Involvement of microsomal fatty aldehyde dehydrogenase in the alpha-oxidation of phytanic acid

Fatty Acid Metabolism in Peroxisomes and Related Disorders

One of the functions of peroxisomes is the oxidation of fatty acids (FAs). The importance of this function in our lives is evidenced by the presence of peroxisomal disorders caused by the genetic deletion of proteins involved in these processes. Unlike mitochondrial oxidation, peroxisomal oxidation is not directly linked to ATP production. What is the role of FA oxidation in peroxisomes? Recent studies have revealed that peroxisomes supply the building blocks for lipid synthesis in the endoplasmic reticulum and facilitate intracellular carbon recycling for membrane quality control. Accumulation of very long-chain fatty acids (VLCFAs), which are peroxisomal substrates, is a diagnostic marker in many types of peroxisomal disorders. However, the relationship between VLCFA accumulation and various symptoms of these disorders remains unclear. Recently, we developed a method for solubilizing VLCFAs in aqueous media and found that VLCFA toxicity could be mitigated by oleic acid replenishment. In this chapter, we present the physiological role of peroxisomal FA oxidation and the knowledge obtained from VLCFA-accumulating peroxisome-deficient cells.

Sjogren-Larsson Syndrome: Mechanisms and Management

Sjogren Larsson syndrome (SLS) is a rare autosomal recessive inborn error of lipid metabolism due to mutations in the ALDH3A2 that result in a deficiency of fatty aldehyde dehydrogenase (FALDH). The syndrome has a high prevalence in Sweden where it was first described, but now known to occur worldwide. The classical triad of ichthyosis, mental retardation and spasticity characterizes clinical features. Preterm birth is common. “Glistening white dots” in the retina is a pathognomic clinical feature. Magnetic resonance imaging of the brain demonstrates leukoencephalopathy predominant in the periventricular region. Cerebral MR spectroscopy reveals a characteristic abnormal lipid peak at 1.3ppm and a small peak at 0.9ppm. The primary role of FALDH is oxidation of medium and long-chain aliphatic aldehydes derived from fatty alcohol, phytanic acid, ether glycerolipids and sphingolipids. The diagnosis is based on the typical phenotype, demonstration of the enzyme deficiency and presence of biallelic mutations in the ALDH3A2. The management of SLS largely remains symptomatic currently. However, several potential therapeutic options are being developed, keeping in view of the fundamental metabolic defects or correcting the genetic defect. This review aims to summarize the clinical, genetic and biochemical findings, pathogenetic mechanisms and the current therapeutic options, in SLS.

Disruption of the Sjögren-Larsson Syndrome Gene Aldh3a2 in Mice Increases Keratinocyte Growth and Retards Skin Barrier Recovery

The fatty aldehyde dehydrogenase (FALDH) ALDH3A2 is the causative gene of Sjögren Larsson syndrome (SLS). To date, the molecular mechanism underlying the symptoms characterizing SLS has been poorly understood. Using Aldh3a2(-/-) mice, we found here that Aldh3a2 was the major FALDH active in undifferentiated keratinocytes. Long-chain base metabolism was greatly impaired in Aldh3a2(-/-) keratinocytes. Phenotypically, the intercellular spaces were widened in the basal layer of the Aldh3a2(-/-) epidermis due to hyperproliferation of keratinocytes. Furthermore, oxidative stress-induced genes were up-regulated in Aldh3a2(-/-) keratinocytes. Upon keratinocyte differentiation, the activity of another FALDH, Aldh3b2, surpassed that of Aldh3a2 As a result, Aldh3a2(-/-) mice were indistinguishable from wild-type mice in terms of their whole epidermis FALDH activity, and their skin barrier function was uncompromised under normal conditions. However, perturbation of the stratum corneum caused increased transepidermal water loss and delayed barrier recovery in Aldh3a2(-/-) mice. In conclusion, Aldh3a2(-/-) mice replicated some aspects of SLS symptoms, especially at the basal layer of the epidermis. Our results suggest that hyperproliferation of keratinocytes via oxidative stress responses may partly contribute to the ichthyosis symptoms of SLS.

Genetics and prospective therapeutic targets for Sjögren-Larsson Syndrome

INTRODUCTION

Sjögren-Larsson syndrome (SLS) is a rare neurocutaneous disease characterized by ichthyosis, spasticity, intellectual disability and a distinctive retinopathy. It is caused by inactivating mutations in ALDH3A2, which codes for fatty aldehyde dehydrogenase (FALDH) and results in abnormal metabolism of long-chain aliphatic aldehydes and alcohols. The potential disease mechanisms leading to symptoms include 1) accumulation of toxic fatty aldehydes that form covalent adducts with lipids and membrane proteins; 2) physical disruption of multi-lamellar membranes in skin and brain; 3) abnormal activation of the JNK cell signaling pathway; and 4) defective farnesol metabolism resulting in abnormal PPAR-α dependent gene expression. Currently, no effective pathogenesis-based therapy is available.

AREAS COVERED

The clinical, pathologic and genetic features of SLS are summarized. The biochemical abnormalities caused by deficient activity of FALDH are reviewed in the context of proposed pathogenic mechanisms and potential therapeutic interventions.

EXPERT OPINION

The most promising pharmacologic approach to SLS involves blocking the formation of potentially harmful fatty aldehyde adducts using aldehyde scavenging drugs, currently in phase 2 clinical trials. Other approaches needing further investigation include: 1) ALDH-specific activator drugs and PPAR-α agonists to increase mutant FALDH activity; 2) inhibitors of the JNK phosphorylation cascade; 3) antioxidants to decrease aldehyde load; 4) dietary lipid modification; and 5) gene therapy.

Phytanoyl-CoA Hydroxylase: A 2-Oxoglutarate-Dependent Dioxygenase Crucial for Fatty Acid Alpha-Oxidation in Humans

Phytanoyl-CoA hydroxylase belongs to the family of 2-oxoglutarate-dependent dioxygenases and plays a crucial role in the α-oxidation of fatty acids. The complete α-oxidation pathway involves five different enzymes localized in peroxisomes. Thus far, phytanoyl-CoA hydroxylase deficiency has remained the only genetically determined inborn error of metabolism affecting the α-oxidation pathway. In this chapter we describe the current state of knowledge on fatty acid α-oxidation with special emphasis on phytanoyl-CoA hydroxylase and its deficiency in Refsum disease.

