Lipoic acid content of Escherichia coli and other microorganisms

α-lipoic acid modulates prostate cancer cell growth and bone cell differentiation

Prostate cancer (PCa) progression leads to bone modulation in approximately 70% of affected men. A nutraceutical, namely, α-lipoic acid (α-LA), is known for its potent anti-cancer properties towards various cancers and has been implicated in treating and promoting bone health. Our study aimed to explore the molecular mechanism behind the role of α-LA as therapeutics in preventing PCa and its associated bone modulation. Notably, α-LA treatment significantly reduced the cell viability, migration, and invasion of PCa cell lines in a dose-dependent manner. In addition, α-LA supplementation dramatically increased reactive oxygen species (ROS) levels and HIF-1α expression, which started the downstream molecular cascade and activated JNK/caspase-3 signaling pathway. Flow cytometry data revealed the arrest of the cell cycle in the S-phase, which has led to apoptosis of PCa cells. Furthermore, the results of ALP (Alkaline phosphatase) and TRAP (tartrate-resistant acid phosphatase) staining signifies that α-LA supplementation diminished the PCa-mediated differentiation of osteoblasts and osteoclasts, respectively, in the MC3T3-E1 and bone marrow macrophages (BMMs) cells. In summary, α-LA supplementation enhanced cellular apoptosis via increased ROS levels, HIF-1α expression, and JNK/caspase-3 signaling pathway in advanced human PCa cell lines. Also, the treatment of α-LA improved bone health by reducing PCa-mediated bone cell modulation.

Metabolic engineering of Escherichia coli for high-level production of free lipoic acid

L-Lipoic acid (LA) is an important antioxidant with various industrial applications as a nutraceutical and therapeutic. Currently, LA is produced by chemical synthesis. Cell factory development is complex as LA and its direct precursors only occur naturally in protein-bound forms. Here we report a rationally engineered LA cell factory and demonstrate de novo free LA production from glucose for the first time in E. coli. The pathway represents a significant challenge as the three key enzymes, native Octanoyltransferase (LipB) and Lipoyl Synthase (LipA), and heterologous Lipoamidase (LpA), are all toxic to overexpress in E. coli. To overcome the toxicity of LipB, functional metagenomic selection was used to identify a highly active and non-toxic LipB and LipA from S. liquefaciens. Using high throughput screening, we balanced translation initiation rates and dual, orthogonal induction systems for the toxic genes, LipA and LpA. The optimized strain yielded 2.5 mg free LA per gram of glucose in minimal media, expressing carefully balanced LipB and LipA, Enterococcus faecalis LpA, and a truncated, native, Dihydrolipoyllysine-residue acetyltransferase (AceF) lipoylation domain. When the optimized cell factory strain was cultivated in a fed-batch fermentation, a titer of 87 mg/L free LA in the supernatant was reached after 48 h. This titer is ∼3000-fold higher than previously reported free LA titer and ∼8-fold higher than the previous best total, protein-bound LA titer. The strategies presented here could be helpful in designing, constructing and balancing biosynthetic pathways that harbor toxic enzymes with protein-bound intermediates or products.

Chylomicron production is repressed by RPTOR knockdown, R-α-lipoic acid and 4-phenylbutyric acid in human enterocyte-like Caco-2 cells

Although the role of mechanistic target of rapamycin complex 1 (mTORC1) in lipid metabolism has been the subject of previous research, its function in chylomicron production is not known. In this study, we created three stable human colorectal adenocarcinoma Caco-2 cell lines exhibiting normal, low or high mTORC1 kinase activity, and used these cells to investigate the consequences of manipulating mTORC1 activity on enterocyte differentiation and chylomicron-like particle production. Constitutively active mTORC1 induced Caco-2 cell proliferation and differentiation (as judged by alkaline phosphatase activity) but weakened transepithelial electrical resistance (TEER). Repressed mTORC1 activity due to the knockdown of RPTOR significantly decreased the expression of lipogenic genes FASN, DGAT1 and DGAT2, lipoprotein assembly genes APOB and MTTP, reduced protein expression of APOB, MTTP and FASN, downregulated the gene expression of very long-chain fatty acyl-CoA ligase (FATP2), acyl-CoA binding protein (DBI), and prechylomicron transport vesicle-associated proteins VAMP7 (vesicle-associated membrane protein 7) and SAR1B (secretion associated Ras related GTPase 1B) resulting in the repression of apoB-containing triacylglycerol-rich lipoprotein secretion. Exposure of Caco-2 cells harboring a constitutively active mTORC1 to short-chain fatty acid derivatives, R-α-lipoic acid and 4-phenylbutyric acid, downregulated chylomicron-like particle secretion by interfering with the lipidation and assembly of the particles, and concomitantly repressed mTORC1 activity with no change to Raptor abundance or PRAS40 (Thr246) phosphorylation. R-α-lipoic acid and 4-phenylbutyric acid may be useful to mitigate intestinal lipoprotein overproduction and associated postprandial inflammation.

R-α-Lipoic Acid and 4-Phenylbutyric Acid Have Distinct Hypolipidemic Mechanisms in Hepatic Cells

The constitutive activation of the mechanistic target of rapamycin complex 1 (mTORC1) leads to the overproduction of apoB-containing triacylglycerol-rich lipoproteins in HepG2 cells. R-α-lipoic acid (LA) and 4-phenylbutyric acid (PBA) have hypolipidemic function but their mechanisms of action are not well understood. Here, we reported that LA and PBA regulate hepatocellular lipid metabolism via distinct mechanisms. The use of SQ22536, an inhibitor of adenylyl cyclase, revealed cAMP’s involvement in the upregulation of CPT1A expression by LA but not by PBA. LA decreased the secretion of proprotein convertase subtilisin/kexin type 9 (PCSK9) in the culture media of hepatic cells and increased the abundance of LDL receptor (LDLR) in cellular extracts in part through transcriptional upregulation. Although PBA induced LDLR gene expression, it did not translate into more LDLR proteins. PBA regulated cellular lipid homeostasis through the induction of CPT1A and INSIG2 expression via an epigenetic mechanism involving the acetylation of histone H3, histone H4, and CBP-p300 at the CPT1A and INSIG2 promoters.

