Immunoaffinity purification of human thromboxane synthase

TBXAS1 Gene Polymorphism Is Associated with the Risk of Ischemic Stroke of Metabolic Syndrome in a Chinese Han Population

Objective

To investigate the association between thromboxane A synthase 1 (TBXAS1) gene polymorphism and metabolic syndrome (MS) and explore whether gene polymorphism could act as biomarkers in MS and its components or whether it could play a role in MS-related damage.

Methods

A total of 3072 eligible subjects were obtained, of which 1079 cases were controls and 1993 cases were MS patients. Subjects were followed up for 5 years, and the endpoint were recorded. The gene polymorphism of TBXAS1 was detected by using the Sequenom MassArray method.

Results

Significant differences were observed in ischemic stroke and NC_000007.14: g.139985896C>T (P < 0.05). The incidence of ischemic stroke was significantly higher in T allele carriers than in C (P < 0.05). C allele was the protective factor of the onset of ischemic stroke. There were negative interactions between C allele and waist circumference (WC), systolic blood pressure (SBP), diastolic blood pressure (DBP), triglycerides (TG), high-density lipoprotein cholesterol (HDL-C), and fasting plasma glucose (FPG).

Conclusion

These findings suggest that NC_000007.14: g.139985896C>T was related to the incidence of ischemic stroke in the whole and MS population, and individuals who carry the C allele have a reduced risk of ischemic stroke, which may be used as a promising biomarker of disease risk in patients with MS.

Molecular evolution of enzymes involved in the arachidonic acid cascade

The metabolic pathway called the arachidonic acid cascade produces a wide range of eicosanoids, such as prostaglandins, thromboxanes and leukotrienes with potent biological activities. Recombinant DNA techniques have made it possible to determine the nucleotide sequences of cDNAs and/or genomic structures for the enzymes involved in the pathway. Sequence comparison analyses of the accumulated sequence data have brought great insights into the structure, function and molecular evolution of the enzymes. This paper reviews the sequence comparison analyses of the enzymes involved in the arachidonic acid cascade.

Identification of genetic variants in the human thromboxane synthase gene (CYP5A1)

Thromboxane synthase (CYP5A1) catalyzes the conversion of prostaglandin H2 to thromboxane A2, a potent mediator of platelet aggregation, vasoconstriction and bronchoconstriction. It has been implicated in the patho-physiological process of a variety of diseases, such as atherosclerosis, myocardial infarction, stroke and asthma. On the basis of the hypothesis that variations of the CYP5A1 gene may play an important role in human diseases, we performed a screening for variations in the human CYP5A1 gene sequence. We examined genomic DNA from 200 individuals, for mutations in the promoter region, the protein encoding sequences and the 3'-untranslated region of the CYP5A1. Eleven polymorphisms have been identified in the CYP5A1 gene including eight missense mutations R61H, D161E, N246S, L357V, Q417E, E450K, T451N and R466Q. This is the first report of genetic variants in the human CYP5A1 altering the protein sequence. The effect of these variants on the metabolic activity of CYP5A1 remains to be further evaluated.

Identification of thromboxane synthase amino acid residues involved in heme-propionate binding

Thromboxane A2 synthase (TXAS) is a member of the cytochrome P450 superfamily and catalyzes an isomerization reaction that converts prostaglandin H2 to thromboxane A2. As a step toward understanding the structure/function relationships of TXAS, we mutated amino acid residues predicted to bind the propionate groups of A- and D-pyrrole rings of the heme. These mutations at each of these residues (Asn-110, Trp-133, Arg-137, Arg-413, and Arg-478) resulted in altered heme binding, as evidenced by perturbation of the absorption spectra and EPR. The mutations, although causing no significant changes in the secondary structure of the proteins, induced tertiary structural changes that led to increased susceptibility to trypsin digestion and alteration of the intrinsic protein fluorescence. Moreover, these mutant proteins lost their binding affinity to the substrate analog, had a lower heme content and retained less than 5% of the wild-type catalytic activity. However, mutations at the neighboring amino acid of the aforementioned residues yielded mutant proteins retaining the biochemical and biophysical properties of the wild type TXAS. Aligning the TXAS sequence with the structurally known P450s, we proposed that in TXAS the A-ring propionate of the heme is hydrogen bonded to Asn-110, Arg-413, and Arg-478, whereas D-ring propionate is hydrogen bonded to Trp-133 and Arg-137. Furthermore, both A- and D-ring propionates bulge away from the heme plane and both lie on the proximal face of heme plane, a structure similar to P450terp.

