Classes of Drugs that Mitigate Radiation Syndromes

We previously reported several vignettes on types and classes of drugs able to mitigate acute and, in at least one case, late radiation syndromes in mice. Most of these had emerged from high throughput screening (HTS) of bioactive and chemical drug libraries using ionizing radiation-induced lymphocytic apoptosis as a readout. Here we report the full analysis of the HTS screen of libraries with 85,000 small molecule chemicals that identified 220 "hits." Most of these hits could be allocated by maximal common substructure analysis to one of 11 clusters each containing at least three active compounds. Further screening validated 23 compounds as being most active; 15 of these were cherry-picked based on drug availability and tested for their ability to mitigate acute hematopoietic radiation syndrome (H-ARS) in mice. Of these, five bore a 4-nitrophenylsulfonamide motif while 4 had a quinoline scaffold. All but two of the 15 significantly (p < 0.05) mitigated H-ARS in mice. We had previously reported that the lead 4-(nitrophenylsulfonyl)-4-phenylpiperazine compound (NPSP512), was active in mitigating multiple acute and late radiation syndromes in mice of more than one sex and strain. Unfortunately, the formulation of this drug had to be changed for regulatory reasons and we report here on the synthesis and testing of active analogs of NPSP512 (QS1 and 52A1) that have increased solubility in water and in vivo bioavailability while retaining mitigator activity against H-ARS (p < 0.0001) and other radiation syndromes. The lead quinoline 057 was also active in multiple murine models of radiation damage. Taken together, HTS of a total of 150,000 bioactive or chemical substances, combined with maximal common substructure analysis has resulted in the discovery of diverse groups of compounds that can mitigate H-ARS and at least some of which can mitigate multiple radiation syndromes when given starting 24 h after exposure. We discuss what is known about how these agents might work, and the importance of formulation and bioavailability.

4-(Nitrophenylsulfonyl)piperazines mitigate radiation damage to multiple tissues

Our ability to use ionizing radiation as an energy source, as a therapeutic agent, and, unfortunately, as a weapon, has evolved tremendously over the past 120 years, yet our tool box to handle the consequences of accidental and unwanted radiation exposure remains very limited. We have identified a novel group of small molecule compounds with a 4-nitrophenylsulfonamide (NPS) backbone in common that dramatically decrease mortality from the hematopoietic acute radiation syndrome (hARS). The group emerged from an in vitro high throughput screen (HTS) for inhibitors of radiation-induced apoptosis. The lead compound also mitigates against death after local abdominal irradiation and after local thoracic irradiation (LTI) in models of subacute radiation pneumonitis and late radiation fibrosis. Mitigation of hARS is through activation of radiation-induced CD11b⁺Ly6G⁺Ly6C⁺ immature myeloid cells. This is consistent with the notion that myeloerythroid-restricted progenitors protect against WBI-induced lethality and extends the possible involvement of the myeloid lineage in radiation effects. The lead compound was active if given to mice before or after WBI and had some anti-tumor action, suggesting that these compounds may find broader applications to cancer radiation therapy.

Interstitial Lung Disease in West Highland White Terriers

Progressive respiratory failure and pulmonary fibrosis in West Highland White Terriers (WHWT) is an apparently genetic disorder of unknown pathogenesis. This study characterizes the light microscopic, ultrastructural, and immunohistochemical features of affected WHWT in comparison with lesions in usual interstitial pneumonia (UIP) of humans. Lesions in WHWT were confined to the expansion of the interstitial space of alveolar septa by extracellular matrix (ECM) determined to be mixtures of type-I and -III collagens. Features of UIP such as intra-alveolar fibroblastic foci, subpleural distribution, and honeycombing were not observed in six WHWT. Comparison with normal dogs showed no apparent increase in septal myofibroblasts. Ultrastructually, the ECM in alveolar septa consisted of large aggregates of periodic collagen filaments underlying alveolar capillaries that were surrounded by thick bands of amorphous to fine fibrillar matrix. This study suggests that chronic pulmonary disease of WHWT is a result of aberrant collagen regulation.