Fatty aldehyde dehydrogenase multigene family involved in the assimilation of n-alkanes in Yarrowia lipolytica

In the n-alkane assimilating yeast Yarrowia lipolytica, n-alkanes are oxidized to fatty acids via fatty alcohols and fatty aldehydes, after which they are utilized as carbon sources. Here, we show that four genes (HFD1-HFD4) encoding fatty aldehyde dehydrogenases (FALDHs) are involved in the metabolism of n-alkanes in Y. lipolytica. A mutant, in which all of four HFD genes are deleted (Δhfd1-4 strain), could not grow on n-alkanes of 12-18 carbons; however, the expression of one of those HFD genes restored its growth on n-alkanes. Production of Hfd2Ap or Hfd2Bp, translation products of transcript variants generated from HFD2 by the absence or presence of splicing, also supported the growth of the Δhfd1-4 strain on n-alkanes. The FALDH activity in the extract of the wild-type strain was increased when cells were incubated in the presence of n-decane, whereas this elevation in FALDH activity by n-decane was not observed in Δhfd1-4 strain extract. Substantial FALDH activities were detected in the extracts of Escherichia coli cells expressing the HFD genes. Fluorescent microscopic observation suggests that Hfd3p and Hfd2Bp are localized predominantly in the peroxisome, whereas Hfd1p and Hfd2Ap are localized in both the endoplasmic reticulum and the peroxisome. These results suggest that the HFD multigene family is responsible for the oxidation of fatty aldehydes to fatty acids in the metabolism of n-alkanes, and raise the possibility that Hfd proteins have diversified by gene multiplication and RNA splicing to efficiently assimilate or detoxify fatty aldehydes in Y. lipolytica.

Fatty aldehyde and fatty alcohol metabolism: review and importance for epidermal structure and function

Normal fatty aldehyde and alcohol metabolism is essential for epidermal differentiation and function. Long-chain aldehydes are produced by catabolism of several lipids including fatty alcohols, sphingolipids, ether glycerolipids, isoprenoid alcohols and certain aliphatic lipids that undergo α- or ω-oxidation. The fatty aldehyde generated by these pathways is chiefly metabolized to fatty acid by fatty aldehyde dehydrogenase (FALDH, alternately known as ALDH3A2), which also functions to oxidize fatty alcohols as a component of the fatty alcohol:NAD oxidoreductase (FAO) enzyme complex. Genetic deficiency of FALDH/FAO in patients with Sjögren-Larsson syndrome (SLS) results in accumulation of fatty aldehydes, fatty alcohols and related lipids (ether glycerolipids, wax esters) in cultured keratinocytes. These biochemical changes are associated with abnormalities in formation of lamellar bodies in the stratum granulosum and impaired delivery of their precursor membranes to the stratum corneum (SC). The defective extracellular SC membranes are responsible for a leaky epidermal water barrier and ichthyosis. Although lamellar bodies appear to be the pathogenic target for abnormal fatty aldehyde/alcohol metabolism in SLS, the precise biochemical mechanisms are yet to be elucidated. Nevertheless, studies in SLS highlight the critical importance of FALDH and normal fatty aldehyde/alcohol metabolism for epidermal function. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Sphingosine 1-phosphate is a key metabolite linking sphingolipids to glycerophospholipids

The sphingolipid metabolite sphingosine 1-phosphate (S1P) is a well-known lipid mediator. As a lipid mediator, S1P must be present in extracellular space and bind to its cell surface receptors (S1P1-5). However, most S1P, synthesized intracellularly, is metabolized without being released into extracellular space, in other words, without functioning as a lipid mediator in the vast majority of cells except those supplying plasma and lymph S1P such as blood cells and endothelial cells. Instead, intracellular S1P plays an important role as an intermediate of the sole sphingolipid-to-glycerophospholipid metabolic pathway. The degradation of S1P by S1P lyase is the first irreversible reaction (committed step) of this pathway. This metabolic pathway is conserved in eukaryotes from yeast to human, indicating its much older origin than the function of S1P as a lipid mediator, which is found to be present only in vertebrates and chordates. The sphingolipid-to-glycerophospholipid metabolism takes place ubiquitously in mammalian tissues, and its defect causes an aberration of several tissue functions as well as abnormal lipid metabolism. Although this metabolic pathway has been known for over four decades, only recently the precise reactions and enzymes involved in this pathway have been revealed. This review will focus on the recent advances in our understanding of the sphingolipid metabolic pathway via S1P and its physiological and pathological roles. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Substrate specificity, plasma membrane localization, and lipid modification of the aldehyde dehydrogenase ALDH3B1

The accumulation of reactive aldehydes is implicated in the development of several disorders. Aldehyde dehydrogenases (ALDHs) detoxify aldehydes by oxidizing them to the corresponding carboxylic acids. Among the 19 human ALDHs, ALDH3A2 is the only known ALDH that catalyzes the oxidation of long-chain fatty aldehydes including C16 aldehydes (hexadecanal and trans-2-hexadecenal) generated through sphingolipid metabolism. In the present study, we have identified that ALDH3B1 is also active in vitro toward C16 aldehydes and demonstrated that overexpression of ALDH3B1 restores the sphingolipid metabolism in the ALDH3A2-deficient cells. In addition, we have determined that ALDH3B1 is localized in the plasma membrane through its C-terminal dual lipidation (palmitoylation and prenylation) and shown that the prenylation is required particularly for the activity toward hexadecanal. Since knockdown of ALDH3B1 does not cause further impairment of the sphingolipid metabolism in the ALDH3A2-deficient cells, the likely physiological function of ALDH3B1 is to oxidize lipid-derived aldehydes generated in the plasma membrane and not to be involved in the sphingolipid metabolism in the endoplasmic reticulum.

Sjögren-Larsson syndrome in clinical practice

This review article gives a state-of-the-art synopsis of current pathophysiological concepts in Sjögren-Larsson syndrome (SLS) mainly based upon original research data of the authors in one of the world's largest clinical SLS study cohorts. Clinical features are discussed in order of appearance, and diagnostic tests are set out to guide the clinician toward the diagnosis SLS. Furthermore, current and future treatment strategies are discussed to render a comprehensive review of the topic.