Production of Bioactive Compounds by Food Associated Galactomyces geotrichum 38, as Determined by Proteome Analysis

Fried cottage cheese is a dairy product, popular in some parts of Poland. Proteomic analysis of a culture of the mold Galactomyces geotrichum 38 isolated from fried cottage cheese was performed using UHPLC/MS. From the proteins identified, we selected those involved in the biosynthesis of bioactive compounds and those useful in industry. In the G. geotrichum 38 culture, the production quantities of vitamin B₂ (224 μg/L), ergosterol (54.63 mg/kg), and trehalose (0.91 g/L) were determined by HPLC. The identified proteins were also used to prepare a hypothetical fatty acid biosynthesis pathway, and the percentage of individual sphingolipids in the culture was determined. Sphingolipids are also bioactive compounds. During culturing of G. geotrichum 38, the percentage of three sphingolipids increased. The last step of the research was to prepare a model of fried cottage cheese. The mold G. geotrichum 38, used in the process of ripening fried cottage cheese, synthesized vitamin B₂ and erogsterol, which influenced the nutritional value of the product.

Overproduction of α-Lipoic Acid by Gene Manipulated Escherichia coli

Alpha-lipoic acid (LA) is an important enzyme cofactor widely used by organisms and is also a natural antioxidant for the treatment of pathologies driven by low levels of endogenous antioxidants. In order to establish a safer and more efficient process for LA production, we developed a new biological method for LA synthesis based on the emerging knowledge of lipoic acid biosynthesis. We first cloned the lipD gene, which encodes the lipoyl domain of the E2 subunit of pyruvate dehydrogenase, allowing high levels of LipD production. Plasmids containing genes for the biosynthesis of LA were subsequently constructed utilizing various vectors and promotors to produce high levels of LA. These plasmids were transformed into the Escherichia coli strain BL21. Octanoic acid (OA) was used as the substrate for LA synthesis. One transformant, YS61, which carried lipD, lplA, and lipA, produced LA at levels over 200-fold greater than the wild-type strain, showing that LA could be produced efficiently in E. coli using genetic engineering methods.

The regulation of FGF21 gene expression by metabolic factors and nutrients

Fibroblast growth factor 21 (FGF21) gene expression is altered by a wide array of physiological, metabolic, and environmental factors. Among dietary factors, high dextrose, low protein, methionine restriction, short-chain fatty acids (butyric acid and lipoic acid), and all-trans-retinoic acid were repeatedly shown to induce FGF21 expression and circulating levels. These effects are usually more pronounced in liver or isolated hepatocytes than in adipose tissue or isolated fat cells. Although peroxisome proliferator-activated receptor α (PPARα) is a key mediator of hepatic FGF21 expression and function, including the regulation of gluconeogenesis, ketogenesis, torpor, and growth inhibition, there is increasing evidence of PPARα-independent transactivation of the FGF21 gene by dietary molecules. FGF21 expression is believed to follow the circadian rhythm and be placed under the control of first order clock-controlled transcription factors, retinoic acid receptor-related orphan receptors (RORs) and nuclear receptors subfamily 1 group D (REV-ERBs), with FGF21 rhythm being anti-phase to REV-ERBs. Key metabolic hormones such as glucagon, insulin, and thyroid hormone have presumed or clearly demonstrated roles in regulating FGF21 transcription and secretion. The control of the FGF21 gene by glucagon and insulin appears more complex than first anticipated. Some discrepancies are noted and will need continued studies. The complexity in assessing the significance of FGF21 gene expression resides in the difficulty to ascertain (i) when transcription results in local or systemic increase of FGF21 protein; (ii) if FGF21 is among the first or second order genes upregulated by physiological, metabolic, and environmental stimuli, or merely an epiphenomenon; and (iii) whether FGF21 may have some adverse effects alongside beneficial outcomes.

Assembly of Lipoic Acid on Its Cognate Enzymes: an Extraordinary and Essential Biosynthetic Pathway

Although the structure of lipoic acid and its role in bacterial metabolism were clear over 50 years ago, it is only in the past decade that the pathways of biosynthesis of this universally conserved cofactor have become understood. Unlike most cofactors, lipoic acid must be covalently bound to its cognate enzyme proteins (the 2-oxoacid dehydrogenases and the glycine cleavage system) in order to function in central metabolism. Indeed, the cofactor is assembled on its cognate proteins rather than being assembled and subsequently attached as in the typical pathway, like that of biotin attachment. The first lipoate biosynthetic pathway determined was that of Escherichia coli, which utilizes two enzymes to form the active lipoylated protein from a fatty acid biosynthetic intermediate. Recently, a more complex pathway requiring four proteins was discovered in Bacillus subtilis, which is probably an evolutionary relic. This pathway requires the H protein of the glycine cleavage system of single-carbon metabolism to form active (lipoyl) 2-oxoacid dehydrogenases. The bacterial pathways inform the lipoate pathways of eukaryotic organisms. Plants use the E. coli pathway, whereas mammals and fungi probably use the B. subtilis pathway. The lipoate metabolism enzymes (except those of sulfur insertion) are members of PFAM family PF03099 (the cofactor transferase family). Although these enzymes share some sequence similarity, they catalyze three markedly distinct enzyme reactions, making the usual assignment of function based on alignments prone to frequent mistaken annotations. This state of affairs has possibly clouded the interpretation of one of the disorders of human lipoate metabolism.

Emerging role of alpha-lipoic acid in the prevention and treatment of bone loss

Osteoporosis is a chronic disease associated with decreased bone density that afflicts millions of people worldwide. Current pharmacological treatments are limited, costly, and linked to several negative side effects. These factors are driving current interest in the clinical use of naturally occurring bioactive compounds to mitigate bone loss. Alpha-lipoic acid, a potent antioxidant and essential member of mitochondrial dehydrogenases, has shown considerable promise as an antiosteoclastogenic agent due to its potent reactive oxygen species-scavenging capabilities along with a proven clinical safety record. Collectively, current data indicate that alpha-lipoic acid protects from bone loss via a 2-pronged mechanism involving inhibition of osteoclastogenic reactive oxygen species generation and upregulation of redox gene expression.

Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation

Two vitamins, biotin and lipoic acid, are essential in all three domains of life. Both coenzymes function only when covalently attached to key metabolic enzymes. There they act as "swinging arms" that shuttle intermediates between two active sites (= covalent substrate channeling) of key metabolic enzymes. Although biotin was discovered over 100 years ago and lipoic acid 60 years ago, it was not known how either coenzyme is made until recently. In Escherichia coli the synthetic pathways for both coenzymes have now been worked out for the first time. The late steps of biotin synthesis, those involved in assembling the fused rings, were well described biochemically years ago, although recent progress has been made on the BioB reaction, the last step of the pathway in which the biotin sulfur moiety is inserted. In contrast, the early steps of biotin synthesis, assembly of the fatty acid-like "arm" of biotin were unknown. It has now been demonstrated that the arm is made by using disguised substrates to gain entry into the fatty acid synthesis pathway followed by removal of the disguise when the proper chain length is attained. The BioC methyltransferase is responsible for introducing the disguise, and the BioH esterase is responsible for its removal. In contrast to biotin, which is attached to its cognate proteins as a finished molecule, lipoic acid is assembled on its cognate proteins. An octanoyl moiety is transferred from the octanoyl acyl carrier protein of fatty acid synthesis to a specific lysine residue of a cognate protein by the LipB octanoyltransferase followed by sulfur insertion at carbons C-6 and C-8 by the LipA lipoyl synthetase. Assembly on the cognate proteins regulates the amount of lipoic acid synthesized, and, thus, there is no transcriptional control of the synthetic genes. In contrast, transcriptional control of the biotin synthetic genes is wielded by a remarkably sophisticated, yet simple, system, exerted through BirA, a dual-function protein that both represses biotin operon transcription and ligates biotin to its cognate proteins.

Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation

Two vitamins, biotin and lipoic acid, are essential in all three domains of life. Both coenzymes function only when covalently attached to key metabolic enzymes. There they act as "swinging arms" that shuttle intermediates between two active sites (= covalent substrate channeling) of key metabolic enzymes. Although biotin was discovered over 100 years ago and lipoic acid was discovered 60 years ago, it was not known how either coenzyme is made until recently. In Escherichia coli the synthetic pathways for both coenzymes have now been worked out for the first time. The late steps of biotin synthesis, those involved in assembling the fused rings, were well described biochemically years ago, although recent progress has been made on the BioB reaction, the last step of the pathway, in which the biotin sulfur moiety is inserted. In contrast, the early steps of biotin synthesis, assembly of the fatty acid-like "arm" of biotin, were unknown. It has now been demonstrated that the arm is made by using disguised substrates to gain entry into the fatty acid synthesis pathway followed by removal of the disguise when the proper chain length is attained. The BioC methyltransferase is responsible for introducing the disguise and the BioH esterase for its removal. In contrast to biotin, which is attached to its cognate proteins as a finished molecule, lipoic acid is assembled on its cognate proteins. An octanoyl moiety is transferred from the octanoyl-ACP of fatty acid synthesis to a specific lysine residue of a cognate protein by the LipB octanoyl transferase, followed by sulfur insertion at carbons C6 and C8 by the LipA lipoyl synthetase. Assembly on the cognate proteins regulates the amount of lipoic acid synthesized, and thus there is no transcriptional control of the synthetic genes. In contrast, transcriptional control of the biotin synthetic genes is wielded by a remarkably sophisticated, yet simple, system exerted through BirA, a dual-function protein that both represses biotin operon transcription and ligates biotin to its cognate protein.

Analysis of Reaction between α-Lipoic Acid and 2-Chloro-1-methylquinolinium Tetrafluoroborate Used as a Precolumn Derivatization Technique in Chromatographic Determination of α-Lipoic Acid

The present study offers results of analysis concerning the course of reaction between reduced α-lipoic acid (LA) and 2-chloro-1-methylquinolinium tetrafluoroborate (CMQT). In water environments, the reaction between CMQT and hydrophilic thiols proceeds very rapidly and the resultant products are stable. For the described analysis, optimum reaction conditions, such as concentration of the reducing agent, environment pH, and concentration of the reagent were carefully selected. The spectrophotometric assay was carried out measuring absorbance at λ = 348 nm (i.e., the spectral band of the obtained reaction product). Furthermore, the calibration curve of lipoic acid was registered. It was concluded that the Lambert-Beer law was observed within the range 1-10 μmol L(-1). Later, the reaction between LA and CMQT was used as precolumn derivatization in a chromatographic determination of the lipoic acid in the range 2.5-50 μmol L(-1). Practical applicability of the designed methods was evaluated by determining lipoic acid in Revitanerv pharmaceutical preparation which contains 300 mg LA in a single capsule. The error of the determination did not exceed 0.5% in relation to the declared value.

α-Lipoic acid as a triglyceride-lowering nutraceutical

Considering the current obesity epidemic in the United States (>100 million adults are overweight or obese), the prevalence of hypertriglyceridemia is likely to grow beyond present statistics of ∼30% of the population. Conventional therapies for managing hypertriglyceridemia include lifestyle modifications such as diet and exercise, pharmacological approaches, and nutritional supplements. It is critically important to identify new strategies that would be safe and effective in lowering hypertriglyceridemia. α-Lipoic acid (LA) is a naturally occurring enzyme cofactor found in the human body in small quantities. A growing body of evidence indicates a role of LA in ameliorating metabolic dysfunction and lipid anomalies primarily in animals. Limited human studies suggest LA is most efficacious in situations where blood triglycerides are markedly elevated. LA is commercially available as dietary supplements and is clinically shown to be safe and effective against diabetic polyneuropathies. LA is described as a potent biological antioxidant, a detoxification agent, and a diabetes medicine. Given its strong safety record, LA may be a useful nutraceutical, either alone or in combination with other lipid-lowering strategies, when treating severe hypertriglyceridemia and diabetic dyslipidemia. This review examines the current evidence regarding the use of LA as a means of normalizing blood triglycerides. Also presented are the leading mechanisms of action of LA on triglyceride metabolism.