Thromboxanes: synthase and receptors

Thromboxane A2 is a biologically potent arachidonate metabolite through the cyclooxygenase pathway. It induces platelet aggregation and smooth muscle contraction and may promote mitogenesis and apoptosis of other cells. Its roles in physiological and pathological conditions have been widely documented. The enzyme that catalyzes its synthesis, thromboxane A2 synthase, and the receptors that mediate its actions, thromboxane A2 receptors, are the two key components critical for the functioning of this potent autacoid. Recent molecular biological studies have revealed the structure-function relationship and gene organizations of these proteins as well as genetic and epigenetic factors modulating their gene expression. Future investigation should shed light on detailed molecular signaling events specifying thromboxane A2 actions, and the genetic underpinning of the enzyme and the receptors in health and disease.

Expression, purification, and spectroscopic characterization of human thromboxane synthase

Thromboxane A2 (TXA2) is a potent inducer of vasoconstriction and platelet aggregation. Large scale expression of TXA2 synthase (TXAS) is very useful for studies of the reaction mechanism, structural/functional relationships, and drug interactions. We report here a heterologous system for overexpression of human TXAS. The TXAS cDNA was modified by replacing the sequence encoding the first 28 amino acid residues with a CYP17 amino-terminal sequence and by adding a polyhistidine tag sequence prior to the stop codon; the cDNA was inserted into the pCW vector and co-expressed with chaperonins groES and groEL in Escherichia coli. The resulting recombinant protein was purified to electrophoretic homogeneity by affinity, ion exchange, and hydrophobic chromatography. UV-visible absorbance (UV-Vis), magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectra indicate that TXAS has a typical low spin cytochrome P450 heme with an oxygen-based distal ligand. The UV-Vis and EPR spectra of recombinant TXAS were essentially identical to those of TXAS isolated from human platelets, except that a more homogenous EPR spectrum was observed for the recombinant TXAS. The recombinant protein had a heme:protein molar ratio of 0.7:1 and a specific activity of 12 micromol of TXA2/min/mg of protein at 23 degreesC. Furthermore, it catalyzed formation of TXA2, 12-hydroxy-5,8,10-heptadecatrienoic acid, and malondialdehyde in a molar ratio of 0.94:1.0:0.93. Spectral binding titrations showed that bulky heme ligands such as clotrimazole bound strongly to TXAS (Kd approximately 0.5 microM), indicating ample space at the distal face of the heme iron. Analysis of MCD and EPR spectra showed that TXAS was a typical low spin hemoprotein with a proximal thiolate ligand and had a very hydrophobic distal ligand binding domain.

P450 inhibitors of use in medical treatment: focus on mechanisms of action

A number of cytochrome P450s are targets for compounds that are clinically used or under clinical evaluation for treatment of patients with mycotic infections, such as dermatophytosis, superficial and systemic candidiasis, cryptococcosis and aspergillosis, with skin diseases, such as psoriasis or ichthyosis, and other retinoid-sensitive malignancies, e.g., neuro-ectodermal glioma. Some of the P450 inhibitors are candidates for the treatment of hirsutism or prostate cancer, others are potent inhibitors of the P450 isomerase involved in the synthesis of thromboxane A2, a potent platelet aggregation inducer and vasoconstrictor.

