A possible role for 15-lipoxygenase in atherogenesis

It is well accepted that high levels of low-density lipoprotein (LDL) cholesterol in the plasma are associated with increased risk of atherosclerosis. The cellular and molecular mechanisms linking the two however, have not been fully resolved. One of the processes involved in atherogensis that has been intensively studied in this regard is the oxidation of LDL. Oxidation may convert LDL into an atherogenic form, which incites an inflammatory and proliferative response characteristic of the atherosclerotic lesion. One of the potential mediators in this process is the lipid peroxidating enzyme 15-lipoxygenase, which has been shown to be induced in the atherosclerotic lesion and is capable of oxidizing LDL. In this article, we review the motivation for looking at mechanisms of LDL oxidation and the proposed involvement of 15-lipoxygenase in the pathogenesis of the disease.

12/15-Lipoxygenase gene disruption attenuates atherogenesis in LDL receptor-deficient mice

BACKGROUND

Human 15-lipoxygenase (LO) and its murine analogue 12/15-LO are capable of directly oxidizing esterified fatty acids in lipoproteins and phospholipids. Because these oxidized products possess atherogenic properties, it was suggested that LOs may be involved in enhancing atherogenesis. Previous in vivo tests of the role of LOs in atherogenesis animal models, however, have yielded conflicting results.

METHODS AND RESULTS

Aiming to study the role of the 12/15-LO in murine atherogenesis, we crossed LDL-receptor-deficient mice (LDL-R(-/-)) with 12/15-LO-knockout mice and evaluated plaque formation 3 to 18 weeks after initiation of a high-fat diet. Atherosclerotic lesions were considerably reduced in the LDL-R/12/15-LO-double-knockout mice compared with LDL-R(-/-) mice at 3, 9, 12, and 18 weeks, at the aortic root as well as throughout the aorta. The cellular composition of plaques from mice deficient in 12/15-LO did not differ with respect to macrophage and T-lymphocyte content compared with plaques from 12/15-LO littermates.

CONCLUSIONS

12/15-LO plays a dominant role in promoting atherogenesis in LDL-R(-/-) mice.

Interleukin (IL)-4 deficiency does not influence fatty streak formation in C57BL/6 mice

Abundant data is present to implicate oxidatively modified low-density lipoprotein (oxLDL) in enhanced atherogenesis. Among the factors involved in LDL oxidation, an important role has been attributed to human 15-lipoxygenase (LO) and its murine analog 12-LO. The expression of these peroxidizing enzymes is under the control of cytokines, the principal of which is IL-4. In the present study we tested the hypothesis that knocking out the IL-4 gene from C57BL/6 mice would result in suppression of fatty streaks. For this purpose, we have fed 45 female IL-4 transgenic knockout (IL-4T KO) and 45 wild-type (WT) mice an atherogenic diet for 15 weeks. Consecutive determinations of the lipid profile from both study groups were performed at monthly intervals, and fatty streak formation was assessed at the aortic sinus level, upon sacrifice. The two study groups did not differ significantly with respect to the lipid profile or the uptake and degradation of iodinated oxLDL by their peritoneal macrophages. We found that the endogenous deficiency of IL-4 did not confer protection from early atherosclerosis in the IL-4T KO as compared to their WT littermates (determined at the aortic sinus). Immunohistochemical studies, Western blots and 12/15-LO activity assays revealed the presence and activity of 12/15-LO in macrophages of WT mice as well as in IL-4T KO mice. Both did not differ significantly between the study groups. The data from this study imply that deficiency in IL-4 does not affect early atherosclerosis in C57BL/6 mice fed a high-cholesterol diet.

Applied genomics: integration of the technology within pharmaceutical research and development

Multiple novel technologies have recently been developed to improve the analysis of genetic sequences, to rapidly assess RNA or protein levels in relevant tissues, and to validate function of potential new drug targets. The challenge facing pharmaceutical research is one of effective integration of these new technologies in ways that can maximally affect the discovery and development pipeline. Although database mining and transcriptional profiling clearly have increased the number of putative targets, the current focus is to assign function to new gene targets in a high-throughput manner. This requires a restructuring of the classical linear progression from gene identification, functional elucidation, target validation and screen development. New approaches are called for that can make this process non-linear and high-throughput.

Overexpression of 15-lipoxygenase in vascular endothelium accelerates early atherosclerosis in LDL receptor-deficient mice

To study the possible role of the human lipid-oxidizing enzyme 15-lipoxygenase (15-LO) in atherosclerosis, we overexpressed it specifically in the vascular wall of C57B6/SJL mice by using the murine preproendothelin-1 promoter. The mice overexpressing 15-LO were crossbred with low density lipoprotein (LDL) receptor-deficient mice to investigate atherogenesis. High levels of 15-LO were expressed in the atherosclerotic lesion in the double-transgenic mice as assessed by immunohistochemistry. The double-transgenic, 15-LO-overexpressing, LDL receptor-deficient mice (LDLR-/-/15LO) developed significantly larger atherosclerotic lesions at the aortic sinus compared with lesions in the LDL receptor-deficient (LDLR-/-) mice after 3 and 6 weeks (107,000 versus 28,000 microm(2) [P:<0.001] and 121,000 versus 87,000 microm(2) [P:<0.05], respectively) of an atherogenic diet. LDL from the LDLR-/-/15LO mice was more susceptible to oxidation than was the LDL from the control LDLR-/- mice, as shown by a shorter lag period for copper-induced conjugated diene formation. On the other hand, no differences were found in the levels of serum anti-oxidized LDL antibodies between the study groups. There were also no differences with respect to the density of macrophages and T lymphocytes infiltrating the lesions in both experimental groups. Taken together, these results support the hypothesis that 15-LO overexpression in the vessel wall is associated with enhanced atherogenesis.