Phytanic acid metabolism in health and disease

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is a branched-chain fatty acid which cannot be beta-oxidized due to the presence of the first methyl group at the 3-position. Instead, phytanic acid undergoes alpha-oxidation to produce pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) plus CO(2). Pristanic acid is a 2-methyl branched-chain fatty acid which can undergo beta-oxidation via sequential cycles of beta-oxidation in peroxisomes and mitochondria. The mechanism of alpha-oxidation has been resolved in recent years as reviewed in this paper, although some of the individual enzymatic steps remain to be identified. Furthermore, much has been learned in recent years about the permeability properties of the peroxisomal membrane with important consequences for the alpha-oxidation process. Finally, we present new data on the omega-oxidation of phytanic acid making use of a recently generated mouse model for Refsum disease in which the gene encoding phytanoyl-CoA 2-hydroxylase has been disrupted.

The role of fatty aldehyde dehydrogenase in epidermal structure and function

The epidermal water barrier resides in the stratum corneum (SC) and is dependent on a highly organized network of multi-lamellar membranes comprised of a critical lipid composition. The SC membranes are formed from precursor membranes packaged in cytoplasmic lamellar bodies in the stratum granulosum and delivered to the SC by exocytosis. An abnormal lipid composition of the SC membranes often results in a disrupted water barrier and the clinical appearance of ichthyosis. This cutaneous feature is characteristic of Sjögren-Larsson syndrome (SLS), an inborn error of lipid metabolism caused by deficiency of fatty aldehyde dehydrogenase (FALDH). The contribution of FALDH to normal epidermal function has become increasingly evident with the recognition that this enzyme has an essential role in metabolism of several lipids, including fatty aldehydes and alcohols, ether glycerolipids, isoprenoid alcohols and certain lipids that undergo ω-oxidation, such as leukotriene B4 and very long-chain fatty acids. In the absence of FALDH, the skin produces lamellar bodies that are empty, lack their surrounding vesicle membranes or contain granular contents rather then the usual cargo membranes. These defective organelles also have impaired exocytosis, which results in structurally abnormal, deficient multi-lamellar membranes in the SC and a leaky water barrier. Although the exact biochemical mechanism for the cutaneous pathology is still unclear, studies in SLS demonstrate the critical importance of FALDH for normal epidermal structure and function.

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.

MR spectroscopy and diffusion tensor imaging of the brain in Sjögren-Larsson syndrome

Diffusion tensor imaging (DTI) is reported for the first time in a patient with Sjögren-Larsson syndrome, an autosomal recessive neurocutaneous disorder. Magnetic resonance spectroscopy (MRS) revealed normal levels of choline, creatine and N-acetyl aspartate (NAA) and the characteristic lipid signals in the white matter brain tissue. Conventional MRI showed increased signal intensity around the lateral ventricles indicating abnormal myelination. DTI revealed normal apparent diffusion coefficient (ADC) values, but reduced fractional anisotropy (FA) in the white matter. After co-registration of the parameters obtained with DTI with the results of MRS (36 voxels), significant correlations were obtained of lipid content with FA (r=0.81), ADC (r=-0.62), choline (r=0.51), and NAA (r=0.44) (P<0.01, all). These results suggest that in Sjögren-Larsson syndrome, the white matter lipid signals originate from the neurons, with NAA and choline reflecting neuron density and myelination. The comparatively high FA/low ADC values in these lipid-rich locations, indicate a loss of diffusion in directions perpendicular to the fibers. The overall loss of FA in the white matter may reflect a loss of brain tissue water content in SLS patients compared with controls and precede the formation of atrophy.

Non-P450 aldehyde oxidizing enzymes: the aldehyde dehydrogenase superfamily

BACKGROUND

Aldehydes are highly reactive molecules. While several non-P450 enzyme systems participate in their metabolism, one of the most important is the aldehyde dehydrogenase (ALDH) superfamily, composed of NAD(P)+-dependent enzymes that catalyze aldehyde oxidation.

OBJECTIVE

This article presents a review of what is currently known about each member of the human ALDH superfamily including the pathophysiological significance of these enzymes.

METHODS

Relevant literature involving all members of the human ALDH family was extensively reviewed, with the primary focus on recent and novel findings.

CONCLUSION

To date, 19 ALDH genes have been identified in the human genome and mutations in these genes and subsequent inborn errors in aldehyde metabolism are the molecular basis of several diseases, including Sjögren-Larsson syndrome, type II hyperprolinemia, gamma-hydroxybutyric aciduria and pyridoxine-dependent seizures. ALDH enzymes also play important roles in embryogenesis and development, neurotransmission, oxidative stress and cancer. Finally, ALDH enzymes display multiple catalytic and non-catalytic functions including ester hydrolysis, antioxidant properties, xenobiotic bioactivation and UV light absorption.

Alpha-methylacyl-CoA racemase--an 'obscure' metabolic enzyme takes centre stage

Branched-chain lipids are important components of the human diet and are used as drug molecules, e.g. ibuprofen. Owing to the presence of methyl groups on their carbon chains, they cannot be metabolized in mitochondria, and instead are processed and degraded in peroxisomes. Several different oxidative degradation pathways for these lipids are known, including alpha-oxidation, beta-oxidation, and omega-oxidation. Dietary branched-chain lipids (especially phytanic acid) have attracted much attention in recent years, due to their link with prostate, breast, colon and other cancers as well as their role in neurological disease. A central role in all the metabolic pathways is played by alpha-methylacyl-CoA racemase (AMACR), which regulates metabolism of these lipids and drugs. AMACR catalyses the chiral inversion of a diverse number of 2-methyl acids (as their CoA esters), and regulates the entry of branched-chain lipids into the peroxisomal and mitochondrial beta-oxidation pathways. This review brings together advances in the different disciplines, and considers new research in both the metabolism of branched-chain lipids and their role in cancer, with particular emphasis on the crucial role played by AMACR. These recent advances enable new preventative and treatment strategies for cancer.