Effects of cyclosporin on kidney glutathione metabolism and cytochrome P-450 in the rabbit: possible implication of eicosanoid metabolism

This study was designed to assess Cyclosporin A (CsA) nephrotoxicity in the rabbit-possibly a more sensitive species than the rat-and to explore the mechanism of this toxicity with special attention to glutathione metabolism disturbances and cytochrome P-450 level in the kidney. CsA given for three days at a daily dose of 50 mg/kg (s.c.) induced nephrotoxicity as assessed by histological abnormalities and by a significant increase in blood urea nitrogen and urinary enzyme activities: N-acetyl-beta-D-glucosaminidase and L-gamma-glutamyl-transferase. This observed renal injury was of the same order as that obtained in the rat. In addition, there was a significant increase in oxidized glutathione content (40%) while reduced glutathione level remained unchanged. Concurrently, there was a significant decrease in renal cortex glutathione reductase (49%) and to a lesser extent in glutathione peroxidase activities (16%) whereas that of glutathione-S-transferase was not modified. A significant increase in renal cortex cytochrome P-450 (3-fold versus controls) was also observed. The mechanism of CsA nephrotoxicity is to be related to a cytochrome P-450 induction. This event could induce the observed impairments in renal glutathione metabolism and Na+K(+)-ATPase activity, via a possible increase in eicosanoid metabolism.

The porcine thromboxane synthase-encoding cDNA: sequence, mRNA expression and enzyme production in Sf9 insect cells

A full-length cDNA encoding porcine thromboxane synthase (TS) was isolated and sequenced. The open reading frame encodes a 534-amino acid (aa) protein (M(r) 60,451) which shares more than 75% identity with TS from other species and is 30% homologous to several enzymes of the cytochrome P-450 III family. Sequence comparison among porcine (p), human (h), and murine (m) TS indicated conservation of eight Cys residues and one putative N-glycosylation site. Several highly conserved regions were identified at the near N terminus, middle and C terminus. The most divergent region lies at aa residues 290-325, within which a Lys308 residue was unique to pTS. Between aa residues 70 and 90, considerable divergence was observed in mTS. Northern analysis showed that the pTS gene was expressed as a 2.3-kb transcript primarily in lung, kidney and thymus. A high-titer recombinant (re-) baculovirus containing pTS cDNA was developed to conduct a time course study of enzyme production in Spodoptera frugiperda (Sf9) cells. TS activity was detectable in the microsomes of Sf9 cells 12-h post-infection and reached maximum by 48 h. The produced TS resembles purified pTS in catalysis, as well as inhibition by a substrate analog inhibitor.

Pregnancy and lipid peroxide-induced alterations of eicosanoid-metabolizing enzymes in the aorta of the rat

OBJECTIVES

We examined whether pregnancy modulates the expression of prostaglandin endoperoxide synthase, prostacyclin synthase, and thromboxane A2 synthase in the systemic vasculature. Further, we examined whether elevated lipid peroxidation during pregnancy (induced by vitamin E deprivation) affects the normal adaptive process to pregnancy.

STUDY DESIGN

Western immunoblotting was performed on aortas from normal and vitamin E-deprived late pregnant (18 to 19 days) and age-matched virgin control rats.

RESULTS

Normal pregnancy resulted in an increased expression of prostaglandin endoperoxide synthase (2.91 vs 1.06 fmol/ng deoxyribonucleic acid, p < 0.05). Surprisingly, the expression for both prostacyclin and thromboxane A2 synthases were significantly decreased by pregnancy: prostacyclin synthase 2.60 versus 13.82 units/ng deoxyribonucleic acid and thromboxane A2 synthase 1.32 versus 9.85 units/ng of deoxyribonucleic acid. Elevation of endogenous lipid peroxidation partially reversed this normal pregnancy trend in enzyme expression: prostaglandin endoperoxide synthase 1.85 fmol/ng deoxyribonucleic acid, prostacyclin synthase 9.38 units/ng deoxyribonucleic acid, thromboxane A2 synthase 4.36 units/ng deoxyribonucleic acid.