Characterization of genome-wide transcriptional changes in liver and adipose tissues of ZDF (fa/fa) rats fed R-α-lipoic acid by next-generation sequencing

We report on the characterization of lipogenic tissue transcriptional networks that support physiological responses of obese rats to a lipid-lowering bioactive food compound, R-α-lipoic acid (LA). Nine-week-old male Zucker diabetic fatty (fa/fa) rats were fed a chow diet supplemented with 3 g LA per kg diet or pair fed for 2 wk. At the end of the trial, high-quality RNA was extracted from the liver and epididymal fat and subjected to transcriptome analysis by RNA-Seq technology. Results showed a substantially higher number of differentially expressed genes [DEG, false discovery rate adjusted P ≤ 0.05 and absolute log2 (fold change) ≥ 1] in the liver (110 genes) vs. epididymal fat (10 genes). Most epididymal fat DEG were also differentially expressed in liver and shared directionality of change. Gene Ontology (GO) analysis of these transcripts revealed significant enrichment of GO categories related to immune response, stress response, lipid metabolism, and carboxylic acid metabolic processes. Of interest, interferon-related genes involved in defense against microorganisms and innate immune response were induced by LA. Lipid metabolism-related transcript changes observed in LA-fed animals included downregulation of lipogenic genes ( Pnpla3, Pnpla5, Elovl6, Acly, Gpam, and Aacs) and concomitant upregulation of short-, medium-, and long-chain fatty acid metabolic processes ( Acot1, Acot2, Acsf2, and Crat). Transcriptional changes were accompanied by the lowering of abdominal adiposity and blood triacylglycerol levels. We conclude that LA dietary supplementation induces prominent gene expression changes in liver in support of significant improvement of whole-body lipid status.

Organic hydroperoxide resistance protein and ergothioneine compensate for loss of mycothiol in Mycobacterium smegmatis mutants

The mshA::Tn5 mutant of Mycobacterium smegmatis does not produce mycothiol (MSH) and was found to markedly overproduce both ergothioneine and an ~15-kDa protein determined to be organic hydroperoxide resistance protein (Ohr). An mshA(G32D) mutant lacking MSH overproduced ergothioneine but not Ohr. Comparison of the mutant phenotypes with those of the wild-type strain indicated the following: Ohr protects against organic hydroperoxide toxicity, whereas ergothioneine does not; an additional MSH-dependent organic hydroperoxide peroxidase exists; and elevated isoniazid resistance in the mutant is associated with both Ohr and the absence of MSH. Purified Ohr showed high activity with linoleic acid hydroperoxide, indicating lipid hydroperoxides as the likely physiologic targets. The reduction of oxidized Ohr by NADH was shown to be catalyzed by lipoamide dehydrogenase and either lipoamide or DlaT (SucB). Since free lipoamide and lipoic acid levels were shown to be undetectable in M. smegmatis, the bound lipoyl residues of DlaT are the likely source of the physiological dithiol reductant for Ohr. The pattern of occurrence of homologs of Ohr among bacteria suggests that the ohr gene has been distributed by lateral transfer. The finding of multiple Ohr homologs with various sequence identities in some bacterial genomes indicates that there may be multiple physiologic targets for Ohr proteins.

Integrative inference of gene-regulatory networks in Escherichia coli using information theoretic concepts and sequence analysis

Background

Although Escherichia coli is one of the best studied model organisms, a comprehensive understanding of its gene regulation is not yet achieved. There exist many approaches to reconstruct regulatory interaction networks from gene expression experiments. Mutual information based approaches are most useful for large-scale network inference.

Results

We used a three-step approach in which we combined gene regulatory network inference based on directed information (DTI) and sequence analysis. DTI values were calculated on a set of gene expression profiles from 19 time course experiments extracted from the Many Microbes Microarray Database. Focusing on influences between pairs of genes in which one partner encodes a transcription factor (TF) we derived a network which contains 878 TF - gene interactions of which 166 are known according to RegulonDB. Afterward, we selected a subset of 109 interactions that could be confirmed by the presence of a phylogenetically conserved binding site of the respective regulator. By this second step, the fraction of known interactions increased from 19% to 60%. In the last step, we checked the 44 of the 109 interactions not yet included in RegulonDB for functional relationships between the regulator and the target and, thus, obtained ten TF - target gene interactions. Five of them concern the regulator LexA and have already been reported in the literature. The remaining five influences describe regulations by Fis (with two novel targets), PhdR, PhoP, and KdgR. For the validation of our approach, one of them, the regulation of lipoate synthase (LipA) by the pyruvate-sensing pyruvate dehydrogenate repressor (PdhR), was experimentally checked and confirmed.

Conclusions

We predicted a set of five novel TF - target gene interactions in E. coli. One of them, the regulation of lipA by the transcriptional regulator PdhR was validated experimentally. Furthermore, we developed DTInfer, a new R-package for the inference of gene-regulatory networks from microarrays using directed information.

The amidase domain of lipoamidase specifically inactivates lipoylated proteins in vivo

BACKGROUND

In the 1950s, Reed and coworkers discovered an enzyme activity in Streptococcus faecalis (Enterococcus faecalis) extracts that inactivated the Escherichia. coli and E. faecalis pyruvate dehydrogenase complexes through cleavage of the lipoamide bond. The enzyme that caused this lipoamidase activity remained unidentified until Jiang and Cronan discovered the gene encoding lipoamidase (Lpa) through the screening of an expression library. Subsequent cloning and characterization of the recombinant enzyme revealed that lipoamidase is an 80 kDa protein composed of an amidase domain containing a classic Ser-Ser-Lys catalytic triad and a carboxy-terminal domain of unknown function. Here, we show that the amidase domain can be used as an in vivo probe which specifically inactivates lipoylated enzymes.

METHODOLOGY/PRINCIPAL FINDINGS

We evaluated whether Lpa could function as an inducible probe of alpha-ketoacid dehydrogenase inactivation using E. coli as a model system. Lpa expression resulted in cleavage of lipoic acid from the three lipoylated proteins expressed in E. coli, but did not result in cleavage of biotin from the sole biotinylated protein, the biotin carboxyl carrier protein. When expressed in lipoylation deficient E. coli, Lpa is not toxic, indicating that Lpa does not interfere with any other critical metabolic pathways. When truncated to the amidase domain, Lpa retained lipoamidase activity without acquiring biotinidase activity, indicating that the carboxy-terminal domain is not essential for substrate recognition or function. Substitution of any of the three catalytic triad amino acids with alanine produced inactive Lpa proteins.

CONCLUSIONS/SIGNIFICANCE

The enzyme lipoamidase is active against a broad range of lipoylated proteins in vivo, but does not affect the growth of lipoylation deficient E. coli. Lpa can be truncated to 60% of its original size with only a partial loss of activity, resulting in a smaller probe that can be used to study the effects of alpha-ketoacid dehydrogenase inactivation in vivo.

