Characterization of recombinant human diamine oxidase (rhDAO) produced in Chinese Hamster Ovary (CHO) cells

Incubation of protonated NADH or NADPH with ortho-aminobenzaldehyde generates a novel fluorescent nicotinamide dihydroquinazoline condensate

Incubation of reduced nicotinamide adenine dinucleotide (NADH) but not oxidized NAD+ with ortho-aminobenzaldehyde (oABA) generated an uncharacterized chromophore with an absorption peak characteristic of a dihydroquinazoline condensate. This chromophore is responsible for a non-specific signal in a diamine oxidase (DAO) activity assay based on the generation of fluorescent dihydroquinazoline structures directly from DAO substrates. Herein we show that at pH values below 3.0 the glycosidic bond of NADH/NADPH is broken releasing double protonated dihydro-nicotinamide (dihydro-NAM), which consequently condensates with oABA to a novel dihydroquinazoline chromophore and fluorophore, namely the 6- or 8-carbamoyl-5H,7H,8H,9H-10λ⁵-pyrido[2,1-b]quinazolin-10-ylium isomer (CMPQ). The second protonation event closely correlates with the pKa of the N1 nitrogen of C5-protonated dihydro-NAM and fluorophore stability. The fusion partner of oABA is likely the iminium of the primary acid product of dihydro-NAM after glycosidic bond hydrolysis and before irreversible cyclization. Trapping of protonated dihydro-NAM from NADH or NADPH with oABA allows quantification of these dinucleotides. Despite almost a century of research studying acid-catalyzed molecular rearrangements of NADH and NADPH, new and surprising details can be discovered.

Recent advances in the application of microbial diamine oxidases and other histamine-oxidizing enzymes

The consumption of foods fraught with histamine can lead to various allergy-like symptoms if the histamine is not sufficiently degraded in the human body. The degradation occurs primarily in the small intestine, naturally catalyzed by the human diamine oxidase (DAO). An inherent or acquired deficiency in human DAO function causes the accumulation of histamine and subsequent intrusion of histamine into the bloodstream. The histamine exerts its effects acting on different histamine receptors all over the body but also directly in the intestinal lumen. The inability to degrade sufficient amounts of dietary histamine is known as the ‘histamine intolerance’. It would be preferable to solve this problem initially by the production of histamine-free or -reduced foods and by the oral supplementation of exogenous DAO supporting the human DAO in the small intestine. For the latter, DAOs from mammalian, herbal and microbial sources may be applicable. Microbial DAOs seem to be the most promising choice due to their possibility of an efficient biotechnological production in suitable microbial hosts. However, their biochemical properties, such as activity and stability under process conditions and substrate selectivity, play important roles for their successful application. This review deals with the advances and challenges of DAOs and other histamine-oxidizing enzymes for their potential application as processing aids for the production of histamine-reduced foods or as orally administered adjuvants to humans who have been eating food fraught with histamine.

Glycosylation site Asn168 is important for slow in vivo clearance of recombinant human diamine oxidase heparin-binding motif mutants

Elevated plasma and tissues histamine concentrations can cause severe symptoms in mast cell activation syndrome, mastocytosis or anaphylaxis. Endogenous and recombinant human diamine oxidase (rhDAO) can rapidly and completely degrade histamine, and administration of rhDAO represents a promising new treatment approach for diseases with excess histamine release from activated mast cells. We recently generated heparin-binding motif mutants of rhDAO with considerably increased in vivo half-lives in rodents compared with the rapidly cleared wildtype protein. Herein, we characterize the role of an evolutionary recently added glycosylation site asparagine 168 in the in vivo clearance and the influence of an unusually solvent accessible free cysteine 123 on the oligomerization of diamine oxidase (DAO). Mutation of the unpaired cysteine 123 strongly reduced oligomerization without influence on enzymatic DAO activity and in vivo clearance. Recombinant hDAO produced in ExpiCHO-S™ cells showed a 15-fold reduction in the percentage of glycans with terminal sialic acid at Asn168 compared with Chinese hamster ovary (CHO)-K1 cells. Capping with sialic acid was also strongly reduced at the other glycosylation sites. The high abundance of terminal mannose and N-acetylglucosamine residues in the four glycans expressed in ExpiCHO-S™ cells compared with CHO-K1 cells resulted in rapid in vivo clearance. Mutation of Asn168 or sialidase treatment also significantly increased clearance. Intact N-glycans at Asn168 seem to protect DAO from rapid clearance in rodents. Full processing of all glycoforms is critical for preserving the improved in vivo half-life characteristics of the rhDAO heparin-binding motif mutants.