High throughput screening of small molecule libraries for modifiers of radiation responses

PURPOSE

An unbiased approach of drug discovery through high-throughput screening (HTS) of libraries of chemically defined and bioactive small molecule compounds was used to identify modulators of radiation injury with an emphasis on radioprotectors and mitigators rather than radiosensitisers. Assay system endpoints included radiation-induced genotoxicity and DNA damage in yeast and apoptosis in murine lymphocytes. Large-scale data mining of chemically diverse libraries identified agents that were effective with all endpoints. HTS of bioactive compound libraries against murine lymphocytes profiled tetracycline and fluoroquinolone antibiotics and cyclopiazonic acid as having activity, and structure-activity analysis showed a common pharmacophore. Purine nucleosides, the interferon inducer tilorone, and linoleic acid were also identified as potential mitigators of radiation damage that often were also radioprotective. Many of these compounds enhance DNA repair, have anti-inflammatory activity, and stimulate hematopoiesis. Selected compounds within these initial verified hits from both types of libraries identified potent mitigators of lethal whole body irradiation (WBI) in mice.

CONCLUSION

In spite of the fact that in vitro HTS has limitations and is unable to fully recapitulate all aspects of the complex in vivo acute radiation response, it identified several classes of molecules that had activity as radioprotectors and radiomitigators of the hematopoietic system in vivo. In the future, addition of 3-dimensional (3-D) or stem cell cultures or pathway analysis, may improve the power of HTS, but our findings indicate that common, evolutionary conserved, canonical pathways can be identified that could be exploited to mitigate radiation-induced defects.

Ionizing radiation activates the Nrf2 antioxidant response

The transcription factor NF-E2-related factor 2 (Nrf2) binds the antioxidant DNA response element (ARE) to activate important cellular cytoprotective defense systems. Recently several types of cancers have been shown to overexpress Nrf2, but its role in the cellular response to radiation therapy has yet to be fully determined. In this study, we report that single doses of ionizing radiation from 2 to 8 Gy activate ARE-dependent transcription in breast cancer cells in a dose-dependent manner, but only after a delay of five days. Clinically relevant daily dose fractions of radiation also increased ARE-dependent transcription, but again only after five days. Downstream activation of Nrf2-ARE-dependent gene and protein markers, such as heme oxygenase-1, occurred, whereas Nrf2-deficient fibroblasts were incapable of these responses. Compared with wild-type fibroblasts, Nrf2-deficient fibroblasts had relatively high basal levels of reactive oxygen species that increased greatly five days after radiation exposure. Further, in vitro clonogenic survival assays and in vivo sublethal whole body irradiation tests showed that Nrf2 deletion increased radiation sensitivity, whereas Nrf2-inducing drugs did not increase radioresistance. Our results indicate that the Nrf2-ARE pathway is important to maintain resistance to irradiation, but that it operates as a second-tier antioxidant adaptive response system activated by radiation only under specific circumstances, including those that may be highly relevant to tumor response during standard clinical dose-fractionated radiation therapy.

High-throughput screening identifies two classes of antibiotics as radioprotectors: tetracyclines and fluoroquinolones

PURPOSE

Discovery of agents that protect or mitigate normal tissue from radiation injury during radiotherapy, accidents, or terrorist attacks is of importance. Specifically, bone marrow insufficiency, with possible infection due to immunosuppression, can occur after total body irradiation (TBI) or regional irradiation and is a major component of the acute radiation syndrome. The purpose of this study was to identify novel radioprotectors and mitigators of the hematopoietic system.

EXPERIMENTAL DESIGN

High-throughput screening of small-molecule libraries was done using viability of a murine lymphocyte line as a readout with further validation in human lymphoblastoid cells. The selected compounds were then tested for their ability to counter TBI lethality in mice.