The effects of N-6 polyunsaturated fatty acid supplementation on the lipid composition and atherogenesis in mouse models of atherosclerosis

Despite numerous studies, the precise role of dietary n-6 polyunsaturated fatty acids in the pathogenesis of atherosclerosis remains controversial. It has been shown that feeding an n-6-enriched diet resulted in decreased atherosclerosis in African green monkeys and was associated with a reduction in LDL levels. However, other authors reported that n-6 supplementation increased the oxidative stress and the susceptibility of LDL to undergo in vitro oxidation, thus potentially enhancing atherosclerosis. The present study was designed to investigate the effect of dietary supplementation of n-6 polyunsaturated fats (safflower oil), as compared with a saturated fat-rich diet (Paigen), on the blood lipid profile and atherosclerosis in two mouse models. In the first experiment, female C57BL/6 mice (n=23-30 per group) were fed a cholate containing Paigen diet, a safflower oil-rich diet (with cholate), or normal chow for 15 weeks. No significant differences between the high fat diet groups were evident with respect to total cholesterol, LDL, HDL or triglyceride levels. The extent of aortic sinus fatty streaks did not differ significantly between the two groups. In the second experiment, LDL-receptor-deficient (LDL-RD) mice (n=20-30 per group) were randomized into similar dietary regimens. Mice consuming a safflower oil-enriched diet developed significantly less atherosclerosis, in comparison with Paigen diet-fed mice. A reduction in LDL levels, although not of a similar magnitude as the reduction in atherosclerosis, was evident in the safflower oil-fed mice when compared to the Paigen diet-fed littermates. In both mouse models of atherosclerosis, LDL isolated from the plasma of mice on the n-6 polyunsaturated diet was rendered slightly more susceptible to oxidation in vitro, as indicated by a shorter lag period for diene formation. Thus, the effects of n-6 fatty acids on the lipoprotein composition and other potential influences may have contributed to the anti-atherogenic effect in the LDL-RD mouse model.

Hereditary hemochromatosis: gene discovery and its implications for population-based screening

OBJECTIVE

To evaluate the role of genetic testing in screening for hereditary hemochromatosis to help guide clinicians, policymakers, and researchers.

PARTICIPANTS

An expert panel was convened on March 3, 1997, by the Centers for Disease Control and Prevention (CDC) and the National Human Genome Research Institute (NHGRI), with expertise in epidemiology, genetics, hepatology, iron overload disorders, molecular biology, public health, and the ethical, legal, and social implications surrounding the discovery and use of genetic information.

EVIDENCE

The group reviewed evidence regarding the clinical presentation, natural history, and genetics of hemochromatosis, including current data on the candidate gene for hemochromatosis (HFE) and on the ethical and health policy implications of genetic testing for this disorder.

CONSENSUS PROCESS

Consensus was achieved by group discussion confirmed by a voice vote. A draft of the consensus statement was prepared by a writing committee and subsequently reviewed and revised by all members of the expert group over a 1-year period.

CONCLUSIONS

Genetic testing is not recommended at this time in population-based screening for hereditary hemochromatosis, due to uncertainties about prevalence and penetrance of HFE mutations and the optimal care of asymptomatic people carrying HFE mutations. In addition, use of a genetic screening test raises concerns regarding possible stigmatization and discrimination. Tests for HFE mutations may play a role in confirming the diagnosis of hereditary hemochromatosis in persons with elevated serum iron measures, but even this use is limited by uncertainty about genotype-phenotype correlations. To address these questions, the expert group accorded high priority to population-based research to define the prevalence of HFE mutations, age and sex-related penetrance of different HFE genotypes, interactions between HFE genotypes and environmental modifiers, and psychosocial outcomes of genetic screening for hemochromatosis.

The hemochromatosis gene product complexes with the transferrin receptor and lowers its affinity for ligand binding

We recently reported the positional cloning of a candidate gene for hereditary hemochromatosis called HFE. The gene product, a member of the major histocompatibility complex class I-like family, was found to have a mutation, Cys-282 --> Tyr (C282Y), in 85% of patient chromosomes. This mutation eliminates the ability of HFE to associate with beta2-microglobulin (beta2m) and prevents cell-surface expression. A second mutation that has no effect on beta2m association, H63D, was found in eight out of nine patients heterozygous for the C282Y mutant. In this report, we demonstrate in cultured 293 cells overexpressing wild-type or mutant HFE proteins that both the wild-type and H63D HFE proteins form stable complexes with the transferrin receptor (TfR). The C282Y mutation nearly completely prevents the association of the mutant HFE protein with the TfR. Studies on cell-associated transferrin at 37 degrees C suggest that the overexpressed wild-type HFE protein decreases the affinity of the TfR for transferrin. The overexpressed H63D protein does not have this effect, providing the first direct evidence for a functional consequence of the H63D mutation. Addition of soluble wild-type HFE/beta2m heterodimers to cultured cells also decreased the apparent affinity of the TfR for its ligand under steady-state conditions, both in 293 cells and in HeLa cells. Furthermore, at 4 degrees C, the added soluble complex of HFE/beta2m inhibited binding of transferrin to HeLa cell TfR in a concentration-dependent manner. Scatchard plots of these data indicate that the added heterodimer substantially reduced the affinity of TfR for transferrin. These results establish a molecular link between HFE and a key protein involved in iron transport, the TfR, and raise the possibility that alterations in this regulatory mechanism may play a role in the pathogenesis of hereditary hemochromatosis.

The structure of mammalian 15-lipoxygenase reveals similarity to the lipases and the determinants of substrate specificity

Here we report the first structure of a mammalian 15-lipoxygenase. The protein is composed of two domains; a catalytic domain and a previously unrecognized beta-barrel domain. The N-terminal beta-barrel domain has topological and sequence identify to a domain in the mammalian lipases, suggesting that these domains may have similar functions in vivo. Within the C-terminal domain, the lipoxygenase substrate binding site is a hydrophobic pocket defined by a bound inhibitor. Arachidonic acid can be docked into this deep hydrophobic pocket with the methyl end extending down into the bottom of the pocket and the acid end tethered by a conserved basic residue on the surface of the enzyme. This structure provides a unifying hypothesis for the positional specificity of mammalian lipoxygenases.

Characterization and sequence of an additional 15-lipoxygenase transcript and of the human gene

15-lipoxygenase is a lipid-peroxidating enzyme that oxidizes fatty acids, such as those esterified to cellular membranes. It has been implicated in the oxidative modification of low-density lipoprotein and is thus thought to contribute to the development of atherosclerosis. The enzyme has also been shown to be specifically induced by interleukin-4 in human blood monocytes. Two 15-lipoxygenase-hybridizing messages were detected in these cells; one (2.7 kb) corresponds to the previously isolated cDNA for 15-lipoxygenase, while the other (4 kb) was of unknown origin. We have isolated and characterized this 4 kb transcript. Our experiments show that it has 1.2 kb additional sequence in its 3' untranslated region, and that it is generated from genomic sequences through differential polyA site selection. We present studies to address the functional significance of the extended 3'UTR. Selection of an upstream polyadenylation signal results in production of the 2.7 kb transcript. In addition, we present here for the first time the cloning and sequence of the human 15-lipoxygenase gene, as well as the identification of regulatory elements in the promoter region of this gene.