Diamine oxidase knockout mice are not hypersensitive to orally or subcutaneously administered histamine

Objective

To evaluate the contribution of endogenous diamine oxidase (DAO) in the inactivation of exogenous histamine, to find a mouse strain with increased histamine sensitivity and to test the efficacy of rhDAO in a histamine challenge model.

Methods

Diamine oxidase knockout (KO) mice were challenged with orally and subcutaneously administered histamine in combination with the β-adrenergic blocker propranolol, with the two histamine-N-methyltransferase (HNMT) inhibitors metoprine and tacrine, with folic acid to mimic acute kidney injury and treated with recombinant human DAO. Core body temperature was measured using a subcutaneously implanted microchip and histamine plasma levels were quantified using a homogeneous time resolved fluorescence assay.

Results

Core body temperature and plasma histamine levels were not significantly different between wild type (WT) and DAO KO mice after oral and subcutaneous histamine challenge with and without acute kidney injury or administration of HNMT inhibitors. Treatment with recombinant human DAO reduced the mean area under the curve (AUC) for core body temperature loss by 63% (p = 0.002) and the clinical score by 88% (p < 0.001). The AUC of the histamine concentration was reduced by 81%.

Conclusions

Inactivation of exogenous histamine is not driven by enzymatic degradation and kidney filtration. Treatment with recombinant human DAO strongly reduced histamine-induced core body temperature loss, histamine concentrations and prevented the development of severe clinical symptoms.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00011-022-01558-2.

Heparin-binding motif mutations of human diamine oxidase allow the development of a first-in-class histamine-degrading biopharmaceutical

Background

Excessive plasma histamine concentrations cause symptoms in mast cell activation syndrome, mastocytosis, or anaphylaxis. Anti-histamines are often insufficiently efficacious. Human diamine oxidase (hDAO) can rapidly degrade histamine and therefore represents a promising new treatment strategy for conditions with pathological histamine concentrations.

Methods

Positively charged amino acids of the heparin-binding motif of hDAO were replaced with polar serine or threonine residues. Binding to heparin and heparan sulfate, cellular internalization and clearance in rodents were examined.

Results

Recombinant hDAO is rapidly cleared from the circulation in rats and mice. After mutation of the heparin-binding motif, binding to heparin and heparan sulfate was strongly reduced. The double mutant rhDAO-R568S/R571T showed minimal cellular uptake. The short α-distribution half-life of the wildtype protein was eliminated, and the clearance was significantly reduced in rodents.

Conclusions

The successful decrease in plasma clearance of rhDAO by mutations of the heparin-binding motif with unchanged histamine-degrading activity represents the first step towards the development of rhDAO as a first-in-class biopharmaceutical to effectively treat diseases characterized by excessive histamine concentrations in plasma and tissues.

Funding

Austrian Science Fund (FWF) Hertha Firnberg program grant T1135 (EG); Sigrid Juselius Foundation, Medicinska Understödsförening Liv och Hälsa rft (TAS and SeV).

Human diamine oxidase is readily released from activated neutrophils ex vivo and in vivo but is rarely elevated in bacteremic patients