RESULTS

All of two major classes of antibiotics, tetracyclines and fluoroquinolones, which share a common planar ring moiety, were radioprotective. Furthermore, tetracycline protected murine hematopoietic stem/progenitor cell populations from radiation damage and allowed 87.5% of mice to survive when given before and 35% when given 24 h after lethal TBI. Interestingly, tetracycline did not alter the radiosensitivity of Lewis lung cancer cells. Tetracycline and ciprofloxacine also protected human lymphoblastoid cells, reducing radiation-induced DNA double-strand breaks by 33% and 21%, respectively. The effects of these agents on radiation lethality are not due to the classic mechanism of free radical scavenging but potentially through activation of the Tip60 histone acetyltransferase and altered chromatin structure.

CONCLUSIONS

Tetracyclines and fluoroquinolones can be robust radioprotectors and mitigators of the hematopoietic system with potential utility in anticancer radiotherapy and radiation emergencies.

Nitroxyl inhibits breast tumor growth and angiogenesis

Nitroxyl (HNO) can inhibit the glycolytic enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Because of the importance of glycolysis in many malignant cells, we thus propose that HNO can adversely affect tumor growth. This hypothesis was tested using in vitro and in vivo models of breast cancer. We report here for the first time that HNO suppresses the proliferation of both estrogen receptor (ER)-positive and ER-negative human breast cancer cell lines, in a dose dependent manner. Mice treated with HNO either injected into the tumor itself or via the intraperitoneal approach had smaller xenograft tumor size. In addition to significantly decreased blood vessel density in the HNO-treated tumors, we observed lower levels of circulating serum vascular endothelial growth factor (VEGF). Accordingly, there was a decrease in total HIF-1alpha (hypoxia-inducible factor) protein in HNO-treated tumor cells. Further studies showed inhibition of GAPDH activity in HNO-treated human breast cancer cell lines and in HNO-treated tumor tissue derived from xenografts. One explanation for the multiplicity of actions observed after HNO treatment could be the effect from the initial inhibition of GAPDH, providing a potential therapeutic avenue based upon blocking glycolysis resulting in decreased HIF-1alpha, thus leading to angiogenesis inhibition. Therefore, HNO appears to act via mechanism(s) different from those of existing breast cancer drugs, making it a potential candidate to overcome known and emerging drug resistance pathways.

Targeted inhibition of EG-1 blocks breast tumor growth

EG-1 is a gene product that is significantly elevated in human breast cancer tissues. Previously, we have shown that EG-1 overexpression stimulates cellular proliferation both in vitro and in vivo. Here, we ask whether this molecule can be targeted for experimental therapeutic purpose. siRNA lentivirus and polyclonal antibodies were designed to suppress EG-1 expression. These agents were then used in cell culture proliferation assays and breast tumor xenograft models. Serum and urine from breast cancer patients were also analyzed for the presence of EG-1 peptide. We report here for the first time that endogenous EG-1 can be targeted to inhibit breast tumor growth. This inhibition, whether delivered via siRNA lentivirus or polyclonal antibody, resulted in decreased cellular proliferation in culture and smaller xenografts in mice. The effects were shown in both ER (estrogen receptor)-positive human breast cancer MCF-7 cells, as well as in ER-negative MDA-MB-231 cells. Furthermore, we detected soluble EG-1 in serum and urine of breast cancer patients. These observations demonstrate that EG-1 is relevant to human breast cancer, and is a molecular target worthy of translational efforts into effective breast cancer therapy.

Fetal gonadotrope cell origin of FSH-secreting pituitary adenoma - insight into human pituitary tumour pathogenesis

OBJECTIVE

The pathogenesis of human pituitary adenomas remains unclear, but we report a case of FSH-secreting pituitary adenoma whose monohormonal phenotype suggests it was of fetal origin.

