alpha-Hydroxylation of fatty acids in brain. Substrate specificity and deuterium isotope effect

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.

The Human FA2H Gene Encodes a Fatty Acid 2-Hydroxylase

2-Hydroxysphingolipids are a subset of sphingolipids containing 2-hydroxy fatty acids. The 2-hydroxylation occurs during de novo ceramide synthesis and is catalyzed by fatty acid 2-hydroxylase (also known as fatty acid alpha-hydroxylase). In mammals, 2-hydroxysphingolipids are present abundantly in brain because the major myelin lipids galactosylceramides and sulfatides contain 2-hydroxy fatty acids. Here we report identification and characterization of a human gene that encodes a fatty acid 2-hydroxylase. Data base searches revealed a human homologue of the yeast ceramide 2-hydroxylase gene (FAH1), which we named FA2H. The FA2H gene encodes a 372-amino acid protein with 36% identity and 46% similarity to yeast Fah1p. The amino acid sequence indicates that FA2H protein contains an N-terminal cytochrome b5 domain and four potential transmembrane domains. FA2H also contains the iron-binding histidine motif conserved among membrane-bound desaturases/hydroxylases. COS7 cells expressing human FA2H contained 3-20-fold higher levels of 2-hydroxyceramides (C16, C18, C24, and C24:1) and 2-hydroxy fatty acids compared with control cells. Microsomal fractions prepared from transfected COS7 cells showed tetracosanoic acid 2-hydroxylase activities in an NADPH- and NADPH: cytochrome P-450 reductase-dependent manner. FA2H lacking the N-terminal cytochrome b5 domain had little activity, indicating that this domain is a functional component of this enzyme. Northern blot analysis showed that the FA2H gene is highly expressed in brain and colon tissues. These results demonstrate that the human FA2H gene encodes a fatty acid 2-hydroxylase. FA2H is likely involved in the formation of myelin 2-hydroxy galactosylceramides and -sulfatides.

Sphingolipid biosynthesis in cultured neurons. Down-regulation of serine palmitoyltransferase by sphingoid bases

Addition of exogenous sphingosine homologues (D-erythro configuration) with different alkyl chain lengths (12 and 18 carbon atoms) to the medium of primary cultured cerebellar cells resulted in a decrease of serine palmitoyltransferase activity in a time- and concentration-dependent manner. This enzyme catalyzes the first committed step in sphingolipid biosynthesis. Half-maximal reduction of enzyme activity occurred after a 4-h treatment with 25 microM sphingoid bases. Maximal decrease (approx. 80%) was obtained after treating the cells for 4-8 h with 50 microM long-chain bases. When a biosynthetically inert sphingoid, azidosphingosine (10-50 microM), was fed to the cells, decrease of 3-ketosphinganine formation was much slower, reaching its maximum (approx. 80%) after 24 h. In contrast to D-erythro-sphingosine, L-threo-C18-sphingosine did not yield any decrease of serine palmitoyltransferase activity when added to the cells under identical experimental conditions. Decrease of serine palmitoyltransferase activity was fully reversible after removal of the long-chain bases from the culture medium. Activities of other enzymes of lipid metabolism, ceramide synthase, long-chain acyl-CoA synthase and choline phosphotransferase, were not affected by the addition of sphingoid bases, indicating that the down regulation of serine palmitoyltransferase is quite specific.

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.

Alpha-hydroxylation of lignoceroyl-CoA by a cyanide-sensitive oxygenase in rat brain microsomes

The enzymatic mechanism of 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. Activity was linear with time (up to 40 min) and with protein (up to 0.8 mg). The apparent Km for lignoceroyl-CoA was about 10 microM. NADPH was a more efficient electron donor than NADH. Oxygen was required for activity, which increased linearly up to 20% O2. In 5 and 10% oxygen, the reaction was inhibited by 0.1 mM cyanide and by electron transfer chain inhibitors, cytochrome c, ferricyanide, menadione, and p-chloromercuriphenyl sulphonate; CO and SKF-525A had no effect. Moreover none of the inhibitors affected the formation of hydroxyceramide. Lignoceroyl-CoA alpha-hydroxylase appears to be an oxygenase requiring NADPH and oxygen, which involves cyanide-sensitive enzyme.