During human diamine oxidase (DAO) ELISA development we noticed that in serum DAO concentrations appear to be higher when compared to plasma. Neutrophils contain DAO in the specific granules and we hypothesized that DAO is released from neutrophils during serum coagulation. If activation of neutrophils can release DAO, its concentrations might be elevated in vivo after lipopolysaccharide (LPS) administration and in bacteremic patients. Using blood from healthy volunteers DAO concentrations were measured ex vivo in serum, citrate, EDTA and heparin plasma over several hours and after activation of neutrophils. Lipopolysaccharide and granulocyte-colony stimulating factor (G-CSF) were administered to 15 and 8 healthy volunteers, respectively and DAO concentrations were measured at different timepoints. DAO antigen levels were also determined in three different subcohorts of patients with culture-proven bacteremia and high C-reactive protein (CRP) levels. DAO concentrations were elevated in a time-dependent manner in serum but not in EDTA or citrate plasma (P < 0.01). Neutrophil activation using phorbol myristate acetate (PMA) and zymosan dose-dependently caused DAO concentrations to be elevated more than 10-fold at both 22°C and 37°C (both P-values <0.001). Administration of LPS to healthy volunteers released DAO from neutrophils (P < 0.001). Of the 55 different bacteremic patients selected from three independent cohorts only 3 (5.4%) showed highly elevated DAO concentrations. Serum DAO concentrations do not accurately reflect circulating enzyme levels but coagulation-induced neutrophil activation and consequently DAO release. Only a few bacteremic patients show high DAO concentrations able to degrade histamine rapidly.

Production of Aldehydes by Biocatalysis

The production of aldehydes, highly reactive and toxic chemicals, brings specific challenges to biocatalytic processes. Absence of natural accumulation of aldehydes in microorganisms has led to a combination of in vitro and in vivo strategies for both, bulk and fine production. Advances in genetic and metabolic engineering and implementation of computational techniques led to the production of various enzymes with special requirements. Cofactor synthesis, post-translational modifications and structure engineering are applied to prepare active enzymes for one-step or cascade reactions. This review presents the highlights in biocatalytical production of aldehydes with the potential to shape future industrial applications.

Human diamine oxidase cellular binding and internalization in vitro and rapid clearance in vivo are not mediated by N-glycans but by heparan sulfate proteoglycan interactions

Human diamine oxidase (hDAO) rapidly inactivates histamine by deamination. No pharmacokinetic data are available to better understand its potential as a new therapeutic modality for diseases with excess local and systemic histamine, like anaphylaxis, urticaria or mastocytosis. After intravenous administration of recombinant hDAO to rats and mice, more than 90% of the dose disappeared from the plasma pool within 10 min. Human DAO did not only bind to various endothelial and epithelial cell lines in vitro, but was also unexpectedly internalized and visible in granule-like structures. The uptake of rhDAO into cells was dependent on neither the asialoglycoprotein-receptor (ASGP-R) nor the mannose receptor (MR) recognizing terminal galactose or mannose residues, respectively. Competition experiments with ASGP-R and MR ligands did not block internalization in vitro or rapid clearance in vivo. The lack of involvement of N-glycans was confirmed by testing various glycosylation mutants. High but not low molecular weight heparin strongly reduced the internalization of rhDAO in HepG2 cells and HUVECs. Human DAO was readily internalized by CHO-K1 cells, but not by the glycosaminoglycan- and heparan sulfate-deficient CHO cell lines pgsA-745 and pgsD-677, respectively. A docked heparin hexasaccharide interacted well with the predicted heparin binding site 568RFKRKLPK575. These results strongly imply that rhDAO clearance in vivo and cellular uptake in vitro is independent of N-glycan interactions with the classical clearance receptors ASGP-R and MR, but is mediated by binding to heparan sulfate proteoglycans followed by internalization via an unknown receptor.

Systematic use of synthetic 5'-UTR RNA structures to tune protein translation improves yield and quality of complex proteins in mammalian cell factories

Predictably regulating protein expression levels to improve recombinant protein production has become an important tool, but is still rarely applied to engineer mammalian cells. We therefore sought to set-up an easy-to-implement toolbox to facilitate fast and reliable regulation of protein expression in mammalian cells by introducing defined RNA hairpins, termed 'regulation elements (RgE)', in the 5'-untranslated region (UTR) to impact translation efficiency. RgEs varying in thermodynamic stability, GC-content and position were added to the 5'-UTR of a fluorescent reporter gene. Predictable translation dosage over two orders of magnitude in mammalian cell lines of hamster and human origin was confirmed by flow cytometry. Tuning heavy chain expression of an IgG with the RgEs to various levels eventually resulted in up to 3.5-fold increased titers and fewer IgG aggregates and fragments in CHO cells. Co-expression of a therapeutic Arylsulfatase-A with RgE-tuned levels of the required helper factor SUMF1 demonstrated that the maximum specific sulfatase activity was already attained at lower SUMF1 expression levels, while specific production rates steadily decreased with increasing helper expression. In summary, we show that defined 5'-UTR RNA-structures represent a valid tool to systematically tune protein expression levels in mammalian cells and eventually help to optimize recombinant protein expression.