Pathogenesis of permeability barrier abnormalities in the ichthyoses: inherited disorders of lipid metabolism

Many of the ichthyoses are associated with inherited disorders of lipid metabolism. These disorders have provided unique models to dissect physiologic processes in normal epidermis and the pathophysiology of more common scaling conditions. In most of these disorders, a permeability barrier abnormality "drives" pathophysiology through stimulation of epidermal hyperplasia. Among primary abnormalities of nonpolar lipid metabolism, triglyceride accumulation in neutral lipid storage disease as a result of a lipase mutation provokes a barrier abnormality via lamellar/nonlamellar phase separation within the extracellular matrix of the stratum corneum (SC). Similar mechanisms account for the barrier abnormalities (and subsequent ichthyosis) in inherited disorders of polar lipid metabolism. For example, in recessive X-linked ichthyosis (RXLI), cholesterol sulfate (CSO(4)) accumulation also produces a permeability barrier defect through lamellar/nonlamellar phase separation. However, in RXLI, the desquamation abnormality is in part attributable to the plurifunctional roles of CSO(4) as a regulator of both epidermal differentiation and corneodesmosome degradation. Phase separation also occurs in type II Gaucher disease (GD; from accumulation of glucosylceramides as a result of to beta-glucocerebrosidase deficiency). Finally, failure to assemble both lipids and desquamatory enzymes into nascent epidermal lamellar bodies (LBs) accounts for both the permeability barrier and desquamation abnormalities in Harlequin ichthyosis (HI). The barrier abnormality provokes the clinical phenotype in these disorders not only by stimulating epidermal proliferation, but also by inducing inflammation.

Abnormal fatty alcohol metabolism in cultured keratinocytes from patients with Sjögren-Larsson syndrome

Sjögren-Larsson syndrome (SLS) is an inherited neurocutaneous disorder characterized by ichthyosis, mental retardation, spasticity, and deficient activity of fatty aldehyde dehydrogenase (FALDH). FALDH is an enzyme component of fatty alcohol:NAD oxidoreductase (FAO), which is necessary for fatty alcohol metabolism. To better understand the biochemical basis for the cutaneous symptoms in this disease, we investigated lipid metabolism in cultured keratinocytes from SLS patients. Enzyme activities of FALDH and FAO in SLS cells were <10% of normal. SLS keratinocytes accumulated 45-fold more fatty alcohol (hexadecanol, octadecanol, and octadecenol) than normal, whereas wax esters and 1-O-alkyl-2,3-diacylglycerols were increased by 5.6-fold and 7.5-fold, respectively. SLS keratinocytes showed a reduced incorporation of radioactive octadecanol into fatty acid (24% of normal) and triglyceride (13% of normal), but incorporation into wax esters and 1-O-alkyl-2,3-diacylglycerol was increased by 2.5-fold and 2.8-fold, respectively. Our results indicate that FALDH deficiency in SLS keratinocytes causes the accumulation and diversion of fatty alcohol into alternative biosynthetic pathways. The striking lipid abnormalities in cultured SLS keratinocytes are distinct from those seen in fibroblasts and may be related to the stratum corneum dysfunction and ichthyosis in SLS.

Peroxisomal disorders affecting phytanic acid α-oxidation: a review

Peroxisomes are involved in the synthesis and degradation of complex fatty acids. They contain enzymes involved in the α-, β- and ω-oxidation pathways for fatty acids. Investigation of these pathways and the diseases associated with mutations in enzymes involved in the degradation of phytanic acid have led to the clarification of the pathophysiology of Refsum's disease, rhizomelic chondrodysplasia and AMACR (α-methylacyl-CoA racemase) deficiency. This has highlighted the role of an Fe(II)- and 2-oxoglutarate-dependent oxygenases [PhyH (phytanoyl-CoA 2-hydroxylase), also known as PAHX], thiamin-dependent lyases (phytanoyl-CoA lyase) and CYP (cytochrome P450) family 4A in fatty acid metabolism. The differential regulation and biology of these pathways is suggesting novel ways to treat the neuro-ophthalmological sequelae of Refsum's disease. More recently, the discovery that AMACR and other peroxisomal β-oxidation pathway enzymes are highly expressed in prostate and renal cell cancers has prompted active investigation into the role of these oxidation pathways and the peroxisome in the progression of obesity- and insulin resistance-related cancers.

Peroxisomes contain a specific phytanoyl-CoA/pristanoyl-CoA thioesterase acting as a novel auxiliary enzyme in alpha- and beta-oxidation of methyl-branched fatty acids in mouse

Phytanic acid and pristanic acid are derived from phytol, which enter the body via the diet. Phytanic acid contains a methyl group in position three and, therefore, cannot undergo beta-oxidation directly but instead must first undergo alpha-oxidation to pristanic acid, which then enters beta-oxidation. Both these pathways occur in peroxisomes, and in this study we have identified a novel peroxisomal acyl-CoA thioesterase named ACOT6, which we show is specifically involved in phytanic acid and pristanic acid metabolism. Sequence analysis of ACOT6 revealed a putative peroxisomal targeting signal at the C-terminal end, and cellular localization experiments verified it as a peroxisomal enzyme. Subcellular fractionation experiments showed that peroxisomes contain by far the highest phytanoyl-CoA/pristanoyl-CoA thioesterase activity in the cell, which could be almost completely immunoprecipitated using an ACOT6 antibody. Acot6 mRNA was mainly expressed in white adipose tissue and was co-expressed in tissues with Acox3 (the pristanoyl-CoA oxidase). Furthermore, Acot6 was identified as a target gene of the peroxisome proliferator-activated receptor alpha (PPARalpha) and is up-regulated in mouse liver in a PPARalpha-dependent manner.

Dual subcellular localization in the endoplasmic reticulum and peroxisomes and a vital role in protecting against oxidative stress of fatty aldehyde dehydrogenase are achieved by alternative splicing

Fatty aldehyde dehydrogenase (FALDH, ALDH3A2) is thought to be involved in the degradation of phytanic acid, a saturated branched chain fatty acid derived from chlorophyll. However, the identity, subcellular distribution, and physiological roles of FALDH are unclear because several variants produced by alternative splicing are present in varying amounts at different subcellular locations. Subcellular fractionation experiments do not provide a clear-cut conclusion because of the incomplete separation of organelles. We established human cell lines heterologously expressing mouse FALDH from each cDNA without tagging under the control of an inducible promoter and detected the variant FALDH proteins using a mouse FALDH-specific antibody. One variant, FALDH-V, was exclusively detected in peroxisomal membranes. Human FALDH-V with an amino-terminal Myc sequence also localized to peroxisomes. The most dominant form, FALDH-N, and other variants examined, however, were distributed in the endoplasmic reticulum. A gas chromatography-mass spectrometry-based analysis of metabolites in FALDH-expressing cells incubated with phytol or phytanic acid showed that FALDH-V, not FALDH-N, is the key aldehyde dehydrogenase in the degradation pathway and that it protects peroxisomes from oxidative stress. In contrast, both FALDHs had a protective effect against oxidative stress induced by a model aldehyde for lipid peroxidation, dodecanal. These results suggest that FALDH variants are produced by alternative splicing and share an important role in protecting against oxidative stress in an organelle-specific manner.