A comparative study of stearic and lignoceric acid oxidation by human skin fibroblasts

Sensitive assays were developed for long chain and very long chain fatty acid oxidation in human skin fibroblast homogenates. Stearic and lignoceric acids were degraded by the fibroblasts by the beta-oxidation pathway. The cofactor requirements for stearic and lignoceric acid beta-oxidation were very similar but not identical. For example, appreciable lignoceric acid oxidation could be demonstrated only in the presence of alpha-cyclodextrin and was inhibited by Triton X-100. In Zellweger's syndrome, stearic acid beta-oxidation was partially reduced whereas lignoceric acid beta-oxidation was reduced dramatically (less than 12% activity compared to the controls). The results presented suggest that stearic acid beta-oxidation occurs in mitochondria as well as in peroxisomes, but lignoceric acid oxidation occurs entirely in the peroxisomes. We suggest that the beta-oxidation systems for stearic acid and lignoceric acid may be different.

Effects of subperineurial injections of very-long-chain and medium-chain fatty acids into rat sciatic nerve

[9,10-3H] palmitic (C16:0) and [1-14C] lignoceric (C24:0) acid dissolved in 10 microL of ethanol were injected subperineurially into the sciatic nerve of rats. Both C16:0 and C24:0 were incorporated into lipids, and in most lipid fractions C16:0 incorporation exceeded that of C24:0. Free ceramide and cholesterol ester were the only lipid moieties in which C24:0 incorporation was equal to or greater than that of C16:0. This finding is of particular interest since the very-long-chain fatty acid excess is by far the most striking in the cholesterol ester fraction in adrenoleukodystrophy. Furthermore, incorporation into cerebroside and sulfatide indicates that at least some of the injected fatty acids were metabolized in the Schwann cell. Subperineurial injections of either very-long-chain fatty acids or medium-chain fatty acids into rat sciatic nerve caused demyelination, and this morphological change does not occur following injection of pure solvent.

Isotope effects: definitions and consequences for pharmacologic studies

The use of stable isotope-labeled compounds for pharmacologic studies requires careful consideration of the nature of the stable isotope label (2H, 13C, 15N, 18O) and its position of incorporation in the molecule. When deuterium is used, improper positioning can lead to significant primary isotope effects. Primary isotope effects occur when the breaking of the bond to the heavy isotope is the rate-limiting step in a reaction (or metabolic transformation). A reaction will proceed slower for the molecule with the heavy isotope label because of the mass difference between the light and the heavy isotope. In addition to these primary isotope effects, smaller but nevertheless important secondary isotope effects, physicochemical isotope effects, active hydrogen/deuterium exchange, or isotope effects associated with either the enzyme-catalyzed biotransformation or the mass spectrometric ionization and fragmentation can be operative. In mechanistic studies, isotope effects are used to their advantage; however, in pharmacokinetic studies, the occurrence of isotope effects can lead to grossly misleading biologic and analytic results: the metabolism of the drug will differ when 'in vivo' isotope effects are operative, and isotope effects occurring during the analysis procedure will obscure the true metabolic profile of the drug.

Characteristics of synthesis of very-long-chain saturated and tetraenoic fatty acids in swine cerebral microsomes

The elongation of arachidoyl-CoA (20:0-CoA) yielded 22:0 and 24:0 concomitantly, whereas the elongation of behenoyl-CoA (22:0-CoA) yielded only a negligible amount of 24:0 in adult swine cerebral microsomes. The dependence on time, pH, and the substrate concentrations were examined for the synthesis of 22:0 and 24:0 from 20:0-CoA. A microcomputer-aided simulation study suggested that there were two parallel pathways in the elongation of 20:0-CoA to 22:0 and 24:0. The elongation of 22:0-CoA could not be observed in adult swine cerebral microsomes; however, it was observed clearly in newborn swine and rat brain microsomes. A dilution experiment with the addition of cold 22:0-CoA in the reaction of elongation of 20:0-CoA confirmed the above suggestion that no intermediate 22:0 appeared during the synthesis of 24:0 from 20:0-CoA. The elongation of endogenous 20:4-CoA to 22:4 and 24:4 was examined in newborn swine cerebral microsomes, and the presence of two parallel pathways in the elongation of 20:4-CoA to 22:4 and 24:4 similar to those involved in the elongation of 20:0-CoA to 22:0 and 24:0 was suggested.