Simple, sensitive and specific quantification of diamine oxidase activity in complex matrices using newly discovered fluorophores derived from natural substrates

Objective

To measure diamine oxidase (DAO) activity with high sensitivity in complex matrices like plasma or tissue extracts radioactive putrescine or horseradish peroxidase (HRP)/hydrogen peroxide (H₂O₂) coupling must be used. The use of radioactive material should be avoided and HRP/H₂O₂ coupling is compromised by antioxidants.

Methods and results

Condensation of ortho-aminobenzaldehyde (oABA) with delta-1-pyrroline and delta-1-piperideine, the autocyclization products of the DAO-oxidized natural substrates putrescine and cadaverine, generates new quinazoline fluorophores with absorption and excitation maxima of 430 and 460 nm, respectively, and peak emission at 620 nm. Fluorescent-based detection limits are 20–40 times lower compared to absorption measurements. This assay can be used to measure DAO activity in human plasma after spiking recombinant human (rh)DAO, in rat plasma after intravenous rhDAO administration, in pregnancy plasma and in tissue extracts of DAO wild-type and knock-out mice. Using rat plasma the correlation between rhDAO activity and ELISA data is 99%. Human and rat plasma without DAO spiking and tissue extracts from DAO knock-out mice showed stable and low fluorescence in the presence of high substrate concentrations.

Conclusions

Incubation of DAO with the natural substrates putrescine and cadaverine and oABA generates novel fluorophores increasing the detection limit compared to absorption measurements at least tenfold. This simple, sensitive and specific assay allows the non-radioactive quantification of DAO activity in complex matrices like plasma and tissue extracts without interference by antioxidants.

Electronic supplementary material

The online version of this article (10.1007/s00011-020-01359-5) contains supplementary material, which is available to authorized users.

Regulation of histamine and diamine oxidase in patients undergoing orthotopic liver transplantation

Increased concentrations of the vasodilator histamine have been observed in patients undergoing abdominal surgery. The role of histamine during orthotopic liver transplantation (OLT) has only been studied in animals. The aim of this study was to measure plasma concentrations of histamine and its degrading enzyme diamine oxidase (DAO) in patients undergoing orthotopic liver transplantation, and assess whether histamine or DAO correlate with intraoperative noradrenaline requirements. Histamine and DAO concentrations were measured in 22 adults undergoing liver transplantation and 22 healthy adults. Furthermore, norepinephrine requirements during liver transplantation were recorded. Baseline concentrations of histamine and DAO were greater in patients, who underwent liver transplantation, than in healthy individuals (Histamine: 6.4 nM, IQR[2.9-11.7] versus 4.3 nM, IQR[3.7-7.1], p = 0.029; DAO: 2.0 ng/mL, IQR[1.5-4.1] versus <0,5 ng/mL, IQR[<0.5-1.1], p < 0.001). During liver transplantation, histamine concentrations decreased to 1.8 nM, IQR[0.5-4.9] in the anhepatic phase (p < 0.0001 versus baseline), and to 1.5 nM, IQR[0.5-2.9] after reperfusion (p < 0.0001 versus baseline). In contrast, DAO concentrations increased to 35.5 ng/ml, IQR[20-50] in the anhepatic phase (p = 0.001 versus baseline) and to 39.5 ng/ml, IQR[23-64] after reperfusion (p = 0.001 versus baseline), correlating inversely with histamine. Norepinephrine requirements during human liver transplantation correlated significantly with DAO concentrations in the anhepatic phase (r = 0.58, p = 0.011) and after reperfusion (r = 0.56; p = 0.022). In patients undergoing orthotopic liver transplantation, histamine concentrations decrease whereas DAO concentrations increase manifold. Diamine oxidase correlates with intraoperative norepinephrine requirements in patients undergoing OLT.

