based on physical and chemical properties, we suggested that peak A (pamitoyl-CoA ligase) and peak C (lignoceroyl-CoA ligase) were two different enzymes (A. Bhushan, R. P. Singh, and I. Singh (1986) Arch. Biochem. Biophys. 246, 374-380). We raised antibodies against purified liver microsomal palmitoyl-CoA ligase (EC 6.2.1.3) and examined the effect of this antibody on acyl-CoA ligase activities for palmitic, arachidonic and lignoceric acids in microsomal enzyme extract and different acyl-CoA ligase peaks from the hydroxyapatite column. In an enzyme activity assay system in microsomal extract, the antisera inhibited the palmitoyl-CoA ligase activity but had very little effect on the acyl-CoA ligase activities for arachidonic and lignoceric acids. This antisera inhibited the acyl-CoA ligase activities for these three fatty acids in peak A and had no effect on these activities in peak B or peak C. Western blot analysis demonstrated that antibody to liver microsomal palmitoyl-CoA ligase cross-reacted with only peak A (palmitoyl-CoA ligase), but not with peak B or peak C. This immunochemical study demonstrates that palmitoyl-CoA ligase does not share immunological determinants with acyl-CoA ligases in peaks B or C, thus demonstrating that palmitoyl-CoA ligase (peak A) is different from the arachidonoyl-CoA and lignoceroyl-CoA ligase activities in peaks B or C.

Myelin lipids: a phylogenetic study

The lipid composition of CNS and PNS myelin was studied in rat, Xenopus, trout and Torpedo. The main difference lay in the proportion of cerebrosides, which decreased in the sequence rat greater than Xenopus greater than Torpedo greater than trout. In addition Torpedo CNS and PNS myelins were extremely rich in sulfatides. In some respects, Torpedo appeared closer to tetrapods than trout. Otherwise the proportion of the different lipid classes did not reveal any clear evolutionary trends. The presence of hydroxylated galactolipids in CNS myelin was investigated in several additional species. Considerable amounts were found in Torpedo, Polypterus, Protopterus, lizard, and chicken, with the highest values in rat and anurans. Only very small amounts of hydroxylated cerebrosides were detected in trout and in axolotl, while newt had none. This parameter appears therefore of doubtful usefulness for phylogenetic studies. In contrast to myelin proteins, myelin lipids are of limited value for establishing phylogenetic relationships among vertebrates.

Lignoceroyl-coenzyme A synthetase from developing rat brain: partial purification, characterization and comparison with palmitoyl-coenzyme A synthetase activity and liver enzyme

As part of a long-term study of sphingolipid metabolism in brain, we have purified and partially characterized a long-chain acyl-CoA synthetase from microsomes of developing rat brain and compared it with the hepatic microsomal enzyme from the same animals. Both enzymes were solubilized from microsomes by treatment with Triton X-100 and then chromatographed successively on Blue-Sepharose and DEAE-Sepharose. Blue-Sepharose chromatography yielded a single peak with acyl-CoA synthetase activity, whereas DEAE-Sepharose chromatography of both brain and liver preparations yielded two peaks. Elution patterns of lignoceroyl-CoA synthetase and palmitoyl-CoA synthetase activities were identical throughout these steps and were similar in brain and liver. Gel filtration of each DEAE-Sepharose fraction on Sephadex G-200 also yielded two peaks of activity. The more rapidly eluted material contained much more lignoceroyl-CoA synthetase activity, while the activity for palmitoyl-CoA synthetase was higher in slower eluting peaks. In all preparations the ratio of lignoceroyl-CoA synthetase activity to palmitoyl-CoA synthetase activity was much higher in brain than in liver. These results suggest that although the brain acyl-CoA synthetase is chromatographically similar to the liver enzyme, there are differences in substrate specificity.

Lignoceroyl-CoA ligase activity in rat brain microsomal fraction: topographical localization and effect of detergents and alpha-cyclodextrin