PATIENTS

A 28-year-old woman presented with abdominal discomfort and irregular menses, enlarged multicystic ovaries and elevated serum oestradiol, with sustained high-normal FSH and low LH levels.

MEASUREMENTS

Endocrine studies were performed before and after curative surgery, with assessment of tumour hormone secretion in vitro, and immunostaining of tumour tissue for a series of gonadotrope proteins.

RESULTS

Immunocytochemistry showed that tumour cells were monohormonal for FSH. Normal components of gonadotrope signalling pathways were expressed, including oestrogen receptor-alpha, activin receptors, secretogranin-II and chromogranin-A. beta-glycan, the putative inhibin coreceptor, was absent. Tumour culture in vitro confirmed secretion of FSH with minimal LH, that was unsuppressed by oestradiol or inhibin-A. Human fetal pituitary tissue contained FSH-only cells at 18 weeks gestation, whereas normal adult pituitary tissue contained only bihormonal gonadotropes.

CONCLUSIONS

We propose that this pituitary adenoma represents an indolent tumour of monohormonal fetal gonadotrope cells that originated early in gestation. Pituitary tumours may therefore arise from abnormal persistence of fetal cell types, with extremely slow growth over many years until reaching a size threshold to generate an endocrine syndrome. Understanding fetal pituitary architecture and function may be more informative for new insights into pituitary tumour pathogenesis than classical theories of cancer biology that invoke unrestrained cell proliferation.

EG-1 interacts with c-Src and activates its signaling pathway

EG-1 is significantly elevated in breast, colorectal, and prostate cancers. Overexpression of EG-1 stimulates cellular proliferation, and targeted inhibition blocks mouse xenograft tumor growth. To further clarify the function of EG-1, we investigated its role in c-Src activation. We observed that EG-1 overexpression results in activation of c-Src, but found no evidence that EG-1 is a direct Src substrate. EG-1 also binds to other members of the Src family. Furthermore, EG-1 shows interaction with multiple other SH3- and WW-containing molecules involved in various signaling pathways. These observations suggest that EG-1 may be involved in signaling pathways including c-Src activation.

Boric acid inhibits adenosine diphosphate-ribosyl cyclase non-competitively

Adenosine diphosphate-ribosyl cyclase (ADP-ribosyl cyclase) is a ubiquitous enzyme in eukaryotes that converts NAD+ to cyclic-ADP-ribose (cADPR) and nicotinamide. A quantitative assay for cADPR was developed using capillary electrophoresis to separate NAD+, cADPR, ADP-ribose, and ADP with UV detection (254 nm). Using this assay, the apparent Km and Vmax for Aplysia ADP-ribosyl cyclase were determined to be 1.24+/-0.05 mM and 131.8+/-2.0 microM/min, respectively. Boric acid inhibited ADP-ribosyl cyclase non-competitively with a Ki of 40.5+/-0.5 mM. Boric acid binding to cADPR, determined by electrospray ionization mass spectrometry, was characterized by an apparent binding constant, KA, of 655+/-99 L/mol at pH 10.3.

A novel mass spectrometric assay for the cerebroside sulfate activator protein (saposin B) and arylsulfatase A

A mass spectrometric method is described for monitoring cerebrosides in the presence of excess concentrations of alkali metal salts. This method has been adapted for use in the assay of arylsulfatase A (ASA) and the cerebroside sulfate activator protein (CSAct or saposin B). Detection of the neutral glycosphingolipid cerebroside product was achieved via enhancement of ionization efficiency in the presence of lithium ions. Assay samples were extracted into the chloroform phase as for the existing assays, dried, and diluted in methanol-chloroform-containing lithium chloride. Samples were analyzed by electrospray ionization mass spectrometry with a triple quadrupole mass spectrometer in the multiple reaction monitoring tandem mass spectrometric mode. The assay has been used to demonstrate several previously unknown or ambiguous aspects of the coupled ASA/CSAct reaction, including an absolute in vitro preference for CSAct over the other saposins (A, C, and D) and a preference for the non-hydroxylated species of the sulfatide substrate over the corresponding hydroxylated species. The modified assay for the coupled ASA/CSAct reaction could find applicability in settings in which the assay could not be performed previously because of the need for radiolabeled substrate, which is now not required.