CONCLUSION

Changes in prostanoid activity in the systemic vasculature during pregnancy may be mediated by concerted induction and down-regulation of specific enzymes. Increased lipid peroxidation interferes with this normal pregnant pattern. Further studies on the cell-specific expression of these genes will help to define the cardiovascular role of prostaglandins in pregnancy and in preeclampsia.

Expression of two different forms of cDNA for thromboxane synthase in insect cells and site-directed mutagenesis of a critical cysteine residue

cDNA coding for human placental thromboxane synthase (EC 5.3.99.5) was amplified by PCR from a human placental cDNA library and sequenced. This cDNA and a shorter cDNA isolated from a human lung cDNA library with a deletion of 163 bp near the 3' end were expressed in Spodoptera frugiperda (Sf9) insect cells using a baculovirus expression system. The cDNA from human placenta was expressed as an active enzyme (60 kDa) with a specific activity higher than those reported from other cell types, whereas the shorter cDNA was expressed in an inactive form (52 kDa). The active recombinant enzyme appeared to be unglycosylated as the molecular mass and the enzyme activity were not altered in the presence of tunicamycin. Site-directed mutagenesis was performed to convert a cysteine at position 480 in thromboxane synthase to a serine. This cysteine is found to be highly conserved in related cytochrome P-450 enzymes. The mutant enzyme was found to be inactive, although Western blot, immunoprecipitation and SDS/PAGE analysis indicated that the mutant enzyme was expressed at a level comparable with the wild-type enzyme. These results suggest that Cys-480 is essential for the enzyme catalytic activity and that the short-form cDNA may be a non-functional transcript.

Expression of human thromboxane synthase using a baculovirus system

Human thromboxane (TX) synthase (EC 5.3.99.5) was produced by the baculovirus expression system using cDNA encoding human TX synthase [(1991) Biochem. Biophys. Res. Commun. 78, 1479-1484]. A recombinant baculovirus TXS7 was expressed in Spodoptera frugiperda Sf9 insect cells. The expressed protein was recognized by monoclonal antibody, Kon 7 raised against human TX synthase [(1990) Blood 76, 80-85]. The recombinant TX synthase catalyzed the conversion of prostaglandin (PG) H2 to TXA2 and 12-hydroxy-heptadecatrienoic acid (HHT). Both conversions of PGH2 to TXA2 and HHT by the expressed TX synthase were completely inhibited by a specific TX synthase inhibitor, OKY-046 (5 microM).

The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature

We provide here a list of 221 P450 genes and 12 putative pseudogenes that have been characterized as of December 14, 1992. These genes have been described in 31 eukaryotes (including 11 mammalian and 3 plant species) and 11 prokaryotes. Of 36 gene families so far described, 12 families exist in all mammals examined to date. These 12 families comprise 22 mammalian subfamilies, of which 17 and 15 have been mapped in the human and mouse genome, respectively. To date, each subfamily appears to represent a cluster of tightly linked genes. This revision supersedes the previous updates [Nebert et al., DNA 6, 1-11, 1987; Nebert et al., DNA 8, 1-13, 1989; Nebert et al., DNA Cell Biol. 10, 1-14 (1991)] in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene and cDNA, we recommend that the italicized root symbol "CYP" for human ("Cyp" for mouse), representing "cytochrome P450," be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen should precede the final number in mouse genes. "P" ("p" in mouse) after the gene number denotes a pseudogene. If a gene is the sole member of a family, the subfamily letter and gene number need not be included. We suggest that the human nomenclature system be used for all species other than mouse. The mRNA and enzyme in all species (including mouse) should include all capital letters, without italics or hyphens. This nomenclature system is identical to that proposed in our 1991 update. Also included in this update is a listing of available data base accession numbers for P450 DNA and protein sequences. We also discuss the likelihood that this ancient gene superfamily has existed for more than 3.5 billion years, and that the rate of P450 gene evolution appears to be quite nonlinear. Finally, we describe P450 genes that have been detected by expressed sequence tags (ESTs), as well as the relationship between the P450 and the nitric oxide synthase gene superfamilies, as a likely example of convergent evolution.