Accumulation and defective beta-oxidation of very long chain fatty acids in Zellweger's syndrome, adrenoleukodystrophy and Refsum's disease variants

The accumulation of very long chain fatty acids in plasma and skin fibroblasts was measured in at least four separate inherited disease states. Both the magnitude and the nature of the fatty acid changes reflected the clinical status of individual patients. In Zellweger's syndrome, and to a lesser extent in infantile Refsum's disease, there was an increase in 24:0, 26:0, 26:1, and a number of even longer chain fatty acids, while in the X-linked form of adrenoleukodystrophy these changes were less pronounced. Zellweger fibroblasts in culture took up lignoceric, phytanic and stearic acids and incorporated them into a variety of lipids in a manner comparable to control fibroblasts. However, these cells were unable to convert phytanic or lignoceric acid to CO2. Infantile Refsum's and X-linked adrenoleukodystrophy fibroblasts showed normal conversion of these acids to CO2. Normal fibroblast homogenates produced radioactive acetate from [1-14C] stearic and [1-14C] lignoceric acids indicating that both substrates were beta-oxidised under these conditions. Homogenates of fibroblasts from all patients patients with biochemical evidence of accumulation of very long chain fatty acids showed normal or near-normal stearic acid beta-oxidation, but were deficient in lignoceric acid beta-oxidation. Residual lignoceric acid beta-oxidation activity varied from approximately 15% in Zellweger syndrome up to 50% in X-linked adrenoleukodystrophy. It is postulated that the accumulation of very long chain fatty acids results from defects in peroxisomal beta-oxidation. In Zellweger's syndrome, and possibly in infantile Refsum's disease, it is probable that this defect is secondary to a primary abnormality affecting the structure and/or function of peroxisomes, while the primary defect in X-linked adrenoleukodystrophy may be confined to a pathway specific for the oxidation of very long chain fatty acids.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Ceramide synthesis from free fatty acids in rat brain: function of NADPH and substrate specificity

At the subcellular level, the synthesis of ceramide from free lignoceric acid and sphingosine in brain required reconstituted enzyme system (particulate fraction, heat-stable and heat-labile factors) and pyridine nucleotide (NADPH). The mitochondrial electron transfer inhibitors (KCN and antimycin A), energy uncouplers (oligomycin and 2,4-dinitrophenol), and carboxyatractyloside, which prevents the transport of ATP and ADP through the mitochondrial wall, inhibit the synthesis of ceramide in the presence of NADPH but have very little effect in the presence of ATP. Similar to the synthesis of ceramide, the synthesis of ATP from NADPH and NADH by the particulate fraction also required cytoplasmic factors (heat-stable and heat-labile factors). Moreover, ATP, but not its analog (AMP-CH2-P-O-P), can replace NADPH, thus suggesting that the function of the pyridine nucleotide is to provide ATP for the synthesis of ceramide. The cytoplasmic factors were not required for the synthesis of ceramide in the presence of ATP. The maximum velocity for synthesis of ceramide from free fatty acids of different chain lengths (C16-C26) was bimodal, with maxima around stearic acid (C18) and behenic acid (C22). The relative rate of synthesis of ceramide parallels the relative distribution of these fatty acids in brain cerebrosides and sulfatides.

alpha-Hydroxylation of fatty acids in brain: characterization of heat-labile factor

The particulate fraction, heat-labile factor, heat-stable factor, and NADPH are essential for the conversion of lignoceric acid (tetracosanoic acid) to cerebronic acid (alpha-hydroxylignoceric acid). The heat-labile factor was extracted from calf cerebellum and partially purified in four steps: ammonium sulfate precipitation, hydroxylapatite chromatography, isoelectric focusing, and NAD-Agarose affinity chromatography. The specific activity of the heat-labile factor was increased 105-fold during the last three steps, with a yield of 37% of the activity. One major and several minor bands were visible when the preparation was examined by SDS-polyacrylamide gel electrophoresis with Coomassie blue staining. The major band corresponded to a protein of molecular weight 32,700, and the minor bands corresponded to proteins of molecular weights 62,000 and 67,000. The activity was lost when the heat-labile factor was incubated with 1 mM-N-ethylmaleimide. This inhibition was prevented by preincubating the heat-labile factor with 1 mM-NADH. These observations indicate that the heat-labile factor contains a sulfhydryl group which is essential for activity, and that it is located at or near the binding site for the pyridine nucleotide.

Lipids of myelin, white matter and gray matter in a case of generalized deficiency of cytochrome b5 reductase in congenital methemoglobinemia with mental retardation

The lipid classes and fatty acid compositions of myelin, white matter and gray matter were analyzed in a case of generalized deficiency of cytochrome b5 reductase in congenital methemoglobinemia with mental retardation. When compared with normal data, the percentage of 24:1 was considerably decreased and diminished unsaturation was observed in cerebrosides, whereas the sum of 24:0 and 24:1 was the same as in normals. The ratio of hydroxy fatty acids to total fatty acids in cerebrosides was low. The contents of cholesterol and phospholipids in white matter were reduced to 80% of the normal, whereas cerebroside was reduced to 48% of the normal.