Lignoceroyl-CoA ligase activity has been detected in microsomal fractions prepared from rat brain. The synthesis of lignoceroyl-CoA from [1-14C]lignoceric acid and CoASH by this enzyme had an absolute dependence on ATP and Mg2+; ATP could not be replaced by GTP [I. Singh, M. S. Kang, and L. Phillips (1982) Fed. Proc. 41, 1192]. The product has been characterized as lignoceroyl-CoA by the following criteria: Rf on thin-layer chromatography; incorporation of [1-14C]lignoceric acid and [3H]CoASH into the product; acid hydrolysis and identification of the radiolabel in lignoceric acid; and methanolysis and identification of the radiolabel in methyl lignocerate by thin-layer chromatography. The optimal concentrations for CoASH, ATP, and Mg2+ were about 100 microM, 10 mM, and 5 mM, respectively. Lignoceric acid, solubilized by alpha-cyclodextrin, Triton X-100, and deoxycholate, was utilized by the lignoceroyl-CoA ligase, but lignoceric acid solubilized by Triton WR-1339 was not. Topographical localization of lignoceroyl-CoA ligase in the plane of rat brain microsomal membranes was determined by the use of Triton X-100, trypsin, and mercury-Dextran, and was compared with the marker enzymes, ethanol acyltransferase and thiamine pyrophosphatase, which are known to be localized on the luminal (inner) surface of the microsomal vesicles. Mercury-Dextran (100 microM) and trypsin (trypsin:microsomes, 1:56 w/w) treatment of the microsomes inhibited the lignoceroyl-CoA ligase activity by 70 and 90% without disrupting the microsomal vesicles. Disruption of the vesicles with Triton X-100 increased the activity of both ethanol acyltransferase and thiamine pyrophosphatase by 400% but there was no increase in lignoceroyl-CoA ligase activity. These results suggest that lignoceroyl-CoA ligase is localized on the cytoplasmic surface of the microsomal vesicles.

Alpha-hydroxylation and oxidation of lignoceric acid in brain: the role of heat-stable and heat-labile factors

Our previous investigations disclosed that the heat-stable and heat-labile factors obtained from brain cytosol are required for alpha-hydroxylation and oxidation of lignoceric acid by rat brain particulate fraction. The heat-stable factor was recently found to contain glucose-6-phosphate, N- acetylaspartate , glutamate, aspartate, glutamine, inorganic phosphate and low levels of adenosine nucleotide as active components. A combination of these compounds was as effective as the crude heat-stable factor for enzymic activity. Using these compounds, we reinvestigated the requirement for the heat-labile factor. With crude heat-stable factor there was an absolute requirement for the heat-labile factor; however, with various combinations of the individual components of the heat-stable factor, some degree of activity was obtained without the heat-labile factor. When aspartate or one of its derivatives, N- acetylaspartate or oxaloacetate, was used in place of the heat-stable factor, the activity was relatively low but highly stimulated by the addition of heat-labile factor. On the other hand, higher activity was obtained when glutamate or one of its derivatives, glutamine or alpha-ketoglutarate, was used without heat-labile factor. The addition of heat-labile factor to this system did not stimulate the activity. When studying the aspartate family, we discovered that the requirement for the heat-labile factor varied in a descending order: N- acetylaspartate greater than aspartate greater than oxaloacetate. Lignoceric acid oxidation was further characterized with rat brain particulate fraction, NADPH, Mg2+, glutamate, inorganic phosphate, and AMP without heat-stable and heat-labile factors. It was found that the requirement for NADPH was also partially eliminated with glutamate but not aspartate. The effects of various inhibitors, such as inhibitors of the electron transfer system, oxidative phosphorylation, the enzymes involved in citric acid cycle, and glycolysis, suggest that the heat-stable factor is involved in producing ATP or other high energy compounds to be used for the activation of lignoceric acid. ATP added to the system in place of heat-stable factor resulted in less than one-half of the lignoceric acid oxidation.

Further characterization of the heat-stable factor in the alpha-hydroxylation and oxidation of lignoceric acid in brain: effect of acidic amino acids and hexose-phosphates on brain fatty acid metabolism

Lignoceric acid and other very long-chain fatty acids are converted to alpha-hydroxy fatty acids and ceramide in brain. These fatty acids are also oxidized and produce glutamic acid and other water-soluble products. All of these metabolic conversions are catalyzed by a rat brain particulate fraction and require NADPH, heat-labile factor, and heat-stable factor. The heat-stable factor was prepared from calf cerebellum. Glucose 6-phosphate and N-acetylaspartic acid have previously been identified as active components of the heat-stable factor. We report in this manuscript that glutamic acid, glutamine, aspartic acid, and gamma-aminobutyric acid as well as inorganic phosphate and adenosine nucleotides are also active components of the heat-stable factor. When the amino acids, glucose 6-phosphate, AMP, and phosphoric acid were combined, full activity of the heat-stable factor for the formation of cerebronate (by alpha-hydroxylation) and glutamate (presumably by beta-oxidation) from lignoceric acid was recovered. The role of the acidic amino acids in the metabolic conversion of lignoceric acid in brain appears to be their conversion to the corresponding alpha-keto acids and then incorporation into the TCA cycle. Glucose 6-phosphate is also likely to be involved in the TCA cycle through the Emden-Meyerhof pathway. Inorganic phosphate and AMP seemingly are used to produce ATP. However, the addition of up to 20 mM ATP alone did not replace the heat-stable factor.