Induction of thromboxane synthase and prostaglandin endoperoxide synthase mRNAs in human erythroleukemia cells by phorbol ester

The effects of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on the mRNA levels of two enzymes, thromboxane synthase (TXS) and prostaglandin endoperoxide synthase (PES), responsible for the synthesis of thromboxane A2 from arachidonic acid, were studied in human erythroleukemia (HEL) cells by RNA blot analysis. TPA induced both TXS and PES mRNAs in HEL cells in a dose-dependent manner at 36 h. The half-maximal and maximal effects for the induction of both mRNAs were at approximately 3 x 10(-9) M and at 10(-8) M, respectively. TXS and PES mRNA levels increased in a time-dependent fashion by TPA, and reached to 7- and 3.5-fold of the control, respectively after 48 h of TPA treatment. These results suggest that expression of TXS and PES genes in HEL cells were simultaneously stimulated by TPA.

Molecular cloning of human platelet thromboxane A synthase

Complementary DNA coding for thromboxane A synthase was amplified by polymerase chain reaction using primers synthesized according to the partial amino acid sequences of human platelet thromboxane A synthase (Nüsing, R., Schneider-Voss, S., and Ullrich, V. (1990) Arch. Biochem. Biophys. 280, 325-330) and cloned into pBluescript SK II(-). The primary structure of human platelet enzyme was deduced from the nucleotide sequence of the cDNA. The enzyme is composed of 533 amino acids with a molecular weight of 60,487. The primary structure of the enzyme exhibited a 34-36% homology to the amino acid sequences of cytochrome P450s classified in the P450 III gene family. The highly conserved cysteine-containing sequence involved in the heme-binding site of P450 was found near the carboxyl terminus (residues 472-492). The size of the major thromboxane A synthase mRNA from human platelets and human erythroleukemia cells was estimated to be approximately 2.2 kilobases by RNA blot analysis.

Isolation of partial complementary DNA encoding human thromboxane synthase

Thromboxane synthase catalyzes the biosynthesis of thromboxane A2 which plays a key role in the proaggregatory and vasoconstrictive processes. In this communication, we reported the successful cloning of thromboxane synthase cDNA from a human lung cDNA library. Oligonucleotides were synthesized according to the direct amino acid sequence of 2 peptides derived from purified human thromboxane synthase. Polymerase chain reaction was carried out using these oligonucleotides as primers to isolate a complementary DNA from human lung cDNA library. The longest cDNA thus obtained was 687 base pairs in length. Amino acid sequences deduced from the cDNA contained all three peptide sequences reported, confirming the authenticity of the cDNA clone.

Prostaglandin and thromboxane biosynthesis

We describe the enzymological regulation of the formation of prostaglandin (PG) D2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2 (prostacyclin), and thromboxane (Tx) A2 from arachidonic acid. We discuss the three major steps in prostanoid formation: (a) arachidonate mobilization from monophosphatidylinositol involving phospholipase C, diglyceride lipase, and monoglyceride lipase and from phosphatidylcholine involving phospholipase A2; (b) formation of prostaglandin endoperoxides (PGG2 and PGH2) catalyzed by the cyclooxygenase and peroxidase activities of PGH synthase; and (c) synthesis of PGD2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2, and TxA2 from PGH2. We also include information on the roles of aspirin and other nonsteroidal anti-inflammatory drugs, dexamethasone and other anti-inflammatory steroids, platelet-derived growth factor (PDGF), and interleukin-1 in prostaglandin metabolism.