Sjögren-Larsson syndrome: molecular genetics and biochemical pathogenesis of fatty aldehyde dehydrogenase deficiency

Sjögren-Larsson syndrome (SLS) is an inherited neurocutaneous disorder caused by mutations in the ALDH3A2 gene that encodes fatty aldehyde dehydrogenase (FALDH), an enzyme that catalyzes the oxidation of fatty aldehyde to fatty acid. Affected patients display ichthyosis, mental retardation and spastic diplegia. More than 70 mutations in ALDH3A2 have been discovered in SLS patients including amino acid substitutions, deletions, insertions and splicing errors. Most mutations are private, but several common mutations reflect founder effects, consanguinity or recurrent mutational events. FALDH oxidizes fatty aldehyde substrates arising from metabolism of fatty alcohols, leukotriene B4, ether glycerolipids and other potential sources such as sphingolipids. The pathogenesis of the cutaneous and neurologic symptoms is thought to result from abnormal lipid accumulation in the membranes of skin and brain; the formation of aldehyde Schiff base adducts with amine-containing lipids or proteins; or defective eicosanoid metabolism. Therapeutic approaches are being developed to target specific metabolic defects associated with FALDH deficiency or to correct the genetic defect by gene transfer.

Alpha-Oxidation

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is a branched chain fatty acid, which is a constituent of the human diet. The presence of the 3-methyl group of phytanic acid prevents degradation by beta-oxidation. Instead, the terminal carboxyl group is first removed by alpha-oxidation. The mechanism of the alpha-oxidation pathway and the enzymes involved are described in this review.

Sjögren-Larsson syndrome: diversity of mutations and polymorphisms in the fatty aldehyde dehydrogenase gene (ALDH3A2)

Sjögren-Larsson syndrome (SLS) is an autosomal recessive disorder characterized by ichthyosis, mental retardation, and spastic diplegia or tetraplegia. The disease is caused by mutations in the ALDH3A2 gene (also known as FALDH and ALDH10) on chromosome 17p11.2 that encodes fatty aldehyde dehydrogenase (FALDH), an enzyme that catalyzes the oxidation of long-chain aldehydes derived from lipid metabolism. In SLS patients, 72 mutations have been identified, with a distribution that is scattered throughout the ALDH3A2 gene. Most mutations are private but several common mutations have been detected, which probably reflect founder effects or recurrent mutational events. Missense mutations comprise the most abundant class (38%) and expression studies indicate that most of these result in a profound reduction in enzyme activity. Deletions account for about 25% of the mutations and range from single nucleotides to entire exons. Twelve splice-site mutations have been demonstrated to cause aberrant splicing in cultured fibroblasts. To date, more than a dozen intragenic ALDH3A2 polymorphisms consisting of SNPs and one microsatellite marker have been characterized, although none of them alter the FALDH protein sequence. The striking mutational diversity in SLS offers a challenge for DNA-based diagnosis, but promises to provide a wealth of information about enzyme structure-function correlations.

Oxidative one-carbon cleavage of the octyl side chain of olanexidine, a novel antimicrobial agent, in dog liver microsomes

1. The oxidative one-carbon cleavage reaction in the octyl side chain of olanexidine [1-(3,4-dichlorobenzyl)-5-octylbiguanide], a new potent biguanide antiseptic, was characterized in dog liver microsomes. 2. Olanexidine was initially biotransformed to a monohydroxylated metabolite, 8-[5-(3,4-dichlorobenzyl)-1-biguanidino]-2-octanol (DM-215), and DM-215 was subsequently oxidized to the diol derivative, 8-[5-(3,4-dichlorobenzyl)-1-biguanidino]-1,2-octandiol (DM-220). DM-220 was further biotransformed to 2-hydroxy aldehyde derivative, 2-hydroxy carboxylic acid derivative, and an oxidative C-1-C-2 bond cleavage metabolite, 7-[5-(3,4-dichlorobenzyl)-1-biguanidino] heptanoic acid [DM-223 (C7), a seven-carbon chain derivative], after incubation with dog liver microsomes. 3. DM-223 formation required NADPH as a cofactor and was inhibited by quinidine and quinine, relatively selective inhibitors of CYP2D subfamilies in dogs. 4. The results suggest that the one-carbon fragment of the octyl side chain of olanexidine could be removed by the oxidative C-C bond cleavage with the possible involvement of cytochrome P450 systems such as CYP2D subfamily. This oxidative C-C bond cleavage reaction by cytochrome P450s could play an important role in the removal of one-carbon fragment of other drugs or endogenous compounds containing aliphatic chains.

Breakdown of 2-hydroxylated straight chain fatty acids via peroxisomal 2-hydroxyphytanoyl-CoA lyase: a revised pathway for the alpha-oxidation of straight chain fatty acids

2-Hydroxyfatty acids, constituents of brain cerebrosides and sulfatides, were previously reported to be degraded by an alpha-oxidation system, generating fatty acids shortened by one carbon atom. In the current study we used labeled and unlabeled 2-hydroxyoctadecanoic acid to reinvestigate the degradation of this class of lipids. Both in intact and broken cell systems formate was identified as a main reaction product. Furthermore, the generation of an n-1 aldehyde was demonstrated. In permeabilized rat hepatocytes and liver homogenates, studies on cofactor requirements revealed a dependence on ATP, CoA, Mg(2+), thiamine pyrophosphate, and NAD(+). Together with subcellular fractionation data and studies on recombinant enzymes, this led to the following picture. In a first step, the 2-hydroxyfatty acid is activated to an acyl-CoA; subsequently, the 2-hydroxy fatty acyl-CoA is cleaved by 2-hydroxyphytanoyl-CoA lyase, to formyl-CoA and an n-1 aldehyde. The severe inhibition of formate generation by oxythiamin treatment of intact fibroblasts indicates that cleavage through the thiamine pyrophosphate-dependent 2-hydroxyphytanoyl-CoA lyase is the main pathway for the degradation of 2-hydroxyfatty acids. The latter protein was initially characterized as an essential enzyme in the peroxisomal alpha-oxidation of 3-methyl-branched fatty acids such as phytanic acid. Our findings point to a new role for peroxisomes in mammals, i.e. the breakdown of 2-hydroxyfatty acids, at least the long chain 2-hydroxyfatty acids. Most likely, the more abundant very long chain 2-hydroxyfatty acids are degraded in a similar manner.