Defining the arachidonic acid binding site of human 15-lipoxygenase. Molecular modeling and mutagenesis

Mammalian lipoxygenases have been implicated in the pathogenesis of several inflammatory disorders and are, therefore, important targets for drug discovery. Both plant and mammalian lipoxygenases catalyze the dioxygenation of polyunsaturated fatty acids, which contain one or more 1,4-cis,cis-pentadiene units to yield hydroperoxide products. At the time this study was initiated, soybean lipoxygenase-1 was the only lipoxygenase for which an atomic resolution structure had been determined. No structure of lipoxygenase with substrate or inhibitor bound is currently available. A model of arachidonic acid docked into the proposed substrate binding site in the soybean structure is presented here. Analysis of this model suggested two residues, an aromatic residue and a positively charged residue, could be critical for substrate binding. Validation of this model is provided by site-directed mutagenesis of human 15-lipoxygenase, despite the low amino acid sequence identity between the soybean and mammalian enzymes. Both a positively charged amino acid residue (Arg402) and an aromatic amino acid residue (Phe414) are identified as critical for the binding of fatty acid substrates in human 15-lipoxygenase. Thus, binding determinants shown to be characteristic of non-enzymatic fatty acid-binding proteins are now implicated in the substrate binding pocket of lipoxygenases.

Expression of cyclooxygenase-2 in human and an animal model of rheumatoid arthritis

An inducible form of cyclooxygenase-2 (COX-2) has been shown to be upregulated in vitro by various pro-inflammatory agents, such as lipopolysaccharide, IL-1 and TNF, COX-2 appears to be responsible for the increase in prostaglandin synthesis at the site of inflammation. To examine the involvement of COX-2 in inflammation, we analysed the expression of this gene in human rheumatoid arthritis (RA) and in rat adjuvant-induced arthritis. Immunocytochemical studies of synovial membrane biopsies from human RA, osteoarthritic (OA) and normal joints using a COX-2 specific antibody showed positive staining in RA, but not in normal synovial membranes. Specifically, expression of COX-2 was detected in synovial lining cells, lymphoid aggregates and endothelial cells of blood vessels. Although some positive staining was observed in the OA joints, the number of stained cells was dramatically lower and the staining of the cells was less intense than in the rheumatoid tissue. By reverse transcription and polymerase chain reaction analysis, COX-2 mRNA was detected in the rat adjuvant arthritic limb, whereas no COX-2 mRNA was detectable in the normal limb. These observations indicate that COX-2 expression is upregulated in inflammatory joint disease and that COX-2 is a potential therapeutic target for specific inhibition.

Identification of a specific methionine in mammalian 15-lipoxygenase which is oxygenated by the enzyme product 13-HPODE: dissociation of sulfoxide formation from self-inactivation

Mammalian 15-lipoxygenases undergo a characteristic self-inactivation. The oxygenation of a single methionine to methionine sulfoxide, by 13(S)-hydroperoxyoctadecadienoic acid (13-HPODE), was previously suggested as the cause of the inactivation of rabbit reticulocyte lipoxygenase. The site of oxygenation is potentially near the enzyme's active site; however, the specific location of the modified amino acid residue has not been identified. To determine which of the methionine residues is oxygenated, we inactivated both human and rabbit 15-lipoxygenases with 13-HPODE and sequentially denatured, reduced, carboxymethylated, and digested the enzymes with trypsin. The digested mixtures were analyzed by reverse-phase HPLC chromatography. Mass spectrometric analysis of each of the methionine-containing fractions enabled us to locate the peptide segments containing the oxidized methionine in both enzymes separately. Tandem electrospray mass spectrometry identified the oxidized methionine residues to be amino acid 590 in the human enzyme and 591 in the rabbit enzyme. To investigate the significance of this oxygenation, Met590 in human 15-lipoxygenase was substituted with leucine by site-directed mutagenesis. The mutant protein was inactivated by 13-HPODE, yet no oxygenated peptide or other modified peptide could be identified by HPLC-MS analysis. We also found that human 15-lipoxygenase was inactivated during arachidonate oxidation and by the reaction product 15(S)-hydroperoxyeicosatetraenoic acid (15-HPETE), and no modified peptide was detected. Thus, methionine oxygenation is not essential for the inactivation of human 15-lipoxygenase. We suggest, however, that Met590 is an amino acid in the substrate binding pocket of human 15-lipoxygenase and interacts with the enzyme product 13-HPODE.

Conversion of human 15-lipoxygenase to an efficient 12-lipoxygenase: the side-chain geometry of amino acids 417 and 418 determine positional specificity

Positional specificity determinants of human 15-lipoxygenase were examined by site-directed mutagenesis and by kinetic analysis of the wild-type and variant enzymes. By comparing conserved differences among sequences of 12- and 15-lipoxygenases, a small region responsible for functional differences between 12- and 15-lipoxygenases has been identified. Furthermore, the replacement of only two amino acids in 15-lipoxygenase (at 417 and 418 in the primary sequence) by those found in certain 12-lipoxygenases results in an enzyme that has activity similar to 12-lipoxygenase. An examination of the activity of nine variants of lipoxygenase demonstrated that the amino acid side-chain bulk and geometry of residues 417 and 418 are the key components of the positional specificity determinant of 15-lipoxygenase. Overexpression of a variant (containing valines at positions 417 and 418) that performs predominantly 12-lipoxygenation was achieved in a baculo-virus-insect cell culture system. This variant was purified to > 90% homogeneity and its kinetics were compared with the wild-type 15-lipoxygenase. The variant enzyme has no change in its apparent KM for arachidonic acid and a minor (3-fold) change in its Vmax. For linoleic acid, the variant has no change in its KM and a 10-fold reduction in its Vmax, as expected for an enzyme performing predominantly 12-lipoxygenation. The results are consistent with a model in which two amino acids of 15-lipoxygenase (isoleucine 417 and methionine 418) constitute a structural element which contributes to the regiospecificity of the enzyme. Replacement of these amino acids with those found in certain 12-lipoxygenases results in an enzyme which can bind arachidonic acid in a catalytic register that prefers 12-lipoxygenation.

Targeting gene expression to the vascular wall in transgenic mice using the murine preproendothelin-1 promoter

To develop a system for overexpressing genes in the vascular wall, we created transgenic mice using the reporter gene luciferase and the murine preproendothelin-1 promoter. In vitro analysis suggested that the murine 5'-flanking region contained endothelial-specific elements in a 5.9-kb fragment. Five transgenic mice colonies established from independent founders all exhibited the highest level of luciferase activity in the aorta with up to 8,540 light units per microgram of protein. Immunohistochemistry with anti-luciferase antisera revealed high levels of expression in the endothelial cells of both large and small arteries and lower levels of expression in veins and capillaries. Significant expression was also seen in arterial smooth muscle cells and in select epithelial surfaces which is consistent with the known distribution of endothelin-1 in mammals. The further demonstrate the targeting capability of this system, we overexpressed the lipid-peroxidating enzyme, human 15-lipoxygenase, in the vessel wall of transgenic mice. As with luciferase, expression of active enzyme and immunohistochemical localization in vascular cells were documented in transgenic animals. Hence, this new system can be used to direct expression of molecules to the vascular wall for the purpose of examining the biological significance of either overexpression or inhibition of select proteins.