Cyclin D3 expression in normal fetal, normal adult and neoplastic feline tissue

Cyclin D3 is a tightly regulated cell cycle protein and member of the cyclin D family-a group of proteins that facilitates the progression of a cell through G(1) and into the S phase of the cell cycle. All cells use at least one of the cyclin D proteins for cell cycle regulation. In this study, feline tissues (normal fetal and adult, and neoplastic) were examined immunohistochemically for expression and topographical distribution of cyclin D3. Its distribution was similar to that in human tissues in health and neoplasia, and suggested a dual role of cyclin D3 in cell proliferation and differentiation. Immature lymphoid tissue and proliferating epithelial cells in health and neoplasia were immunoreactive for cyclin D3, whereas expression of the protein in other immunoreactive tissues reflected differentiated cell types. Immunoreactivity for cyclin D3 was particularly striking in germinal centre cells of normal lymph nodes and B-cell lymphomas, and in normal suprabasal epithelial cells of the skin and mucous membranes of the oropharynx and in squamous cell carcinomas at these sites.

Inhibition kinetics of carba- and C-fucosyl analogues of GDP-fucose against fucosyltransferase V: implication for the reaction mechanism

Inhibition kinetics of two isosteric analogues of GDP-fucose (GDP-Fuc) were investigated against fucosyltransferase V using electrospray ionization mass spectrometry coupled to multiple reaction monitoring. The carba-Fuc analogue was found to be a competitive inhibitor with a K(i) value of 67.1+/-9.8 microM, similar to the K(m) value for GDP-Fuc (50.4+/-5.5 microM), while the C-Fuc analogue exhibited significantly weak competitive inhibition with a K(i) value of 889+/-93 microM.

Borate-nucleotide complex formation depends on charge and phosphorylation state

Flow injection analysis with electrospray ionization mass spectrometry was used to investigate borate-nucleotide complex formation. Solutions containing 100 microM nucleotide and 500 microM boric acid in water-acetonitrile-triethylamine (50:50:0.2, v/v/v; pH 10.3) showed that borate complexation with nicotinamide nucleotides was significantly influenced by the charge on the nicotinamide group and the number of phosphate groups on the adenine ribose. Borate binding decreased in the order of NAD(+), NADH, NADP(+) and NADPH. To investigate the relationship between complex formation and phosphorylation, association constants (K(A)) of borate-adenine (AMP, ADP, ATP), -guanine (GMP, GDP, GTP), -cytidine (CMP, CDP, CTP) and -uridine (UMP, UDP, UTP) complexes were compared. The results showed that the number of nucleotide phosphate groups was inversely proportional to the relative abundance of the borate complexes, with the K(A) of borate-nucleotide complex decreasing in the order mono-, di- and tri-phosphates (AMP approximately GMP approximately CMP approximately UMP > ADP approximately GDP approximately CDP approximately UDP > GTP > ATP approximately CTP approximately UTP). At pH 7.4, using ammonium bicarbonate buffer, only borate-NAD(+) complex was observed. This indicates that the borate-NAD(+) complex may be the most physiologically relevant of those studied.