Lipoic acid synthesis and attachment in yeast mitochondria

Lipoic acid is a sulfur-containing cofactor required for the function of several multienzyme complexes involved in the oxidative decarboxylation of alpha-keto acids and glycine. Mechanistic details of lipoic acid metabolism are unclear in eukaryotes, despite two well defined pathways for synthesis and covalent attachment of lipoic acid in prokaryotes. We report here the involvement of four genes in the synthesis and attachment of lipoic acid in Saccharomyces cerevisiae. LIP2 and LIP5 are required for lipoylation of all three mitochondrial target proteins: Lat1 and Kgd2, the respective E2 subunits of pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, and Gcv3, the H protein of the glycine cleavage enzyme. LIP3, which encodes a lipoate-protein ligase homolog, is necessary for lipoylation of Lat1 and Kgd2, and the enzymatic activity of Lip3 is essential for this function. Finally, GCV3, encoding the H protein target of lipoylation, is itself absolutely required for lipoylation of Lat1 and Kgd2. We show that lipoylated Gcv3, and not glycine cleavage activity per se, is responsible for this function. Demonstration that a target of lipoylation is required for lipoylation is a novel result. Through analysis of the role of these genes in protein lipoylation, we conclude that only one pathway for de novo synthesis and attachment of lipoic acid exists in yeast. We propose a model for protein lipoylation in which Lip2, Lip3, Lip5, and Gcv3 function in a complex, which may be regulated by the availability of acetyl-CoA, and which in turn may regulate mitochondrial gene expression.

Chaperone-aided expression of LipA and LplA followed by the increase in alpha-lipoic acid production

Alpha-lipoic acid (LA), a naturally occurring cofactor reported to be present in a diverse group of microorganisms, plants, and animal tissues, has been widely and successfully used as a therapy for a variety of diseases, including diabetes and heart disease. However, to date, recombinant DNA technology has not been applied for higher LA production due mainly to difficulties in the functional expression of key enzymes involved in LA production. Here, we report a study for higher LA production with the aid of chaperone plasmids, DnaKJE and trigger factor (Tf). The lipA and lplA genes encoding lipoate synthase and lipoate protein ligase in Pseudomonas fluorescens, respectively, were cloned and transformed into Escherichia coli K12. When they were overexpressed in E. coli, both LipA and LplA were expressed as inclusion bodies leading to no increase in LA production. However, when chaperone plasmids DnaKJE and Tf were coexpressed with lipA and lplA, the resulting recombinant E. coli strains showed higher LA production than the wild-type E. coli by 32-111%, respectively.

Using gene expression data and network topology to detect substantial pathways, clusters and switches during oxygen deprivation of Escherichia coli

Background

Biochemical investigations over the last decades have elucidated an increasingly complete image of the cellular metabolism. To derive a systems view for the regulation of the metabolism when cells adapt to environmental changes, whole genome gene expression profiles can be analysed. Moreover, utilising a network topology based on gene relationships may facilitate interpreting this vast amount of information, and extracting significant patterns within the networks.

Results

Interpreting expression levels as pixels with grey value intensities and network topology as relationships between pixels, allows for an image-like representation of cellular metabolism. While the topology of a regular image is a lattice grid, biological networks demonstrate scale-free architecture and thus advanced image processing methods such as wavelet transforms cannot directly be applied. In the study reported here, one-dimensional enzyme-enzyme pairs were tracked to reveal sub-graphs of a biological interaction network which showed significant adaptations to a changing environment. As a case study, the response of the hetero-fermentative bacterium E. coli to oxygen deprivation was investigated. With our novel method, we detected, as expected, an up-regulation in the pathways of hexose nutrients up-take and metabolism and formate fermentation. Furthermore, our approach revealed a down-regulation in iron processing as well as the up-regulation of the histidine biosynthesis pathway. The latter may reflect an adaptive response of E. coli against an increasingly acidic environment due to the excretion of acidic products during anaerobic growth in a batch culture.

Conclusion

Based on microarray expression profiling data of prokaryotic cells exposed to fundamental treatment changes, our novel technique proved to extract system changes for a rather broad spectrum of the biochemical network.

Function, attachment and synthesis of lipoic acid in Escherichia coli

A series of genetic, biochemical, and physiological studies in Escherichia coli have elucidated the unusual pathway whereby lipoic acid is synthesized. Here we describe the results of these investigations as well as the functions of enzyme proteins that are modified by covalent attachment of lipoic acid and the enzymes that catalyze the modification reactions. Some aspects of the synthesis and attachment mechanisms have strong parallels in the pathways used in synthesis and attachment of biotin and these are compared and contrasted. Homologues of the lipoic acid metabolism proteins are found in all branches of life, save the Archea, and thus these findings seem to have wide biological relevance.

Determination of lipoic acid in dietary supplement preparations by capillary electrophoresis

The present study deals with the development of a method for the quantitative determination of lipoic acid in a dietary supplement preparation. A rapid capillary electrophoretic method is developed using UV detection at 208 nm. Although lipoic acid is only weakly UV-absorbing, at this wavelength it could be detected with sufficient sensitivity with an LOD and LOQ of 0.8 and 2.5 microg/ml, respectively. Analysis time was less than 9 min. The compound was extracted from tablets with a recovery of 98.3% and a precision of 2.8% RSD.

Syntheses of Novel Indole Lipoic Acid Derivatives and Their Antioxidant Effects on Lipid Peroxidation

The aim of the study was to examine antioxidant properties of conjugates based on indole and lipoic acid moieties. The design and syntheses of novel indole alpha-lipoic acid derivatives were performed. The antioxidant properties of target compounds were investigated using rat liver microsomal, NADPH-dependent lipid peroxidation inhibition. Some of the target compounds, especially those containing amide linker at position 5 of indole ring, proved to be highly effective in inhibiting lipid peroxidation as compared to alpha-lipoic acid.