Immunohistochemical demonstration of 15-lipoxygenase in transplant coronary artery disease

15-Lipoxygenase (15-LO) catalyzes the oxygenation of arachidonic and linoleic acids and has been implicated in the oxidative modification of low-density lipoproteins (LDL). 15-LO mRNA and protein have previously been demonstrated in macrophages of rabbit and human atherosclerotic lesions. The purpose of this study was to investigate whether 15-LO is also present in the accelerated form of coronary artery disease that can complicate cardiac transplantation (TCAD). Immunohistochemical analysis of coronary arteries with TCAD was carried out by using a rabbit polyclonal antibody raised against human recombinant 15-LO and an avidin-biotin-immunoperoxidase system. Normal coronary and pulmonary arteries showed no immunostaining for 15-LO. Two different types of TCAD were observed. One type consisted of concentric intimal proliferation of smooth muscle cells, without lipid or calcium deposits. No immunoreactivity for 15-LO was present in these lesions. The second type of graft arteriosclerosis consisted of complex atheromatous lesions, containing myointimal cells, lipid-laden foam cells, fragmented internal elastic laminae, and calcifications. 15-LO immunostaining of myointimal cells, lipid-laden foam cells, and endothelial cells was consistently present in these atheromatous lesions. The majority of the myointimal and foam cells positive for 15-LO were recognized by antisera to alpha-smooth muscle actin; the others were identified as macrophages. The results indicate that 15-LO expression is present in endothelial, myointimal, and foam cells in complex atheromatous lesions of TCAD, and suggest that 15-LO may play a role in the pathogenesis of this form of the disease.

Cloning and characterization of a murine macrophage lipoxygenase

We have isolated a murine macrophage cDNA encoding a 12-lipoxygenase, that represents the homolog of the human 15-lipoxygenase. The predicted amino acid sequence of this lipoxygenase is highly similar to the rat 12-lipoxygenase isolated from brain and human 15-lipoxgenase. The recombinant enzyme expressed in Cos-7 cells oxidizes arachidonic acid to 12- and 15-HETE with a profile similar to that obtained from peritoneal macrophages. A polyclonal antibody generated against a putative peptide recognizes a 75 kDa protein in cell extracts from mouse peritoneal macrophages and transfected Cos-7 cells. The lipoxygenase cDNA hybridizes to a 2.5 kb mRNA present in peritoneal macrophages, lung, spleen, heart and liver. RT-PCR analysis indicates that the same lipoxygenase is expressed in mouse reticulocytes. A partial genomic clone for this lipoxygenase has also been characterized. Southern blot analysis of mouse genomic DNA indicates that this is a single copy gene.

On the positional specificity of 15-lipoxygenase

The lipoxygenases comprise a family of fatty acid dioxygenases that are involved in a variety of inflammatory conditions. Various approaches have been taken in order to understand the different regiospecificities of the different lipoxygenases. Here we have reviewed the current knowledge of the structural features of the substrate and of the enzyme that form the basis of the regiospecificity of 15-lipoxygenase. Earlier experiments on the structural features of the substrate were reviewed, as well as more recent results of site-directed mutagenesis studies. The structure of the soybean lipoxygenase isoform-1 was also briefly reviewed.

Eosinophil 15-lipoxygenase is a leukotriene A4 synthase

5-Lipoxygenase is the first committed enzyme in the leukotriene biosynthetic pathway and is known to catalyze not only the first oxygenation of arachidonate to form 5(S)-hydroperoxyeicosatetraenoic acid (5(S)-HPETE), but also dehydration of this intermediate into leukotriene A4 (LTA4) by an activity termed leukotriene A4 synthase. Inhibition of cytosolic 5-lipoxygenase prepared from human blood granulocytes with zileuton (100 microM) was virtually complete, but LTA4 synthase activity was only inhibited by 47%. Structural characterization of eicosanoids synthesized in these preparations revealed an abundance of 15-lipoxygenase metabolites including 15-HETE when arachidonate was used as substrate and 5(S),15(S)-dihydroxy-6,8,11,13(E,E,Z,Z)-eicosatetraenoic acid when 5(S)-HPETE was used as substrate. When neutrophils were prepared that contained less than 1% eosinophil contamination, zileuton was found to almost completely inhibit all 5-lipoxygenase, as well as LTA4 synthase products. Immunochemical analysis of the supernatants from purified neutrophils and eosinophils confirmed the previous observation that neutrophils do not express 15-lipoxygenase. Incubation of 5(S)-HPETE with recombinant mammalian 15-lipoxygenase resulted in the formation of 6-trans-LTB4 and 6-trans-12-epi-LTB4 as LTA4 products, as well as the 12-lipoxygenase product 5(S),12(S)-diHPETE. The mechanism of action of 15-lipoxygenase acting as an LTA4 synthase is proposed to involve removing the pro-R hydrogen atom at carbon-10 of 5(S)-HPETE, which is antarafacial to the hydroperoxy group to yield LTA4.

Oxidation, lipoxygenase, and atherogenesis

Mounting evidence suggests that oxidative processes contribute to the pathogenesis of atherosclerosis and that antioxidants may represent a strategy to complement the lowering of lipids in the therapy of this disease. Although multiple molecular events have been identified in vitro and although it is tempting to ascribe multiple atherogenic properties to oxidized LDL, our understanding of this process remains incomplete. Further research is warranted in several areas. First, it will be important to selectively inhibit different aspects of the process to determine the relative contribution of various biological targets. In this regard pharmacological inhibition of 15-lipoxygenase in vivo in relevant animal models is required to address the question of the contribution of this enzyme to significant oxidative events. The lack of specific inhibitors has made this task more difficult. It will also be important to define the biologically active moiety of oxidized LDL to begin to determine the mechanisms through which it exerts its atherogenic effects. It is likely that alternate protein targets can be identified both downstream and upstream of the oxidative process. Research is only now beginning to elucidate the inflammatory mechanisms that account for the cellular response. Further research into adhesion events, cytokine profiles, and downstream effector molecules of the oxidative process are likely to identify alternate targets for therapeutic intervention.