A cross-species whole genome siRNA screen in suspension-cultured Chinese hamster ovary cells identifies novel engineering targets

High-throughput siRNA screens were only recently applied to cell factories to identify novel engineering targets which are able to boost cells towards desired phenotypes. While siRNA libraries exist for model organisms such as mice, no CHO-specific library is publicly available, hindering the application of this technique to CHO cells. The optimization of these cells is of special interest, as they are the main host for the production of therapeutic proteins. Here, we performed a cross-species approach by applying a mouse whole-genome siRNA library to CHO cells, optimized the protocol for suspension cultured cells, as this is the industrial practice for CHO cells, and developed an in silico method to identify functioning siRNAs, which also revealed the limitations of using cross-species libraries. With this method, we were able to identify several genes that, upon knockdown, enhanced the total productivity in the primary screen. A second screen validated two of these genes, Rad21 and Chd4, whose knockdown was tested in additional CHO cell lines, confirming the induced high productivity phenotype, but also demonstrating the cell line/clone specificity of engineering effects.

The contributions of individual galactosyltransferases to protein specific N-glycan processing in Chinese Hamster Ovary cells

Galactosylation as part of N-glycan processing is conducted by a set of beta-1,4-galactosyltransferases (B4GALTs), with B4GALT1 as the dominant isoenzyme for this reaction. Nevertheless, the exact contributions of this key-player as well as of the other isoenzymes involved in N-glycosylation, B4GALT2, B4GALT3 and B4GALT4, have not been studied in-depth. To increase the understanding of the protein- and site-specific activities of individual galactosyltransferases in Chinese Hamster Ovary cells, a panel of triple deletion cell lines was generated that expressed only one isoform of B4GALT each. Two model proteins were selected for this study to cover a large spectrum of possible N-glycan structures: erythropoietin and deamine-oxidase. They were expressed as Fc-fusion constructs (EPO-Fc and Fc-DAO) and their N-glycan processing status was analyzed by site-specific mass spectrometry. The sole activity of B4GALT1 resulted in a decrease of 15-21 % of fully galactosylated structures for erythropoietin, emphasizing the involvement of other isoenzymes. Interestingly, the contributions of B4GALT2 and B4GALT3 differed for the two model proteins. Unexpectedly, removal of galactosyltransferases influenced the overall process of N-glycan maturation, with the result of a higher occurrence of poorly processed oligosaccharides. In the context of high productivity cell lines, which can push N-glycan maturation towards incomplete galactosylation, galactosyltransferases are potential targets to ensure stable product quality. In view of our results, specifically engineered "designer" cell lines may be required for different proteins.

Pregnancy-associated diamine oxidase originates from extravillous trophoblasts and is decreased in early-onset preeclampsia

Human extravillous trophoblast (EVT) invasion of the pregnant uterus constitutes a pivotal event for the establishment of the maternal-fetal interface. Compromised EVT function manifesting in inadequate arterial remodeling is associated with the severe pregnancy disorder early-onset preeclampsia (eoPE). Recent studies suggest that EVTs invade the entire uterine vasculature including arteries, veins and lymphatics in the first trimester of pregnancy. We therefore hypothesized that EVT-derived factors accumulate in the circulation of pregnant women early in gestation and may serve to predict eoPE. In contrast to published literature, we demonstrate that placenta-associated diamine oxidase (DAO) is not expressed by maternal decidual cells but solely by EVTs, especially when in close proximity to decidual vessels. Cultures of primary EVTs express and secret large amounts of bioactive DAO. ELISA measurements indicate a pregnancy-specific rise in maternal DAO plasma levels around gestational week (GW) 7 coinciding with vascular invasion of EVTs. Strikingly, DAO levels from eoPE cases were significantly lower (40%) compared to controls in the first trimester of pregnancy but revealed no difference at mid gestation. Furthermore, DAO-containing pregnancy plasma rapidly inactivates pathophysiologically relevant histamine levels. This study represents the first proof of concept suggesting EVT-specific signatures as diagnostic targets for the prediction of eoPE.