Determination of lipoic acid in rat plasma by LC-MS/MS with electrospray ionization: Assay development, validation and application to a pharamcokinetic study

A simple, sensitive and specific LC-MS/MS method for the determination of lipoic acid was developed and validated over the linearity range 5-1000 ng/mL (r2 > 0.99) with 200 microL rat plasma using rosigliatzone as an internal standard (IS). The assay procedure involved a simple one-step liquid-liquid extraction of lipoic acid and IS from plasma into ethyl acetate. The organic layer was separated and evaporated under a gentle stream of nitrogen at 40 degrees C. The residue was reconstituted in the mobile phase and injected onto a Hichrom RPB column (4.6 x 250 mm, 5 microm). Separation of lipoic acid and IS was achieved with a mobile phase consisting of 0.05 M formic acid:acetonitrile (40:60, v/v) at a flow rate of 1.0 mL/min. The API-3000 LC-MS/MS was operated under the multiple reaction monitoring mode (MRM) using the electrospray ionization technique. Positive and negative ion acquisition within the same chromatographic run was used in the present method. For lipoic acid a pseudo-molecular ion transition pair was acquired in negative polarity, whereas for IS the transition pair was acquired in positive polarity. Quantitation was determined for both analyte and IS in MRM scan mode. Absolute recovery of lipoic acid and IS was >70 and 97%, respectively. The lower limit of quantification (LLOQ) of lipoic acid was 5.0 ng/mL. The inter- and intra-day precision in the measurement of quality control (QC) samples 5, 15, 400 and 800 ng/mL were in the range 2.18-5.99% relative standard deviation (RSD) and 0.93-13.77% RSD, respectively. Accuracy in the measurement of QC samples was in the range 87.40-114.40% of the nominal values. Analyte and IS were stable in the battery of stability studies, viz. bench-top, auto-sampler and freeze-thaw cycles. Stability of lipoic acid was established for 1 month at -80 degrees C. The application of the assay to a pharmacokinetic study in rats confirmed the utility of the assay.

A combination of alpha-tocopherol, vitamin C and N-acetyl cysteine increases unsaturated fatty acid levels in hydrogen peroxide-induced Candida tropicalis (ATCC 13803)

This research was aimed at evaluating the antioxidant effects of combinations of alpha lipoic acid (LA), vitamin C (VC), N-acetyl cysteine (NAC) and alpha-tocopherol (TOC) on lipid level and fatty acid composition of C. tropicalis (ATCC 13803) against hydrogen peroxide toxicity. According to the experimental results, the cell density of C. tropicalis increased significantly in NAC+LA+H2O2, NAC+TOC+ H2O2 and NAC+VC+H2O2 groups (p<0.001) at the end of 48 and 72 h incubation times. The total lipid level in H2O2 and H2O2 + antioxidant-supplemented groups was lower than that of the control group. In the fatty acid composition of C. tropicalis, the palmitic acid level was raised in the NAC group (p<0.05), whereas its level was reduced in the other supplemented groups. While the oleic acid level increased in NAC+TOC+H2O2 and NAC+VC+H2O2 (p<0.001) groups, its level slightly decreased in the H2O2 group. The linolenic acid level was low in all the supplemented groups, but linoleic acid and total mono-unsaturated fatty acid (MUFA) levels were high in these groups compared with the control group. Total polyunsaturated fatty acid level (PUFA) decreased in NAC and H2O2 groups (p<0.01), but its level increased in NAC+LA+H2O2 and NAC+TOC+H2O2 groups (respectively, p<0.01, p<0.001). Total saturated fatty acid level decreased significantly in NAC+TOC+H2O2, NAC+H2O2 and NAC+VC+H2O2 (p<0.001) groups (p<0.01), whereas total unsaturated fatty acid level increased in NAC, NAC+H2O2, NAC+LA+H2O2, NAC+TOC+H2O2 and NAC+VC+H2O2 groups. In conclusion, our data showed that the levels of total unsaturated fatty acid, MUFA and PUFA were raised with the combinations of NAC and TOC, LA and VC in C. tropicalis cells subjected to hydrogen peroxide toxicity.

Chromosomal amplification of the Escherichia coli lipB region confers high-level resistance to selenolipoic acid

One of the mutants (slr7 mutant) of a wild-type Escherichia coli strain resistant to selenolipoic acid reported previously (K. E. Reed, T. W. Morris, and J. E. Cronan, Jr., Proc. Natl. Acad. Sci. USA 91:3720-3724, 1994) unexpectedly grew on minimal medium following transductional introduction of a lipA null mutation. We report that the slr7 strain carries a duplication of the lip chromosomal region that causes the phenotype of the mutant strain.

Biosynthesis of biotin and lipoic acid

The genetics and mechanistic enzymology of biotin biosynthesis have been the subject of much investigation in the last decade, owing to the interest for biotin production by fermentation, on the one hand, and for the design of inhibitors with potential herbicidal properties, on the other hand. Four enzymes are involved in the synthesis of biotin from its two precursors, alanine and pimeloyl-CoA. They are now well-characterized and the X-ray structures of the first three have been published. 8-Amino-7-oxopelargonic acid synthase is a pyridoxal 5'-phosphate (PLP) enzyme, very similar to other acyl-CoA alpha-oxoamine synthases, and its detailed mechanism has been determined. The origin of its specific substrate, pimeloyl-CoA, however, is not completely established. It could be produced by a modified fatty acid pathway involving a malonyl thioester as the starter. 7,8-Diaminopelargonic acid (DAPA) aminotransferase, although sharing sequence and folding homologies with other transaminases, is unique as it uses S-adenosylmethionine (AdoMet) as the NH2 donor. The mechanism of dethiobiotin synthethase is also now well understood. It catalyzes the formation of the ureido ring via a DAPA carbamate activated with ATP. On the other hand, the mechanism of the last enzyme, biotin synthase, which has long raised a very puzzling problem, is only starting to be unraveled and appears indeed to be very complex. Biotin synthase belongs to the family of AdoMet-dependent enzymes that reductively cleave AdoMet into a deoxyadenosyl radical, and it is responsible for the homolytic cleavage of C-H bonds. A first radical formed on dethiobiotin is trapped by the sulfur donor, which was found to be the iron-sulfur (Fe-S) center contained in the enzyme, and cyclization follows in a second step. Two important features come from these results: (1) a new role for an Fe-S center has been revealed, and (2) biotin synthase is not only a catalyst but also a substrate for the reaction. Lipoate synthase, which catalyzes the formation of two C-S bonds from octanoic acid, has a very high sequence similarity with biotin synthase. Although no in vitro enzymology has been carried out with lipoate synthase, the sequence homology as well as the results of in vivo studies support the conclusion that both enzymes are strongly mechanistically related.

The isolation and characterisation of a Saccharomyces cerevisiae gene (LIP2) involved in the attachment of lipoic acid groups to mitochondrial enzymes

Lipoic acid is an essential cofactor for a variety of mitochondrial enzymes. We have characterised a gene from Saccharomyces cerevisiae which appears to encode a protein involved in the attachment of lipoic acid groups to the pyruvate dehydrogenase and glycine decarboxylase complexes. The predicted protein product of this gene has significant identity to the lipoyl ligase B of both Escherichia coli and Kluyveromyces lactis. A strain harbouring a null allele of this S. cerevisiae gene is respiratory deficient due to inactive pyruvate dehydrogenase, and is unable to utilise glycine as a sole nitrogen source.