Mechanism of inhibition of cAMP-dependent epithelial chloride secretion by phorbol esters

In T84 cells, we investigated how stimulation of protein kinase C leads to an inhibition of cAMP-dependent chloride secretion. Specifically, we tested the hypothesis that the inhibition was caused by loss of the cystic fibrosis transmembrane regulator (CFTR), an apical membrane chloride channel. As described by others (Trapnell, B. C., Zeitlin, P. L., Chu, C.-S., Yoshimura, K., Nakamura, H., Guggino, W. B., Bargon, J., Banks, T. C., Dalemans, W., Pavirani, A., Lecocq, J.-P., and Crystal, R. G. (1991) J. Biol. Chem. 266, 10319-10323), we found that treatment with the phorbol ester, phorbol myristate acetate (PMA), reduced CFTR mRNA levels by approximately 80% with a t 1/2 of approximately 2 h. Chloride secretion, measured as forskolin-induced short circuit current, was also abolished by PMA with a t 1/2 of approximately 2 h. Levels of mature glycosylated CFTR measured by Western blotting also declined to 50 +/- 8% (n = 7) of control after a 12-h PMA treatment. However, a 12-h exposure to PMA did not affect the forskolin-stimulated efflux of 125I into high potassium medium, a measure of apical membrane CFTR activity. We conclude that increased turnover of apical membrane CFTR in PMA-treated cells compensates for the decline in anion channel numbers. By contrast to its lack of effect on 125I effluxes, PMA reduced the cAMP-induced increase in 86Rb efflux, suggesting that it inhibits chloride secretion mainly by an action on basolateral potassium channels.

Overexpression, purification and characterization of human recombinant 15-lipoxygenase

Human 15-lipoxygenase was expressed to high levels (approx. 20% of cellular protein) in a baculovirus/insect cell expression system. Catalytically active enzyme was readily purified (90-95% pure) from cytosolic fractions by anion-exchange chromatography on a Mono Q column with approx. 95% recovery of enzymatic activity. Routinely, a yield of 25-50 mg of pure enzyme per L of culture and a specific activity of 7.1-21 mumol 13-hydroxyoctadecadienoic acid (13-HODE)/mg.min (turnover rate of 8.4-25 s-1) were obtained. Both the specific activity and the enzyme's iron content was significantly increased by the addition of ferrous ions to either the purified enzyme or to the insect cell culture medium during production. An isoelectric point of 5.85 was determined and the N-terminal amino acid sequence was found to be identical to that predicted from the cDNA. The purified recombinant enzyme exhibits a dual positional specificity with arachidonic acid (formation of 15S- and 12S-hydroxyeicosatetraenoic acid (12S-HETE) in a ratio of 12:1). Double oxygenation products 14R,15S- and various 8,15-DiHETE isomers were also identified. With linoleic acid as substrate, a pH-optimum of 7.0 and a KM of 3 microM were determined. The enzyme undergoes suicidal inactivation during fatty acid oxygenation, is sensitive to standard lipoxygenase inhibitors, and oxygenates phospholipids, cholesterol esters, biomembranes and human low-density lipoprotein. Contrary to prior studies on the rabbit enzyme, no glycosylation was detected.

Oxygenation of lipoproteins by mammalian lipoxygenases

Oxidative modification converts low-density lipoprotein (LDL) into its atherogenic form and appears to be a necessary precondition for LDL uptake by macrophages during foam cell formation. Cellular lipoxygenases have been implicated in this process. We studied the interaction of purified mammalian lipoxygenases with human LDL in vitro and found that the arachidonate 15-lipoxygenases of rabbit and man are capable of oxygenating lipoproteins as indicated by oxygen uptake and by the formation of thiobarbituric-acid-reactive substances. Furthermore, oxygenated polyenoic fatty acids, such as 13-hydro(pero)xy-9Z,11E-octadecadienoic acid and 15-hydro(pero)xy-5,8,11,13(Z,Z,Z,E)-eicosatetraenoic acid were detected in the lipid compartment of various lipoproteins classes after lipoxygenase treatment. More than 90% of the oxygenated polyenoic fatty acids were found in the ester-lipid fraction, particularly in the cholesterol esters, whereas only small amounts of free hydro(pero)xy polyenoic fatty acids were detected. Lipoxygenase-catalyzed oxygenation of LDL is not restricted to the lipid compartment but also leads to a cooxidative modification of the apoproteins as indicated by changes in the electrophoretic mobility and by the formation of carbonyl derivatives of amino acid side chains. The possible biological significance of lipoxygenase-induced oxidative modification of lipoproteins in the pathogenesis of atherosclerosis is discussed.

Human 15-lipoxygenase: induction by interleukin-4 and insights into positional specificity

Arachidonate 15-lipoxygenase (15-lipoxygenase) is a lipid-peroxidizing enzyme associated with specific inflammatory cells seen in asthma and atherosclerosis. In atherosclerosis, 15-lipoxygenase is induced in the macrophages of human and rabbit lesions and has been implicated in foam cell formation. In human lung, 15-lipoxygenase is preferentially expressed in airway epithelial cells and eosinophils. Our studies have focused both on the regulation of expression and on the structure-function relationships of the enzyme. To determine factors that could regulate expression, peripheral blood monocytes were purified and cultured with combinations of 18 factors. Only interleukin-4 (60 pM) induced 15-lipoxygenase mRNA, protein and enzymatic activity. Interferon-gamma (100 pM) inhibited the interleukin-4 dependent induction of 15-lipoxygenase. Results with cultured human airway cells were similar. These data suggest that expression of 15-lipoxygenase is regulated by interleukin-4, and that 15-lipoxygenase is a potential downstream effector molecule for this potent cytokine. In parallel studies, we have investigated determinants of positional specificity using site-directed mutagenesis and bacterial expression of human 15-lipoxygenase. Hypotheses for mutagenesis were derived from an analysis of conserved differences among multiple lipoxygenase sequences. Switching four amino acids in 15-lipoxygenase to their counterparts in 12-lipoxygenase resulted in a variant enzyme that produced equal 12- and 15-lipoxygenation. Further analysis has identified two amino acids that completely control the positional specificity of 15-lipoxygenase. These data have led to a preliminary model of the enzyme's active site region.