Real-time imaging of gene promoter activity using an adenoviral reporter construct demonstrates transcriptional dynamics in normal anterior pituitary cells

Although analysis of luciferase activity using luminescence imaging has provided new insights into the dynamic regulation of gene expression in living tIssues, studies in vitro have relied on stably transfected clonal cell lines, limiting the choice of cell type and species, or DNA microinjection, which is arduous and highly selective. We report here the first use of a recombinant adenovirus in which the firefly luciferase reporter gene was regulated by the prolactin gene promoter, to study temporal dynamics of promoter activity. This vector was used to infect the pituitary GH3 cell line, and also primary cultures of Syrian hamster pituitary cells. We show that adenovirally transduced cells retained normal regulation of the promoter-reporter transgene by appropriate signals. Furthermore, microscopic imaging studies indicated that both clonal and primary pituitary cells were transduced efficiently, giving readily detectable luminescence signals in real-time over long periods. Finally, analysis of single-cell expression patterns indicated that prolactin promoter activity was highly dynamic with pulses in gene expression, revealing that the transcriptional instability seen in clonal cells is a feature of normal pituitary cells. Adenoviral vectors offer a valuable tool for studies of gene regulation where conventional transgenesis and clonal cell lines are not available.

Characterization of a recombinant molecule covalently indistinguishable from human cerebroside-sulfate activator protein (CSAct or Saposin B)

Humans deficient in the cerebroside-sulfate activator protein (CSAct or Saposin B) are unable to catabolize sulfatide and other glycosphingolipids leading to their accumulation and neurodegenerative disease. Clinically this usually manifests as a form of metachromatic leukodystrophy (MLD). CSAct is a small water-soluble glycoprotein that apparently functions in the lysosome to solubilize sulfatide and other lipids enabling their interaction with soluble lysosomal hydrolases. CSAct activity can be measured in vitro by assay of its ability to activate sulfatide-sulfate hydrolysis by arylsulfatase A or ex vivo by its ability to functionally complement CSAct deficient fibroblast cell lines derived from MLD patients. A recombinant form of CSAct has been expressed in E. coli and processed in vitro to a form covalently indistinguishable from deglycosylated human CSAct isolated from human urine. Size-exclusion chromatography in combination with multi-angle laser-light scattering (SEC-MALLS) measurements demonstrate that both native and recombinant forms of the molecule behave as a dimer in the pH range 7.0-4.5. The CSAct activity assay showed that both recombinant and deglycosylated human urine CSAct efficiently activated sulfatide sulfate hydrolysis and provided functional complementation of CSAct-deficient cells. However, a D21N mutant form of recombinant CSAct could not functionally complement these cells despite full activity in the in vitro assay. It is concluded that while glycosylation is unnecessary for in vitro and ex vivo activity of CSAct, modification of the native N21 is necessary to prevent loss of ex vivo activity, possibly via protection from degradation.

Dynamic patterns of growth hormone gene transcription in individual living pituitary cells

Real-time imaging of the GH gene promoter linked to luciferase in living pituitary cells has revealed surprising heterogeneity and variety of dynamic patterns of gene expression. Cells treated with either forskolin or thyroid hormone generated a consistent and characteristic temporal response from cell populations, but detailed analysis of individual cells revealed different patterns. Approximately 25-26% of cells displayed no response, 25-33% of cells exhibited a sustained progressive rise in luciferase activity, and 41-50% showed a transient phasic, or oscillatory response, after given stimuli. In cells treated consecutively with the two stimuli, the population response to the second stimulus was augmented. Single-cell analysis revealed that this was partly due to an increased number of cells responding, but also that the prevalence of response patterns changed: cells that responded to an initial stimulus were more likely to respond subsequently in a progressive sustained manner. In conclusion, these studies have indicated that GH promoter activity in individual living pituitary cells is unstable and possibly stochastic, with dynamic variations from hour to hour. The prevalence of different temporal patterns of response to hormonal stimulation among a population of cells is altered by the endocrine history of those cells.