The lipoate synthase from Escherichia coli is an iron-sulfur protein

Lipoate synthase catalyzes the last step of the biosynthesis of lipoic acid in microorganisms and plants. The protein isolated from an overexpressing Escherichia coli strain was purified from inclusion bodies. Spectroscopic (UV-visible and electron paramagnetic resonance) properties of the reconstituted protein demonstrate the presence of a (2Fe-2S) center per protein. As observed in biotin synthase, these clusters are converted to (4Fe-4S) centers during reduction under anaerobic conditions. The possible involvement of the cluster in the insertion of sulfur atoms into the octanoic acid backbone is discussed.

Chromatographic analysis of lipoic acid and related compounds

The analysis of lipoic acid and related compounds, such as its reduced form dihydrolipoic acid, its amide form lipoamide and other analogues, in biological and food samples is important in biochemistry, nutritional and clinical chemistry. This review summarizes the chromatographic methods for the determination of lipoic acid and related compounds, and their applications to various samples such as bacteria, tissues, drugs and food. Gas chromatographic methods with flame ionization detection and flame photometric detection are commonly used for the quantification of lipoic acid present as its protein-bound form, after acid or base hydrolysis of these samples. High-performance liquid chromatographic methods with ultraviolet, fluorescence and electrochemical detection are mainly used for the determination of free lipoic acid and related compounds, such as dihydrolipoic acid, lipoamide and other analogues. Moreover, gas chromatography-mass spectrometry and capillary electrophoresis methods are also developed.

The pharmacology of the antioxidant lipoic acid

1. Lipoic acid is an example of an existing drug whose therapeutic effect has been related to its antioxidant activity. 2. Antioxidant activity is a relative concept: it depends on the kind of oxidative stress and the kind of oxidizable substrate (e.g., DNA, lipid, protein). 3. In vitro, the final antioxidant activity of lipoic acid is determined by its concentration and by its antioxidant properties. Four antioxidant properties of lipoic acid have been studied: its metal chelating capacity, its ability to scavenge reactive oxygen species (ROS), its ability to regenerate endogenous antioxidants and its ability to repair oxidative damage. 4. Dihydrolipoic acid (DHLA), formed by reduction of lipoic acid, has more antioxidant properties than does lipoic acid. Both DHLA and lipoic acid have metal-chelating capacity and scavenge ROS, whereas only DHLA is able to regenerate endogenous antioxidants and to repair oxidative damage. 5. As a metal chelator, lipoic acid was shown to provide antioxidant activity by chelating Fe2+ and Cu2+; DHLA can do so by chelating Cd2+. 6. As scavengers of ROS, lipoic acid and DHLA display antioxidant activity in most experiments, whereas, in particular cases, pro-oxidant activity has been observed. However, lipoic acid can act as an antioxidant against the pro-oxidant activity produced by DHLA. 7. DHLA has the capacity to regenerate the endogenous antioxidants vitamin E, vitamin C and glutathione. 8. DHLA can provide peptide methionine sulfoxide reductase with reducing equivalents. This enhances the repair of oxidatively damaged proteins such as alpha-1 antiprotease. 9. Through the lipoamide dehydrogenase-dependent reduction of lipoic acid, the cell can draw on its NADH pool for antioxidant activity additionally to its NADPH pool, which is usually consumed during oxidative stress. 10. Within drug-related antioxidant pharmacology, lipoic acid is a model compound that enhances understanding of the mode of action of antioxidants in drug therapy.

Lipoic acid metabolism in Escherichia coli: the lplA and lipB genes define redundant pathways for ligation of lipoyl groups to apoprotein

Lipoic acid is a covalently bound disulfide-containing cofactor required for function of the pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and glycine cleavage enzyme complexes of Escherichia coli. Recently we described the isolation of the lplA locus, the first gene known to encode a lipoyl-protein ligase for the attachment of lipoyl groups to lipoate-dependent apoenzymes (T. W. Morris, K. E. Reed, and J. E. Cronan, Jr., J. Biol. Chem. 269:16091-16100, 1994). Here, we report an unexpected redundancy between the functions of lplA and lipB, a gene previously identified as a putative lipoate biosynthetic locus. First, analysis of lplA null mutants revealed the existence of a second lipoyl ligase enzyme. We found that lplA null mutants displayed no growth defects unless combined with lipA (lipoate synthesis) or lipB mutations and that overexpression of wild-type LplA suppressed lipB null mutations. Assays of growth, transport, lipoyl-protein content, and apoprotein modification demonstrated that lplA encoded a ligase for the incorporation of exogenously supplied lipoate, whereas lipB was required for function of the second lipoyl ligase, which utilizes lipoyl groups generated via endogenous (lipA-mediated) biosynthesis. The lipB-dependent ligase was further shown to cause the accumulation of aberrantly modified octanoyl-proteins in lipoate-deficient cells. Lipoate uptake assays of strains that overproduced lipoate-accepting apoproteins also demonstrated coupling between transport and the subsequent ligation of lipoate to apoprotein by the LplA enzyme. Although mutations in two genes (fadD and fadL) involved in fatty acid failed to affect lipoate utilization, disruption of the smp gene severely decreased lipoate utilization. DNA sequencing of the previously identified slr1 selenolipoate resistance mutation (K. E. Reed, T. W. Morris, and J. E. Cronan, Jr., Proc. Natl. Acad. Sci. USA 91:3720-3724, 1994) showed this mutation (now called lplA1) to be a G76S substitution in the LplA ligase. When compared with the wild-type allele, the cloned lplA1 allele conferred a threefold increase in the ability to discriminate against the selenium-containing analog. These results support a two-pathway/two-ligase model of lipoate metabolism in E. coli.

Analysis of lipoic acid in biological samples by gas chromatography with flame photometric detection

A selective and sensitive gas chromatographic method for the analysis of lipoic acid in biological samples has been developed. After base hydrolysis of the sample, the liberated lipoic acid was converted into its S,S-diethoxycarbonyl methyl ester derivative and measured by gas chromatography using a DB-210 capillary column and a flame photometric detector. The calibration curve was linear in the range 20-500 ng, and the detection limit was ca. 50 pg injected. The best hydrolysis conditions for the biological samples were obtained by using 2 M potassium hydroxide containing 4% bovine serum albumin at 110 degrees C for 3 h. Using this method, lipoic acid in the hydrolysate could be selectively determined without any interference from matrix substances. Analytical results for the determination of lipoic acid in the mouse tissue and bacterial cell samples are presented.