The chemical biology of branched-chain lipid metabolism

Mammalian metabolism of some lipids including 3-methyl and 2-methyl branched-chain fatty acids occurs within peroxisomes. Such lipids, including phytanic and pristanic acids, are commonly found within the human diet and may be derived from chlorophyll in plant extracts. Due to the presence of a methyl group at its beta-carbon, the well-characterised beta-oxidation pathway cannot degrade phytanic acid. Instead its alpha-methylene group is oxidatively excised to give pristanic acid, which can be metabolised by the beta-oxidation pathway. Many defects in the alpha-oxidation pathway result in an accumulation of phytanic acid, leading to neurological distress, deterioration of vision, deafness, loss of coordination and eventual death. Details of the alpha-oxidation pathway have only recently been elucidated, and considerable progress has been made in understanding the detailed enzymology of one of the oxidative steps within this pathway. This review summarises these recent advances and considers the roles and likely mechanisms of the enzymes within the alpha-oxidation pathway.

Alpha-oxidation of 3-methyl-substituted fatty acids and its thiamine dependence

3-Methyl-branched fatty acids, as phytanic acid, undergo peroxisomal alpha-oxidation in which they are shortened by 1 carbon atom. This process includes four steps: activation, 2-hydroxylation, thiamine pyrophosphate dependent cleavage and aldehyde dehydrogenation. The thiamine pyrophosphate dependence of the third step is unique in peroxisomal mammalian enzymology. Human pathology due to a deficient alpha-oxidation is mostly linked to mutations in the gene coding for the second enzyme of the sequence, phytanoyl-CoA hydroxylase.

Human metabolism of phytanic acid and pristanic acid

Phytanic acid is a methyl-branched fatty acid present in the human diet. Due to its structure, degradation by beta-oxidation is impossible. Instead, phytanic acid is oxidized by alpha-oxidation, yielding pristanic acid. Despite many efforts to elucidate the alpha-oxidation pathway, it remained unknown for more than 30 years. In recent years, the mechanism of alpha-oxidation as well as the enzymes involved in the process have been elucidated. The process was found to involve activation, followed by hydroxylase, lyase and dehydrogenase reactions. Part, if not all of the reactions were found to take place in peroxisomes. The final product of phytanic acid alpha-oxidation is pristanic acid. This fatty acid is degraded by peroxisomal beta-oxidation. After 3 steps of beta-oxidation in the peroxisome, the product is esterified to carnitine and shuttled to the mitochondrion for further oxidation. Several inborn errors with one or more deficiencies in the phytanic acid and pristanic degradation have been described. The clinical expressions of these disorders are heterogeneous, and vary between severe neonatal and often fatal symptoms and milder syndromes with late onset. Biochemically, these disorders are characterized by accumulation of phytanic and/or pristanic acid in tissues and body fluids. Several of the inborn errors involving phytanic acid and/or pristanic acid metabolism have been characterized on the molecular level.

Identification of pristanal dehydrogenase activity in peroxisomes: conclusive evidence that the complete phytanic acid alpha-oxidation pathway is localized in peroxisomes

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is a branched-chain fatty acid which, due to the methyl-group at the 3-position, can not undergo beta-oxidation unless the terminal carboxyl-group is removed by alpha-oxidation. The structure of the phytanic acid alpha-oxidation machinery in terms of the reactions involved, has been resolved in recent years and includes a series of four reactions: (1) activation of phytanic acid to phytanoyl-CoA, (2) hydroxylation of phytanoyl-CoA to 2-hydroxyphytanoyl-CoA, (3) cleavage of 2-hydroxyphytanoyl-CoA to pristanal and formyl-CoA, and (4) oxidation of pristanal to pristanic acid. The subcellular localization of the enzymes involved has remained enigmatic, with the exception of phytanoyl-CoA hydroxylase and 2-hydroxyphytanoyl-CoA lyase which are both localized in peroxisomes. The oxidation of pristanal to pristanic acid has been claimed to be catalysed by the microsomal aldehyde dehydrogenase FALDH encoded by the ALDH10-gene. Making use of mutant fibroblasts deficient in FALDH activity, we show that phytanic acid alpha-oxidation is completely normal in these cells. Furthermore, we show that pristanal dehydrogenase activity is not fully deficient in FALDH-deficient cells, implying the existence of one or more additional aldehyde dehydrogenases reacting with pristanal. Using subcellular localization studies, we now show that peroxisomes contain pristanal dehydrogenase activity which leads us to conclude that the complete phytanic acid alpha-oxidation pathway is localized in peroxisomes.

Corneal and stomach expression of aldehyde dehydrogenases: from fish to mammals

We have studied the distribution of the ALDH3A1, ALDH1A1 and ALDH2 proteins in the cornea and stomach of several animal species, including mammals (C57BL/6J and SWR/J mice, rat and pig), birds (chicken and turkey), amphibians (frog) and fish (trout and zebrafish). High ALDH3A1 protein levels and catalytic activities were detected in C57BL/6J mouse, rat and pig. We found complete absence of the ALDH3A1 protein in SWR/J mice, which carry the Aldh3a1(c) allele characterized by four amino acid substitutions (G88R, I154N, H305R and I352V) and lack of enzymatic activity. This indicates that the SWR/J mouse strain is a natural gene knockout model for ALDH3A1. Traces of ALDH3A1 were detected in rabbit, whereas expression was absent from chicken, turkey, frog, trout, and zebrafish. Interestingly, significant levels of the cytosolic ALDH1A1 and mitochondrial ALDH2 proteins were detected by immunoblot analysis in all examined species that are deficient in ALDH3A1 expression. In contrast, no ALDH1A1 or ALDH2 protein was detected in the species expressing ALDH3A1. It can, therefore, be concluded that corneal expression of ALDH3A1 or ALDH1A1/ALDH2 occurs in a taxon-specific manner, supporting the protective role of these ALDHs in cornea against the UV-induced oxidative damage.