HPLC-UV assays for evaluation of inhibitors of mono and diamine oxidases using novel phenyltetrazolylalkanamine substrates

Recently, we have described an HPLC-UV assay for the evaluation of inhibitors of plasma amine oxidase (PAO) using 6-(5-phenyl-2H-tetrazol-2-yl)hexan-1-amine (4) as a new type of substrate. Now we studied, whether this compound or homologues of it can also function as substrate for related amine oxidases, namely diamine oxidase (DAO), monoamine oxidase A (MAO A) and monoamine oxidase B (MAO B). Among these substances, 4 was converted by DAO with the highest rate. The best substrate for MAO A and B was 4-(5-phenyl-2H-tetrazol-2-yl)butan-1-amine (2). To validate the new assays, the inhibition values of known enzyme inhibitors were determined and the data were compared with those obtained with the substrate benzylamine, which is often used in amine oxidase assays. For the DAO inhibitor 2-(4-phenylphenyl)acetohydrazide an about 10fold lower IC₅₀-value against DAO was obtained when benzylamine was applied instead of 4, indicating that 4 binds to the enzyme with higher affinity than benzylamine. The IC₅₀-values of clorgiline and selegiline against MAO A and B, respectively, also decreased (two- and 30fold) replacing 2 by benzylamine. The discrepancies largely disappeared, when the enzymes were pre-incubated with the inhibitors for 15 min. This can be explained with the covalent inhibition mechanism of the inhibitors.

OPP Labeling Enables Total Protein Synthesis Quantification in CHO Production Cell Lines at the Single-Cell Level

Accurate measurement of global and specific protein synthesis rates is becoming increasingly important, especially in the context of biotechnological applications such as process modeling or selection of production cell clones. While quantification of total protein translation across whole cell populations is easily achieved, methods that are capable of tracking population dynamics at the single-cell level are still lacking. To address this need, we apply O-propargyl-puromycin (OPP) labeling to assess total protein synthesis in single recombinant Chinese hamster ovary (CHO) cells by flow cytometry. Thereby we demonstrate that global protein translation rates slightly increase with progression through the cell cycle during exponential growth. Stable CHO cell lines producing recombinant protein display similar levels of total protein synthesis as their parental CHO host cell line. Global protein translation does not correlate with intracellular product content of three model proteins, but the host cell line with high transient productivity has a higher OPP signal. This indicates that production cell lines with increased overall protein synthesis capacity can be identified by our method at the single-cell level. In conclusion, OPP-labeling allows rapid and reproducible assessment of global protein synthesis in single CHO cells, and can be multiplexed with DNA staining or any type of immunolabeling of specific proteins or markers for organelles.

Oligomannosidic glycans at Asn-110 are essential for secretion of human diamine oxidase

N-Glycosylation plays a fundamental role in many biological processes. Human diamine oxidase (hDAO), required for histamine catabolism, has multiple N-glycosylation sites, but their roles, for example in DAO secretion, are unclear. We recently reported that the N-glycosylation sites Asn-168, Asn-538, and Asn-745 in recombinant hDAO (rhDAO) carry complex-type glycans, whereas Asn-110 carries only mammalian-atypical oligomannosidic glycans. Here, we show that Asn-110 in native hDAO from amniotic fluid and Caco-2 cells, DAO from porcine kidneys, and rhDAO produced in two different HEK293 cell lines is also consistently occupied by oligomannosidic glycans. Glycans at Asn-168 were predominantly sialylated with bi- to tetra-antennary branches, and Asn-538 and Asn-745 had similar complex-type glycans with some tissue- and cell line-specific variations. The related copper-containing amine oxidase human vascular adhesion protein-1 also exclusively displayed high-mannose glycosylation at Asn-137. X-ray structures revealed that the residues adjacent to Asn-110 and Asn-137 form a highly conserved hydrophobic cleft interacting with the core trisaccharide. Asn-110 replacement with Gln completely abrogated rhDAO secretion and caused retention in the endoplasmic reticulum. Mutations of Asn-168, Asn-538, and Asn-745 reduced rhDAO secretion by 13, 71, and 32%, respectively. Asn-538/745 double and Asn-168/538/745 triple substitutions reduced rhDAO secretion by 85 and 94%. Because of their locations in the DAO structure, Asn-538 and Asn-745 glycosylations might be important for efficient DAO dimer formation. These functional results are reflected in the high evolutionary conservation of all four glycosylation sites. Human DAO is abundant only in the gastrointestinal tract, kidney, and placenta, and glycosylation seems essential for reaching high enzyme expression levels in these tissues.