Lipoic acid metabolism in Escherichia coli: sequencing and functional characterization of the lipA and lipB genes

Two genes, lipA and lipB, involved in lipoic acid biosynthesis or metabolism were characterized by DNA sequence analysis. The translational initiation site of the lipA gene was established, and the lipB gene product was identified as a 25-kDa protein. Overproduction of LipA resulted in the formation of inclusion bodies, from which the protein was readily purified. Cells grown under strictly anaerobic conditions required the lipA and lipB gene products for the synthesis of a functional glycine cleavage system. Mutants carrying a null mutation in the lipB gene retained a partial ability to synthesize lipoic acid and produced low levels of pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase activities. The lipA gene product failed to convert protein-bound octanoic acid moieties to lipoic acid moieties in vivo; however, the growth of both lipA and lipB mutants was supported by either 6-thiooctanoic acid or 8-thiooctanoic acid in place of lipoic acid. These data suggest that LipA is required for the insertion of the first sulfur into the octanoic acid backbone. LipB functions downstream of LipA, but its role in lipoic acid metabolism remains unclear.

Lipoic acid metabolism in Escherichia coli: isolation of null mutants defective in lipoic acid biosynthesis, molecular cloning and characterization of the E. coli lip locus, and identification of the lipoylated protein of the glycine cleavage system

We report the isolation and genetic characterization of novel Tn10dTc and Tn1000dKn insertion mutations in and near the lip locus of the Escherichia coli chromosome. The Tn10dTc and Tn1000dKn mutations define two genes, lipA and lipB, involved in lipoic acid biosynthesis. Two representative alleles (lip-2 and lip-9) from the previously reported genetic class of lipoic acid auxotrophic mutants (A. A. Herbert and J. R. Guest, J. Gen. Microbiol. 53:363-381, 1968) were assigned to the lipA complementation group. We have cloned the E. coli lip locus and developed a recombinant plasmid-based genetic system for fine-structure physical-genetic mapping of mutations in this region of the E. coli chromosome. We also report that a recombinant plasmid containing a 5.2-kbp PvuII restriction fragment from the E. coli lip locus produced three proteins of approximately 8, 12, and 36 kDa by using either a maxicell or in vitro transcription translation expression system. The 36-kDa protein was identified as the gene product encoded by the lipA locus. Finally, we have identified a previously unreported lipoylated protein that functions in the glycine cleavage system of E. coli.

Subcellular localisation of dihydrolipoamide dehydrogenase and detection of lipoic acid in bloodstream forms of Trypanosoma brucei

In the long-slender bloodstream form of Trypanosoma brucei, the enzyme dihydrolipoamide dehydrogenase exists in the absence of the 2-oxo-acid dehydrogenase complexes of which it is normally a component, and appears to be associated with the plasma membrane of the organism [Danson, M. J., Conroy, K., McQuattie, A. & Stevenson, K. J. (1987) Biochem. J. 243, 661-665]. In the present paper, a complete subcellular fractionation of T. brucei has been carried out and, by comparison with marker enzymes, it is confirmed that the dihydrolipoamide dehydrogenase is indeed associated with the plasma membrane. In addition, we now provide evidence that the distribution of the enzyme is over the whole surface of the membrane, including the flagellar pocket region, and that the enzyme is not found in any other cellular fraction. A study of the latency of the enzyme suggests that it is located on the cytoplasmic surface of the plasma membrane. The discovery of the presumed substrate of dihydrolipoamide dehydrogenase, lipoic acid, is reported for T. brucei. Using a biological assay involving a strain of Escherichia coli that requires lipoic acid for growth, we have found that acid-hydrolysed extracts of T. brucei contain 1.7 (+/- 0.2) ng of the cofactor/mg protein. The chemical nature of the lipoic acid was confirmed by gas chromatography/mass spectrometry.

Isolation and characterization of lipoylated and unlipoylated domains of the E2p subunit of the pyruvate dehydrogenase complex of Escherichia coli

The dihydrolipoamide acetyltransferase subunit (E2p) of the pyruvate dehydrogenase complex of Escherichia coli has three highly conserved and tandemly repeated lipoyl domains, each containing approx. 80 amino acid residues. These domains are covalently modified with lipoyl groups bound in amide linkage to the N6-amino groups of specific lysine residues, and the cofactors perform essential roles in the formation and transfer of acetyl groups by the dehydrogenase (E1p) and acetyltransferase (E2p) subunits. A subgene encoding a hybrid lipoyl domain was previously shown to generate two products when overexpressed, whereas a mutant subgene, in which the lipoyl-lysine codon is replaced by a glutamine codon, expresses only one product. A method has been devised for purifying the three types of independently folded domain from crude extracts of E. coli, based on their pH-(and heat-)stabilities. The domains were characterized by: amino acid and N-terminal sequence analysis, lipoic acid content, acetylation by E1p, tryptic peptide analysis and immunochemical activity. This has shown that the two forms of domain expressed from the parental subgene are lipoylated (L203) and unlipoylated (U203) derivatives of the hybrid lipoyl domain, whereas the mutant subgene produces a single unlipoylatable domain (204) containing the Lys-244----Gln substitution.

Octanoylation of the lipoyl domains of the pyruvate dehydrogenase complex in a lipoyl-deficient strain of Escherichia coli

The overexpression of a subgene encoding a hybrid lipoyl domain of the dihydrolipoamide acetyltransferase component of the pyruvate dehydrogenase complex of Escherichia coli has previously been shown to result in the formation of lipoylated and unlipoylated products. Overexpression of the same subgene in a lipoic acid biosynthesis mutant growing under lipoate-deficient conditions has now been shown to produce domains modified by octanoylation as well as unmodified domains. It was concluded from the mass of a lipoyl-binding-site peptide that the modification involves N6-octanoylation of the lysine residue (Lys244) that is normally lipoylated, and this was confirmed by the trypsin-insensitivity of the corresponding Lys244-Ala-245 bond, and the absence of modification in a mutant domain in which Lys244 is replaced by Gln. This novel protein modification raises interesting questions concerning the pathway of lipoic acid biosynthesis and the mechanism of enzyme lipoylation.