Cloning and expression of an airway epithelial 12-lipoxygenase

Arachidonate 12-lipoxygenase generates metabolites that may regulate airway function. To further characterize this enzyme, we isolated a cDNA corresponding to 12-lipoxygenase from a bovine tracheal epithelium cDNA library using human reticulocyte 15-lipoxygenase cDNA as a probe. The resulting 2.9-kb cDNA, the identity of which was confirmed by expression of active catalytic function in Escherichia coli has a 2.0-kb open reading frame encoding a protein of 75,000 kDa and includes 5 bp of 5'-untranslated region and 0.9 kb of 3'-untranslated region. On Northern blots, the 12-lipoxygenase cDNA hybridized to one band (3.5 kb) of bovine tracheal epithelium RNA. Polyclonal antibodies that recognize human tracheal 15-lipoxygenase cross-reacted on immunoblots to the expressed bovine tracheal 12-lipoxygenase. Further, the deduced amino acid sequence is 86% identical (93% similar) to human 15-lipoxygenase but 64% identical to human platelet 12-lipoxygenase, suggesting that the bovine tracheal enzyme is the homologue of the human 15-lipoxygenase. This is the first sequence of an epithelial lipoxygenase from any species. A comparison of the bovine sequence with other lipoxygenase sequences shows that there are only four amino acids which are conserved differences between a 12-lipoxygenase and a 15-lipoxygenase. We hypothesize that these four amino acids may be responsible for the positional specificity of the enzyme.

Cloning of human airway 15-lipoxygenase: identity to the reticulocyte enzyme and expression in epithelium

Lipoxygenases constitute a family of enzymes which are implicated in a variety of inflammatory disorders including asthma. Although the 15-lipoxygenase has been identified as the major route of arachidonic acid metabolism in human lung, airway epithelial cells, eosinophils, and developing red cells, the localization of the enzyme within lung has not been clearly defined. Furthermore, the existence of isoforms of 15-lipoxygenase in different tissues has recently been proposed. To address these issues, we isolated a 2.6-kb cDNA encoding human airway 15-lipoxygenase from a human bronchus cDNA library using a previously characterized reticulocyte 15-lipoxygenase cDNA as a probe. The airway 15-lipoxygenase sequence was found to be identical to that of the reticulocyte-derived clone. Immunocytochemical studies using an antibody to human recombinant 15-lipoxygenase specifically localizes the enzyme to the basal and ciliated cells of the trachea, bronchi, and bronchioles. Staining for 15-lipoxygenase is not present in the airway secretory epithelial cells, epithelial cells located in the gas-exchanging regions of lung, vascular structures, or inflammatory cells. Taken together these results suggest that the 15-lipoxygenase of human lung is identical to that of the reticulocyte enzyme and is preferentially expressed in airway epithelium.

Specific inflammatory cytokines regulate the expression of human monocyte 15-lipoxygenase

Arachidonate 15-lipoxygenase (arachidonate:oxygen 15-oxidoreductase, EC 1.13.11.33) is a lipid-peroxidating enzyme that is implicated in oxidizing low density lipoprotein to its atherogenic form. Monocyte/macrophage 15-lipoxygenase is present in human atherosclerotic lesions. To pursue a basis for induction of the enzyme, which is not present in blood monocytes, the ability of relevant cytokines to regulate its expression was investigated. Interleukin 4 (IL-4), among 16 factors tested, specifically induced 15-lipoxygenase mRNA and protein in cultured human monocytes. Interferon gamma and hydrocortisone inhibited this induction. High-performance liquid chromatography analysis of lipid extracts from IL-4-treated monocytes detected 15-lipoxygenase products esterified to the cellular membrane lipids, indicating enzymatic action on endogenous substrates. Stimulation of IL-4-treated monocytes with calcium ionophore or opsonized zymosan A enhanced the formation of 15-lipoxygenase products. These data identify IL-4 and interferon gamma as physiological regulators of lipoxygenase expression and suggest an important link between 15-lipoxygenase function and the immune/inflammatory response in atherosclerosis as well as other diseases.

A primary determinant for lipoxygenase positional specificity

The three mammalian lipoxygenases are named according to the carbon position (5, 12 or 15) at which they catalyse the oxygenation of arachidonic acid; they are implicated in inflammatory disorders, for example 15-lipoxygenase is induced in atherosclerosis and can oxidize low-density lipoprotein to its atherogenic form. To identify what determines this positional specificity, we have exchanged conserved differences in the isoforms of 12- and 15-lipoxygenases. Substitution of methionine with valine at position 418 of human 15-lipoxygenase results in an enzyme that performs 12- and 15-lipoxygenation equally. This effect can be mimicked by incubating wild-type 15-lipoxygenase with a synthetically altered substrate which has its doubly allylic methylene carbons shifted by one carbon relative to arachidonic acid. Other mutations at the neighbouring amino acids 416 and 417 give an enzyme which performs 12- and 15-lipoxygenation in a ratio of 15:1. These results indicate that this region might position the substrate in the active site.

Immunocytochemical localization of arachidonate 15-lipoxygenase in erythrocytes, leukocytes, and airway cells

In reticulocytes, the enzyme 15-lipoxygenase (15-LO) is believed to contribute to cellular differentiation, and in leukocytes and airway cells 15-LO generates inflammatory mediators. The recent availability of antibodies to 15-LO now allows us to determine which specific cells contain the enzyme, to characterize its subcellular localization, and to determine its expression at the translational level. A polyclonal antibody to recombinant human reticulocyte 15-LO was used with a standard immunofluorescent technique. In rabbit red blood cells, fluorescence appeared during the course of anemia. Early reticulocytes did not fluoresce, but more mature reticulocytes showed increased fluorescent intensity. Late reticulocytes contained little fluorescence. Among human leukocytes, only eosinophils fluoresced. In human trachea, 15-LO immunofluorescence was localized to epithelial cells, and both basal and ciliated cells fluoresced. In all cells studied, fluorescence was localized to the cytoplasm and was variable in degree among cells in each preparation. We conclude that the 15-LO of airway cells and eosinophils is immunologically related to the reticulocyte 15-LO. Furthermore, the variable fluorescence among cells (e.g., in epithelium) and during development (e.g., reticulocytes) suggests a role of 15-LO in cell growth and development.

Gene expression in macrophage-rich human atherosclerotic lesions. 15-lipoxygenase and acetyl low density lipoprotein receptor messenger RNA colocalize with oxidation specific lipid-protein adducts

Oxidatively modified low density lipoprotein (LDL) exhibits several potentially atherogenic properties, and inhibition of LDL oxidation in rabbits decreases the rate of the development of atherosclerotic lesions. In vitro studies have suggested that cellular lipoxygenases may be involved in LDL oxidation, and we have shown previously that 15-lipoxygenase and oxidized LDL are present in rabbit atherosclerotic lesions. We now report that epitopes of oxidized LDL are also found in macrophage-rich areas of human fatty streaks as well as in more advanced human atherosclerotic lesions. Using in situ hybridization and immunostaining techniques, we also report that 15-lipoxygenase mRNA and protein colocalize to the same macrophage-rich areas. Moreover, these same lesions express abundant mRNA for the acetyl LDL receptor but no detectable mRNA for the LDL receptor. We suggest that atherogenesis in human arteries may be linked to macrophage-induced oxidative modification of LDL mediated by 15-lipoxygenase, leading to subsequent enhanced macrophage uptake, partly by way of the acetyl LDL receptor.