Structural alterations of peripheral nerve monogalactosylceramides during development and Wallerian degeneration

The structural alterations of monogalactosylceramides in peripheral nerve were investigated during development, nerve fiber degeneration and regeneration. During early development, hydroxy cerebrosides and sulfatides were the main constituents of the monogalactosylceramides of immature rat sciatic endoneurium. The ratio of hydroxy to nonhydroxy cerebrosides decreased rapidly as myelination proceeded but remained fairly constant throughout adulthood. More than 50% of the adult content of endoneurial monogalactosylceramides was achieved before 21 days of age. The long-chain nonhydroxy fatty acids (above C21) had increased from under 20% to over 80% by day 20, while 24h:0 (h, hydroxy) had already reached approximately 50% of hydroxy cerebrosides by day 12. These results suggest that the biosynthesis of endoneurial monogalactosylceramides and fatty acid elongation take place preferentially at the time when peripheral nerve is undergoing active myelination. During Wallerian degeneration, the maximum decrease of monogalactosylceramides was associated temporally with axonal degeneration and demyelination and particularly with myelin conversion to sudanophilic lipids. By the time that nerve fiber regeneration was well established, both the cerebroside and sulfatide contents had returned to near control values. Cerebrosides and long-chain fatty acids (above C21) appear to be the most sensitive to fiber degeneration while fatty acid elongation is selectively increased during nerve regeneration.

Purification and characterization of the heat-stable factors essential for the conversion of lignoceric acid to cerebronic acid and glutamic acid: identification of N-acetyl-L-aspartic acid

The conversion of lignoceric acid to cerebronic acid, ceramides, cerebrosides, and glutamic acid is catalyzed by a rat brain particulate preparation. The heat-stable factor, prepared from calf cerebellum, together with the heat-labile factor, a pyridine nucleotide, and Mg2+ are essential to all of these metabolic pathways. Our previous work showed that the heat-stable factor is composed of at least two components, HSF-1 and HSF-2, and identified HSF-2 as D-glucose-6-phosphate. In the current investigation, HSF-1 was further purified and found to be N-acetyl-L-aspartic acid. In addition, it was discovered that a third component, HSF-3, is also required for heat-stable factor activity. A reconstituted system composed of N-acetylaspartic acid, glucose-6-phosphate, and HSF-3 fully replaced the heat-stable factor essential for the conversion of lignoceric acid to cerebronic acid and glutamic acid. The reconstituted heat-stable factor did not show the initial time lag always observed with the crude heat-stable factor.

Change of galactolipids and metabolism of fatty acids in the organotypic culture of myelinating mouse brain

The concentrations of cerebrosides and sulfatides, myelin-characteristic galactolipids, can be determined in organotypic culture of newborn mouse cerebellum by high-performance liquid chromatography. These galactolipids can be detected at 9-days in vitro explants and increased steadily until the explants were more tan 3 weeks old. The levels of nonhydroxycerebroside, hydroxycerebroside, nonhydroxysulfatide, hydroxysulfatide, and monogalactosyl diacylglycerol were 0.3, 0.6, 0.3, 0.2, and 0.3 nmol/mg protein at 9 days in vitro, respectively, and 1.0, 2.7, 0.9, 0.4, and 0.6 at 21 days in vitro, respectively. When serum was removed from the feeding medium after 9 days, the levels of these lipids did not increase and myelination failed to occur. When a 17 day explant was kept in medium containing [1-14C]lignoceric acid for 4 days, considerable radioactivity was taken up by the explant and incorporated into nonhydroxy- and hydroxycerebrosides, sulfatides and sphingomyelin. Most of the radioactivity in the alpha-hydroxy fatty acids was found in cerebronic acid, the product of lignoceric acid alpha-hydroxylation. Similar explants also took up [1-14C]palmitic acid when it was added to the medium. The radioactivity was, however, mostly incorporated into neutral lipids and glycerophospholipids. These observations indicate that the cultured mouse cerebellum explants synthesize and accumulate myelin-characteristic lipids as does brain in vivo.

Triethyllead treatment of cultured brain cells. Effect on accumulation of radioactive precursors in galactolipids

Cultured cells from chick embryo brains were studied for their sensitivity to triethyllead. Triethyllead chloride (3.16 microM) was added to the nutrient medium and incubated for 48 hr with the cells. Morphological changes in light microscope and radioactive labeling of galactolipids were assayed. Triethyllead treatment reduced the number of neuronal cells with processes. Morphological changes were not observed in glial cells. The [35S]sulfate labeling of sulfatides was reduced to 50%. The [3H]serine labeling of cerebrosides with alpha-hydroxy fatty acids was not influenced, while the [3H]serine labeling of cerebrosides with nonhydroxy fatty acids was inhibited 40% in one- and two- but not in three-week-old cultures. The results indicate that the nerve cell response to triethyllead in cultures is selective, since the neurons are more sensitive than the glia cells and the labeling of sulfatides is more sensitive than that of cerebrosides.