Kinetic characterization of enzyme inhibitors using electrospray-ionization mass spectrometry coupled with multiple reaction monitoring

Electrospray ionization mass spectrometry coupled to multiple reaction monitoring (ESI-MS/MRM) has been applied for the first time to analyze enzyme inhibitor kinetics. Specifically, a known competitive inhibitor, guanosine 5'-monophosphate (GMP), and a synthetic, transition-state analogue inhibitor, guanosine 5'-[1D-(1,3,4/2)-5-methyl-5-cyclohexene-1,2,3,4-tetrol 1-diphosphate] (1) have been characterized against recombinant fucosyltransferase (Fuc-T) V using ESI-MS/MRM. Dixon analysis with GMP yielded a signature plot for competitive inhibition. Nonlinear regression analysis gave a Ki of 211.8+/-24.7 microM. The conventional analysis using GDP-[U-14C]-Fuc yielded a similar Ki value of 235.6+/-59.4 microM, confirming the validity of the MS-based method. The synthetic inhibitor 1 showed potent competitive inhibition with a Ki of 25.6+/-2.8 microM. Although 1 possesses a chemically reactive allyl phosphate group, ESI-MS/MRM showed that there was no reduction in the concentration of 1 and no production of a predicted metabolite GDP during the assay. MS/MS also confirmed the absence of a possible pseudo-trisaccharide product. The results clearly show that 1 is neither a slow-reacting donor nor does it act as a suicide-type inhibitor toward Fuc-T V. ESI-MS/MRM is therefore a powerful tool for the kinetic characterization of enzyme inhibitors, providing complete disclosure of the mechanism of action of 1 as an inhibitor.

Persistent synchronized oscillations in prolactin gene promoter activity in living pituitary cells

PRL gene expression in the anterior pituitary gland responds rapidly to different hormonal signals. We have investigated the long-term timing of transcriptional activation from the PRL, GH, and cytomegalovirus promoters in response to different stimulus duration, using real-time imaging of luciferase expression in living stably transfected GH3 cells. Long-term stimulation of serum-starved cells with 50% serum induced a homogeneous rise in PRL promoter activity, with subsequent heterogeneous fluctuations in luciferase activity in individual cells. When cells were subjected to a 2-h pulse of 50% serum, followed by serum-free medium, there were long-term (approximately 50 h) synchronized, homogeneous oscillations in PRL promoter activity. This response was PRL-specific, because in GH3 cells expressing luciferase from the GH or cytomegalovirus promoters, a serum pulse elicited no oscillations in luciferase expression after an initial transient response to serum. The PRL promoter may therefore be a template for an unstable transcription complex subject to stochastic regulation, allowing an oscillatory transcriptional response to physiological signals. This suggests that precise timing and coordination of cell responses to different signal-duration may represent a novel mechanism for coordinating long-term dynamic changes in transcription in cell populations.

Analysis of enzyme kinetics using electrospray ionization mass spectrometry and multiple reaction monitoring: fucosyltransferase V

An accurate, rapid, and versatile method for the analysis of enzyme kinetics using electrospray ionization mass spectrometry (ESI-MS) has been developed and demonstrated using fucosyltransferase V. Reactions performed in primary or secondary amine-containing buffers were diluted in an ESI solvent and directly analyzed without purification of the reaction products. Decreased mass resolution was used to maximize instrument sensitivity, and multiple reaction monitoring (MRM), in the tandem mass spectrometric mode, was used to enhance selectivity of detection. The approach allowed simultaneous monitoring of multiple processes, including substrate consumption, product formation, and the intensity of an internal standard. MRM gave an apparent K(m) for GDP-L-fucose (GDP-Fuc) of 50.4 +/- 5.5 microM and a k(cat) of 1.46 +/- 0.044 s(-1). Under the same conditions, the conventional radioactivity-based assay using GDP-[U-(14)C]Fuc as substrate gave virtually identical results: K(m) = 54.3 +/- 4.6 microM and k(cat) = 1.49 +/- 0.039 s(-1). The close correlation of the data showed that ESI-MS coupled to MRM is a valid approach for the analysis of enzyme kinetics. Consequently, this method represents a valuable alternative to existing analytic methods because of the option of simultaneously monitoring multiple species, the high degree of specificity, and rapid analysis times and because it does not rely on the availability of radioactive or chromogenic substrates.