The airway epithelium and arachidonic acid 15-lipoxygenase

Pulmonary epithelial cells may be primarily responsible for initiating or regulating inflammatory responses in the airways, in part by releasing chemical mediators. Among the most potent mediators of inflammation are the lipoxygenase metabolites of arachidonic acid, including the leukotrienes and other mono and dihydroxyeicosatetraenoic acids (HETES). The human airway epithelium contains significant 15-lipoxygenase activity. Although some biologic functions of 15-lipoxygenase metabolites are known, further understanding of the role of this enzyme in the airway requires localization in tissue and studies of expression, regulation, and biologic activity. Towards these aims, we purified and characterized 15-lipoxygenase from eosinophil-enriched leukocytes. First, we studied cofactors that may be involved in regulating enzymatic activity. Second, we isolated to homogeneity, for the first time, human 15-lipoxygenase. This led to the determination of the N-terminal amino acid sequence and the discovery of homology among various mammalian lipoxygenases. Finally, we utilized this structural information to isolate a cDNA that encodes for human 15-lipoxygenase. The availability of a clone will permit studies of expression and the development of antibodies for tissue localization. Further research using molecular and antibody probes is expected to increase our understanding of the biologic roles of 15-lipoxygenase in airway epithelium.

The molecular biology of mammalian arachidonic acid metabolism

The metabolism of arachidonic acid by cyclooxygenase and lipoxygenase enzymes results in a wide range of oxidized products with potent biological activities. These metabolites, which include the prostaglandins and leukotrienes, have been implicated in the pathogenesis of a variety of inflammatory diseases. Research over the last decade has focused primarily on the elucidation of the chemical structure of the metabolites and their biological effects in vitro and in vivo. Recently, research on the enzymes that produce these bioactive metabolites through oxidization of arachidonic acid has intensified. Recombinant DNA techniques have enabled investigators to determine the nucleotide sequences for several of the enzymes in the arachidonic acid cascade. The resulting cDNAs are now being used to further investigate the biochemical and biological features of arachidonic acid metabolism. The purpose of this paper is to review how the cDNAs for these enzymes were obtained, what information they convey, and how they are being applied in current research.

Purification and crystallization of 15-lipoxygenase from rabbit reticulocytes

We report a new purification of rabbit reticulocyte 15-lipoxygenase that has resulted in the first crystallization of a mammalian lipoxygenase. The enzyme was purified to homogeneity (greater than 98% pure by SDS-PAGE) using high pressure liquid chromatography on hydrophobic-interaction, hydroxyapatite and cation-exchange columns. Crystals were grown by the vapor diffusion method from concentrated solutions of the protein in sodium phosphate buffer, pH 7.0. The hexagonal, rod-shaped crystals were on average 0.09 mm x 0.09 mm x 0.4 mm, with approximate unit cell dimensions of a = b = 260 A, c = 145 A. The crystals diffract to 5 A resolution.

Colocalization of 15-lipoxygenase mRNA and protein with epitopes of oxidized low density lipoprotein in macrophage-rich areas of atherosclerotic lesions

Oxidation of low density lipoprotein (LDL) enhances its atherogenicity, and inhibition of such oxidation decreases the rate of progression of atherosclerotic lesions. The mechanism of LDL oxidation in vivo remains uncertain, but in vitro studies have suggested that cellular lipoxygenases may play a role by initiating lipid peroxidation in LDL. In situ hybridization studies using a 15-lipoxygenase riboprobe and immunostaining using antibodies against 15-lipoxygenase showed strongly positive reactivity largely confined to macrophage-rich areas of atherosclerotic lesions. Polymerase chain reaction with 15-lipoxygenase-specific oligonucleotides and restriction enzyme digestions of the amplified fragment were used to confirm the presence of 15-lipoxygenase message in the reverse-transcribed lesion mRNA. Immunostaining with antibodies reactive with oxidized LDL (but not with native LDL) indicated that the lipoxygenase colocalizes with epitopes of oxidized LDL, compatible with a role for macrophage lipoxygenase in the oxidation of LDL in vivo. Since oxidized LDL is chemotactic for blood monocytes, early lesions might progress at a markedly accelerated rate because of further recruitment of more monocytes which, in turn, would increase further the rate of oxidation of LDL. These data suggest that therapy targeted to block macrophage lipoxygenase activity might decrease the rate of development of atherosclerotic lesions.

Expression of cloned human reticulocyte 15-lipoxygenase and immunological evidence that 15-lipoxygenases of different cell types are related

Cloned 15-lipoxygenase has been expressed for the first time in eukaryotic and prokaryotic cells. Transfection of osteosarcoma cells with a mammalian expression plasmid containing the cDNA for human reticulocyte 15-lipoxygenase resulted in cell lines that were capable of oxidizing body arachidonic acid and linoleic acid. The lipoxygenase metabolites were identified by reverse-phase and straight-phase high pressure liquid chromatography, ultraviolet spectroscopy, and direct mass spectrometry, verifying that the cDNA for 15-lipoxygenase encodes an enzyme with authentic 15-lipoxygenase activity. Incubation of the transformed cells with arachidonic acid generated 15-hydroxyeicosatetraenoic acid (HETE) and 12-HETE in a ratio of 8.6 to 1, demonstrating that 15-lipoxygenase can also perform 12-lipoxygenation. Lesser amounts of 15-keto-ETE, four isomers of 8,15-diHETE, and one isomer of 14,15-diHETE were observed. Incubation with linoleic acid generated predominantly 13-hydroxy linoleic acid. The reaction was inhibited by eicosatetraynoic acid but not by indomethacin. Antibodies to a peptide corresponding to a unique region of the predicted amino acid sequence were generated and shown to react with one major band of 70 kDa on immunoblots of human leukocyte 15-lipoxygenase. To obtain antibodies to the full length enzyme, the cDNA was subcloned into a bacterial expression vector and was expressed as a fusion with the CheY protein. The overexpressed protein was readily purified from bacteria and was shown to be immunoreactive to the peptide-derived antibody. Antibodies raised to this recombinant enzyme did not cross-react with human leukocyte 5-lipoxygenase but did identify 15-lipoxygenase in rabbit reticulocytes, human leukocytes, and tracheal epithelial cells, suggesting that the 15-lipoxygenases from these different cell types are structurally related.