Fatty aldehyde dehydrogenase: genomic structure, expression and mutation analysis in Sjögren-Larsson syndrome

Fatty aldehyde dehydrogenase (FALDH) is a microsomal enzyme that catalyzes the oxidation of medium- and long-chain aliphatic aldehydes derived from metabolism of fatty alcohol, phytanic acid, ether glycerolipids and leukotriene B4. The FALDH gene (ALDH3A2) in man and mouse consists of 11 exons and is closely linked to the gene for ALDH3. In both species, alternative splicing results in formation of a second minor protein, FALDHv, that has a unique carboxy-terminal end. The functional significance of this alternate protein is not known. In humans, mutations in the FALDH gene cause Sjögren-Larsson syndrome (SLS), which is characterized by ichthyosis, mental retardation and spasticity. Missense mutations involving 24 amino acid positions in FALDH have been identified. These amino acids are more highly conserved among related class 3 aldehyde dehydrogenase enzymes than expected, suggesting that they are critically important for protein folding, catalysis or stability. Studies of mutations in SLS should prove useful for understanding structure-function correlations in FALDH and other aldehyde dehydrogenase proteins.

Defective metabolism of leukotriene B4 in the Sjögren-Larsson syndrome

The Sjögren-Larsson Syndrome (SLS) is a neurocutaneous disorder, caused by deficient activity of the microsomal enzyme fatty aldehyde dehydrogenase (FALDH). FALDH catalyzes the oxidation of medium- and long-chain fatty aldehydes to their corresponding carboxylic acids. SLS is diagnosed by demonstrating the enzyme deficiency or by mutation analysis of the FALDH gene, while laboratory investigations of plasma, urine, and cerebrospinal fluid do not reveal any diagnostic abnormality. Leukotriene (LT) B4 is a pro-inflammatory mediator synthesized from arachidonic acid. LTB4 is inactivated by microsomal omega-oxidation, successively yielding 20-OH-LTB4, 20-CHO-LTB4 and 20-COOH-LTB4. Since FALDH is involved in LTB4 degradation, we have analyzed LTB4 and its metabolites in urine and cerebrospinal fluid as well as the degradation capacity for LTB4 in fresh polymorphonuclear leukocytes (PMN) of SLS patients. The urinary concentrations of LTB4, 20-OH-LTB4 and 20-COOH-LTB4 are below the detection limit in healthy controls. The urine of all SLS patients (n=13) exhibited highly elevated concentrations of LTB4 and 20-OH-LTB4, while 20-COOH-LTB4 was absent. Cerebrospinal fluid levels of LTB4, 20-OH-LTB4 and 20-COOH-LTB4 were found to be normal (n=7). PMN isolated from four patients were shown to be unable to convert 20-OH-LTB4 to 20-COOH-LTB4. Our findings provide unambiguous evidence for defective LTB4 degradation in SLS patients, and offer new and non-invasive diagnostic tools. Moreover, they open new pathophysiological considerations, with the prospect of rational treatment strategies.

Microsomal fatty aldehyde dehydrogenase catalyzes the oxidation of aliphatic aldehyde derived from ether glycerolipid catabolism: implications for Sjögren-Larsson syndrome

The enzyme that catalyzes the oxidation of fatty aldehyde derived from ether glycerolipid catabolism has not been identified. To determine whether microsomal fatty aldehyde dehydrogenase (FALDH) is responsible, we investigated the metabolism of 1-O-[9, 10-(3)H-octadecyl]-glycerol ([(3)H]OG) in FALDH-deficient cultured cells from patients with Sjögren-Larsson syndrome (SLS) and in mutant Chinese hamster ovary (CHO) cells. Intact fibroblasts from SLS patients incubated with [(3)H]OG showed a selective deficiency (38+/-7% of normal) in the incorporation of radioactivity into fatty acid, but no decrease in incorporation of radioactivity into fatty alcohol, total lipids and phosphatidylethanolamine (PE). Consistent with fatty aldehyde accumulation, incorporation of radioactivity into N-alkyl-phosphatidylethanolamine, which is derived from Schiff base formation of free aldehyde with PE, was 4-fold higher in SLS fibroblasts compared to normal controls. Similar results were seen with SLS keratinocytes, whereas FALDH-deficient CHO cells showed a more profound reduction in radioactive fatty acid to 12+/-2% of normal. These results implicate FALDH in the oxidation of ether-derived fatty aldehyde in human and rodent cells. Metabolism of ether glycerolipids is a previously unrecognized source of fatty aldehyde that may contribute to the pathogenesis of SLS.

Juvenile macular dystrophy associated with deficient activity of fatty aldehyde dehydrogenase in Sjögren-Larsson syndrome

PURPOSE

To report the ocular manifestations associated with the Sjögren-Larsson syndrome in a series of patients with proven fatty aldehyde dehydrogenase deficiency. To emphasize the clinical importance of the ophthalmological features of the Sjögren-Larsson syndrome. To discuss the metabolic disturbances that might give rise to the ophthalmological picture.

METHODS

Fifteen patients with Sjögren-Larsson syndrome underwent a standardized ophthalmological examination. In patients of appropriate age, and who were able to cooperate, additional investigations were performed.

RESULTS

All patients exhibited bilateral, glistening yellow-white crystalline deposits that were located in the innermost retinal layers and appeared during the first 2 years of life. Repeated fundus photography in individual patients showed that the dots became more numerous as the patients got older. Photophobia, subnormal visual acuity, myopia, and astigmatism were found in most of the patients. Fluorescein angiography was performed in three patients and showed a mottled hyperfluorescence of the retinal pigment epithelium, without leakage. Color vision, electroretinography, and electro-oculography could be performed in only a small number of patients and showed no abnormalities. Visual evoked potentials were found to be abnormal in six of eight patients.

CONCLUSIONS

In Sjögren-Larsson syndrome, patients exhibit highly characteristic bilateral, glistening yellow-white retinal dots from the age of 1 to 2 years onward. The number of dots increases with age. The extent of the macular abnormality does not correlate with the severity of the ichthyosis or with the severity of the neurological abnormalities. A high percentage of patients shows additional ocular signs and symptoms, notably marked photophobia.