Quantification of human diamine oxidase

OBJECTIVES

Diamine oxidase (DAO) is essential for extracellular degradation of histamine. For decades activity assays with inherent limitations were used to quantify the relative amounts of DAO. No reference DAO standard is available. Absolute DAO amounts cannot be determined. Controversy exists, whether DAO is circulating or not in non-pregnant individuals. The role of DAO as biomarker in various diseases is ambiguous. It is not clear, whether precise quantification of human DAO antigen using commercially available enzyme-linked immunosorbent assays (ELISAs) is possible. The objective was to develop a precise and robust ELISA to quantify DAO in various biological fluids.

DESIGN AND METHODS

A research prototype ELISA was established using a mouse monoclonal antibody for capturing and a polyclonal rabbit serum IgG fraction for the detection of human DAO. The limit of blank (LoB), limit of detection (LoD) and estimated limit of quantification (eLoQ) and normal DAO concentrations in serum and plasma were determined.

RESULTS

The LoB, LoD and eLoQ derived from 42 standard curves are 0.27, 0.48 and 0.7ng/mL respectively. The detection range using the LoD as the lower and the highest DAO standard as the upper boundary is 0.5 to 450ng/mL. Serum and plasma mean/median concentrations are between 0.5 and 1.5ng/mL in healthy volunteers (n=58) and mastocytosis patients (n=19) and plateau at approximately 145ng/mL (n=16) during pregnancy. Accurate quantification was not influenced by heparin (DAO is a heparin-binding protein), lipaemic or hemolytic serum. The measured DAO antigen concentrations are in close agreement with published enzymatic activity data using radioactive putrescine as substrate.

CONCLUSIONS

This research prototype ELISA is able to reliably and accurately quantify human DAO in different biological fluids. The potential of DAO as biomarker in various diseases can be evaluated.

Label-free live cell imaging by Confocal Raman Microscopy identifies CHO host and producer cell lines

As a possible viable and non-invasive method to identify high producing cells, Confocal Raman Microscopy was shown to be able to differentiate CHO host cell lines and derivative production clones. Cluster analysis of spectra and their derivatives was able to differentiate between different producer cell lines and a host, and also distinguished between an intracellular region of high lipid and protein content that in structure resembles the Endoplasmic Reticulum. This ability to identify the ER may be a major contributor to the identification of high producers. PCA enabled the discrimination even of host cell lines and their subclones with inherently higher production capacity. The method is thus a promising option that may contribute to early, non-invasive identification of high potential candidates during cell line development and possibly could also be used for proof of identity of established production clones.

Recombinant human diamine oxidase activity is not inhibited by ethanol, acetaldehyde, disulfiram, diethyldithiocarbamate or cyanamide

Human diamine oxidase (hDAO, EC 1.4.3.22) is the key enzyme in the degradation of extracellular histamine. Consumption of alcohol is a known trigger of mast cell degranulation in patients with mast cell activation syndrome. Ethanol may also interfere with enzymatic histamine degradation, but reports on the effects on DAO activity are controversial. There are also conflicting reports whether disulfiram, an FDA-approved agent in the treatment of alcohol dependence, inhibits DAO. We therefore investigated the inhibitory potential of ethanol and disulfiram and their metabolites on recombinant human DAO (rhDAO) in three different assay systems. Relevant concentrations of ethanol, acetaldehyde, and acetate did not inhibit rhDAO activity in an in vitro assay system using horseradish peroxidase (HRP) -mediated luminol oxidation. The aldehyde dehydrogenase (ALDH; EC 1.2.1.3) inhibitors cyanamide and its dimer dicyanamide also had no effect on DAO activity. In one assay system, the irreversible ALDH inhibitor disulfiram and its main metabolite diethyldithiocarbamate seemed to inhibit DAO activity. However, the decreased product formation was not due to a direct block of DAO activity but resulted from inhibition of peroxidase employed in the coupled system. Our in vitro data do not support a direct blocking effect of ethanol, disulfiram, and their metabolites on DAO activity in vivo.