Molecular cloning and primary structure of human 15-lipoxygenase

A full-length cDNA encoding 15-lipoxygenase has been isolated from a human reticulocyte cDNA library. The predicted primary structure of the enzyme exhibits a sequence similarity of 61% and 45% with human 5-lipoxygenase and the soybean lipoxygenase isoenzyme I, respectively. When all three lipoxygenases are aligned, there are two distinct regions of significant sequence identity including a cluster of five histidine residues conserved in all three lipoxygenases. Because histidines can serve as ligands for the enzymatically active iron, this region may be critical to enzymatic function. These results provide a basis for exploring functional domains of lipoxygenases.

Arachidonic acid 15-lipoxygenase and airway epithelium. Biologic effects and enzyme purification

Pulmonary epithelial cells may be primarily responsible for initiating or regulating inflammatory responses in the airways, in part by releasing chemical mediators. Among the most potent mediators of inflammation are the lipoxygenase metabolites of arachidonic acid, including the leukotrienes and the other mono- and dihydroxyeicosatetraenoic acids (HETEs). The human airway epithelium contains significant 15-lipoxygenase activity. Although some biologic functions of 15-lipoxygenase metabolites are known, further understanding of the role of this enzyme in the airway requires localization in tissue and studies of expression, regulation, and biologic activity. Towards these aims, we purified and characterized 15-lipoxygenase from eosinophil-enriched leukocytes. First, we studied cofactors that may be involved in regulating enzymatic activity. We discovered that calcium and phosphatidylcholine both enhanced, but ATP inhibited, the 15-lipoxygenase activity of highly enriched enzyme. Second, we isolated to homeogeneity, for the first time, human 15-lipoxygenase. This led to the determination of the N-terminal amino acid sequence and the discovery of homology among various mammalian lipoxygenases. Further research using purified lipoxygenase is expected to increase our understanding of the biologic roles and biochemical features of 15-lipoxygenation of arachidonic acid.

Inhibition of human leukocyte 5-lipoxygenase by 15-HPETE and related eicosanoids

The inhibition of human leukocyte 5-lipoxygenase by 15-hydroperoxyeicosatetraenoic acid and its chemical or enzymatic rearrangement products was investigated. 15-Hydroperoxyeicosatetraenoic acid was the most potent inhibitor tested. The inhibition was found to be time dependent and is not due to chemical or enzymatic decomposition products nor metabolism of 15-hydroperoxyeicosatetraenoic acid to 5,15-dihydroperoxyeicosatetraenoic acid.

An in vivo chemotaxis assay in the dog trachea: evidence for chemotactic activity of 8,15-diHETE

We describe a new in vivo chemotaxis assay in the dog trachea using a double-balloon endotracheal catheter. When inflated, the two balloons isolate a segment of trachea, which is perfused through Silastic tubes using a peristaltic pump. After instilling a chemotactic agent, the perfusate is sampled periodically to permit characterization of the chemotactic response. We anesthetized four mongrel dogs and ventilated them mechanically through the double-balloon catheter. Two mediators, leukotriene B4 (LTB4) and 8S,15S-dihydroxyeicosatetraenoic acid (8,15-diHETE) were tested in each dog by perfusing the trachea with each mediator in Hanks' balanced salt solution (HBSS) containing ethanol and antibiotics. Aliquots were removed for differential cell counts at fixed time intervals over a 4-h period. Control experiments performed in each dog with the identical concentrations of ethanol and antibiotics in HBSS showed no cellular response before 180 min. At 240 min, the cell counts were 86 +/- 28 (SE) granulocytes/microliter (n = 4). In contrast, both LTB4 and 8,15-diHETE gave a significant cellular response at 120 min (309 +/- 125 and 141 +/- 41 granulocytes/microliter, respectively; P less than 0.05) but did not differ significantly from each other. These results suggest that both LTB4 and 8,15-diHETE can incite inflammatory responses in the dog trachea in vivo. Furthermore, the double-balloon catheter technique promises to be a useful in vivo chemotaxis assay.

Arachidonate 15-lipoxygenase (omega-6 lipoxygenase) from human leukocytes. Purification and structural homology to other mammalian lipoxygenases

The enzyme responsible for 15-lipoxygenation of arachidonic acid was purified to homogeneity from human eosinophil-enriched leukocytes using a combination of ammonium sulfate precipitation, hydrophobic interaction chromatography, and high pressure liquid chromatography on hydroxyapatite and cation-exchange columns. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified protein revealed a single major band (apparent Mr 70,000). Amino acid sequence analysis yielded a single N-terminal sequence. Comparison of the N-terminal 15 residues reveals 71% sequence identity to the rabbit reticulocyte lipoxygenase and 36% sequence identity to the rat basophilic leukemia 5-lipoxygenase. In contrast, sequence identity to the soybean lipoxygenase-1 is not observed. These results demonstrate that human 15-lipoxygenase can be isolated from eosinophil-enriched leukocytes and is accessible for direct sequence analysis. Furthermore, we present initial evidence that the mammalian lipoxygenases constitute an homologous family of enzymes. The availability of homogeneous human 15-lipoxygenase will play a key role in elucidating other relationships in this family of enzymes.

Arachidonate 15-lipoxygenase from human eosinophil-enriched leukocytes: partial purification and properties

Arachidonate 15-lipoxygenase was purified from human eosinophil-enriched leukocytes after showing that 15-lipoxygenase activity was 100-fold greater in eosinophils than in neutrophils. Partial purification was achieved using ammonium sulfate precipitation, cation-exchange and hydrophobic-interaction chromatography. New evidence is presented suggesting that 15-lipoxygenase has electrostatic and hydrophobic properties distinct from 5-lipoxygenase. In addition, ATP is shown to inhibit, and phosphatidylcholine is shown to stimulate, 15-lipoxygenase, suggesting a regulatory role for these compounds in the lipoxygenation of arachidonic acid.

Marked hepatic congestion caused by a thoracoabdominal aneurysm

A 70-yr-old man with a large thoracoabdominal aortic aneurysm developed marked congestive hepatomegaly. Evaluation by arteriography, venography, and computed tomography revealed compression and venous obstruction at the confluence of the hepatic veins and inferior vena cava. Although obstruction of the inferior vena cava caused by rupture of an abdominal aortic aneurysm is known to occur, obstruction without rupture and involving the hepatic veins has not been previously reported.