Genomic organization, expression, and alternate splicing of the mouse fatty aldehyde dehydrogenase gene

Fatty aldehyde dehydrogenase (FALDH) is a microsomal enzyme that catalyzes the oxidation of aliphatic aldehydes to fatty acids. Mutations in the FALDH gene are responsible for the human genetic disorder Sjögren-Larsson syndrome (SLS) which is characterized by ichthyosis, mental retardation, and spasticity. To better understand SLS and the expression of FALDH in mammalian tissues, we investigated the organization and expression of the mouse FALDH gene (recently named ALDH3A2). The mouse gene consists of 11 exons and spans about 25 kb. Primer extension experiments identified the transcription initiation site at nt -121 relative to the translation initiating codon. The major FALDH transcript was 3 kb long and was composed of exons 1-10. A less abundant alternately spliced transcript contained an additional exon (exon 9') inserted between exons 9 and 10 and encodes a protein (FALDHv) with a variant carboxy-terminal domain of unknown function. Northern analysis usingRNA from different tissues showed widespread but variable expression of the gene, which generally correlated with FALDH enzyme activity. Expression of the alternate exon 9' transcript in tissues often differed from that of the major transcript and did not reflect enzyme activity. These results provide a basis for investigating the in vivo expression of FALDH in response to physiologic and pharmacologic manipulation, and are essential for the development of an animal model of SLS.

beta-oxidation - strategies for the metabolism of a wide variety of acyl-CoA esters

Living organisms are exposed to a number of different fatty acids and their various derivatives arising either via endogenous synthesis or from exogenous sources. These hydrophobic compounds can play specific metabolic, structural or endocrinic functions in the organisms before their elimination, which can be metabolism to CO(2) or to more polar lipid metabolites allowing their excretion. Quantitatively, one of the major pathways metabolizing fatty acids is beta-oxidation, which consists of a set of four reactions operating at the carbons 2 or 3 of acyl-CoA esters and shortening of the acyl-chain. To allow the beta-oxidation of acyl groups with various steric variants to proceed, different strategies have been developed. These strategies include evolution of beta-oxidation enzymes as paralogues showing specificity with respect to either chain-length or modified acyl-chain, metabolic compartmentalization in eukaryotic cells, controlling of substrate transport across membranes, development of auxiliary enzyme systems, acquisition of enzymes with adaptive active sites and recruiting and optimizing enzymes from non-homologous sources allowing them to catalyze a parallel set of reactions with different substrate specificities.

Defective inactivation of leukotriene B4 in patients with Sjögren-Larsson syndrome

In 6 patients with Sjögren-Larsson syndrome (SLS), the urinary excretion of leukotriene B(4) (LTB(4)) and omega-hydroxy-LTB(4) was found to be highly elevated, whereas omega-carboxy-LTB(4) was absent. This abnormal pattern of urinary excretion of LTB(4) and its metabolites appears to be unique to patients with SLS and offers a new approach to the diagnosis of this disorder. Moreover, defective inactivation of LTB(4) might be of pathophysiologic significance in the disease.

Phytanic acid alpha-oxidation: identification of 2-hydroxyphytanoyl-CoA lyase in rat liver and its localisation in peroxisomes

Phytanic acid is broken down by alpha-oxidation in three steps finally yielding pristanic acid. The first step occurs in peroxisomes and is catalysed by phytanoyl-CoA hydroxylase. We have studied the second step in the alpha-oxidation pathway, which involves conversion of 2-hydroxyphytanoyl-CoA to pristanal catalysed by 2-hydroxyphytanoyl-CoA lyase. To this end, we have developed a stable isotope dilution gas chromatography-mass spectrometry assay allowing activity measurements in rat liver homogenates. Cell fractionation studies demonstrate that in rat liver 2-hydroxyphytanoyl-CoA lyase is localised in peroxisomes. This finding may have important implications for inherited diseases in man characterised by impaired phytanic acid alpha-oxidation.

Purification, molecular cloning, and expression of 2-hydroxyphytanoyl-CoA lyase, a peroxisomal thiamine pyrophosphate-dependent enzyme that catalyzes the carbon-carbon bond cleavage during alpha-oxidation of 3-methyl-branched fatty acids

In the third step of the alpha-oxidation of 3-methyl-branched fatty acids such as phytanic acid, a 2-hydroxy-3-methylacyl-CoA is cleaved into formyl-CoA and a 2-methyl-branched fatty aldehyde. The cleavage enzyme was purified from the matrix protein fraction of rat liver peroxisomes and identified as a protein made up of four identical subunits of 63 kDa. Its activity proved to depend on Mg(2+) and thiamine pyrophosphate, a hitherto unrecognized cofactor of alpha-oxidation. Formyl-CoA and 2-methylpentadecanal were identified as reaction products when the purified enzyme was incubated with 2-hydroxy-3-methylhexadecanoyl-CoA as the substrate. Hence the enzyme catalyzes a carbon-carbon cleavage, and we propose calling it 2-hydroxyphytanoyl-CoA lyase. Sequences derived from tryptic peptides of the purified rat protein were used as queries to recover human expressed sequence tags from the databases. The composite cDNA sequence of the human lyase contained an ORF of 1,734 bases that encodes a polypeptide with a calculated molecular mass of 63,732 Da. Recombinant human protein, expressed in mammalian cells, exhibited lyase activity. The lyase displayed homology to a putative Caenorhabditis elegans protein that resembles bacterial oxalyl-CoA decarboxylases. Similarly to the decarboxylases, a thiamine pyrophosphate-binding consensus domain was present in the C-terminal part of the lyase. Although no peroxisome targeting signal, neither 1 nor 2, was apparent, transfection experiments with constructs encoding green fluorescent protein fused to the full-length lyase or its C-terminal pentapeptide indicated that the C terminus of the lyase represents a peroxisome targeting signal 1 variant.

Human very-long-chain acyl-CoA synthetase: cloning, topography, and relevance to branched-chain fatty acid metabolism

Very-long-chain acyl-CoA synthetases (VLCS) activate very-long-chain fatty acids (VLCFA) containing 22 or more carbons to their CoA derivatives. We cloned the human ortholog (hVLCS) of the gene encoding the rat liver enzyme (rVLCS). Both hVLCS and rVLCS contain 620 amino acids, are expressed primarily in liver and kidney, and have a potential peroxisome targeting signal 1 (-LKL) at their carboxy termini. When expressed in COS-1 cells, hVLCS activated the VLCFA lignoceric acid (C24:0), a long-chain fatty acid (C16:0), and two branched-chain fatty acids, phytanic acid and pristanic acid. Immunofluorescence and immunoblot studies localized hVLCS to both peroxisomes and endoplasmic reticulum. In peroxisomes of HepG2 cells, hVLCS was topographically oriented facing the matrix and not the cytoplasm. This orientation, coupled with the observation that hVLCS activates branched-chain fatty acids, suggests that hVLCS could play a role in the intraperoxisomal reactivation of pristanic acid produced via alpha-oxidation of phytanic acid.