Europium-labelled recombinant protein G. A fast and sensitive universal immunoreagent for time-resolved immunofluorometry

A Novel Universal Detection Agent for Time-Gated Luminescence Bioimaging

Luminescent lanthanide chelates have been used to label antibodies in time-gated luminescence (TGL) bioimaging. However, it is a challenging task to label directly an antibody with lanthanide-binding ligands and achieve control of the target ligand/protein ratios whilst ensuring that affinity and avidity of the antibody remain uncompromised. We report the development of a new indirect detection reagent to label antibodies with detectable luminescence that circumvents this problem by labelling available lysine residues in the linker portion of the recombinant fusion protein Linker-Protein G (LPG). Succinimide-activated lanthanide chelating ligands were attached to lysine residues in LPG and Protein G (without Linker) and the resulting Luminescence-Activating (LA-) conjugates were compared for total incorporation and conjugation efficiency. A higher and more efficient incorporation of ligands at three different molar ratios was observed for LPG and this effect was attributed to the presence of eight readily available lysine residues in the linker region of LPG. These Luminescence-Activating (LA-) complexes were subsequently shown to impart luminescence (upon formation of europium(III) complexes) to cell-specific antibodies within seconds and without the need for any complicated bioconjugation procedures. The potential of this technology was demonstrated by direct labelling of Giardia cysts and Cryptosporidium oocysts in TGL bioimaging.

Europium tetracycline as a luminescent probe for nucleoside phosphates and its application to the determination of kinase activity

The determination of enzyme activities and the screening of enzyme regulators is a major task in clinical chemistry and drug development. A broad variety of enzymatic reactions is associated with the consumption of adenosine triphosphate (ATP), including, in particular, phosphorylation reactions catalyzed by kinases, formation of adenosine cyclic monophosphate (cAMP) by adenylate cyclases, and ATP decomposition by ATPase. We have studied the effect of a series of adenosine (ATP, ADP, AMP, cAMP) and guanosine (GTP, GDP) phosphoric esters, and of pyrophosphate (PP) on the fluorescence emission of the europium tetracycline (EuTC) complex. We found that these compounds have strongly different quenching effects on the luminescence emission of EuTC. The triphosphates ATP and GTP behave as strong quenchers in reducing the fluorescence intensity of EuTC to 25 % of its initial value by formation of a ternary 1:1:1 complex. All other phosphate esters showed a weak quenching effect only. The applicability of this fluorescent probe to the determination of the activity of phosphorylation enzymes is demonstrated by means of creatine kinase as a model for non-membrane-bound kinases. In contrast to other methods, this approach does not require the use of radioactively labeled ATP substrates, additional enzymes, or of rather complex immunoassays.

A Novel Method for Time-Resolved Fluorimetric Determination and Imaging of the Activity of Peroxidase, and Its Application to an Enzyme-Linked Immunosorbent Assay

A new type of fluorescence assay for the determination of peroxidase (POx) activity is presented. The assay is based on the indication of the enzymatic consumption of H(2)O(2) (HP), using a fluorescent europium-tetracycline (Eu(3)TC) complex as indicator. On addition of HP, this complex forms a highly fluorescent adduct (Eu(3)TC-HP), which is decomposed in the presence of POx to form the weakly fluorescent europium-tetracycline (Eu(3)TC). Hence, the activity of the enzyme can be directly determined by means of the luminescent Eu(3)TC complex as indicator. The POx assay demonstrated herein was elaborated starting from a spectral characterization of the complex systems involved. Due to the long lifetime of lanthanide luminescence, both steady-state and time-resolved luminescence assays can easily be performed. The time-resolved assay can quantify POx in the range from 4.0 x 10(-5) to 5.9 x 10(-3) U mL(-1), with a limit of detection of 1.0 x 10(-5) U mL(-1). The effects of POx inhibitors such as cyanide, hydroxylamine, and azide have also been studied. In addition, a time-resolved fluorescent detection method for a POx-based enzyme-linked immunosorbent assay (ELISA) has been developed, which is demonstrated in a sandwich model assay with bovine IgG serving as analyte. Furthermore, a time-resolved fluorescent imaging method is demonstrated that makes use of a straightforward imaging set-up adjusted to the optical properties of the europium reagent.

Fluorescence quenching of the europium tetracycline hydrogen peroxide complex by copper (II) and other metal ions

The europium-tetracycline complex [Eu(Tc)] is known to show only weak fluorescence with an emission maximum at 615 nm. On addition of hydrogen peroxide (HP), the strongly fluorescent [Eu(Tc)(HP)] complex is formed, which displays a 15-fold stronger luminescence intensity. This study describes the decrease in luminescence intensity of the [Eu(Tc)(HP)] complex in aqueous solution in the presence of Cu2+, Fe3+, Ag+, Al3+, Zn2+, Co2+, Ni2+, Mn2+, Ca2+, and Mg2+. Static and dynamic quenching can be induced by Cu2+, and these processes were quantified by means of their quenching constants. Stern-Volmer plots were also derived from lifetime imaging measurements accomplished by the rapid lifetime determination (RLD) technique based on microwell plate assays, and also by the time-correlated single photon counting (TCSPC) technique. According to those data, a time-resolved fluorescent method for copper determination can be proposed that is based on dynamic quenching of the [Eu(Tc)(HP)] complex by Cu2+ ions. The response to copper concentrations is linear up to 1.6 micromol L(-1), providing a detection limit of 0.2 micromol L(-1).

The combined effect of triple therapy with rosiglitazone, metformin, and insulin aspart in type 2 diabetic patients

OBJECTIVE

Type 2 diabetes is caused by reduced insulin secretion and insulin resistance in skeletal muscle and liver. We tested the combination therapy with insulin aspart, rosiglitazone, and metformin with the purpose of treating all three defects in order to test the hypothesis that this "triple therapy" will normalize glucose metabolism.

RESEARCH DESIGN AND METHODS

Sixteen obese type 2 diabetic outpatients on human NPH or MIX (regular + NPH insulin) insulin twice daily were randomized to either triple therapy, i.e., insulin aspart (a rapid-acting insulin analog) at meals, metformin (which improves hepatic insulin sensitivity), and rosiglitazone (which improves peripheral insulin sensitivity), or to continue their NPH or MIX insulin twice daily for 6 months. Insulin doses were adjusted in both groups based on algorithms. HbA(1c), insulin dose, hypoglycemic episodes, insulin sensitivity (clamp), hepatic glucose production (tracer), and diurnal profiles of plasma glucose and insulin were used in evaluating treatment.

RESULTS

In the triple therapy group, HbA(1c) declined from 8.8 to 6.8% (P < 0.01) without inducing severe hypoglycemic events. Postprandial hyperglycemia was generally avoided, and the diurnal profile of serum insulin showed fast and high peaks without any need to increase insulin dose. In the control group, the insulin dose was increased by 50%, but nevertheless both HbA(1c) and 24-h blood glucose profiles remained unchanged. Insulin sensitivity improved in both skeletal muscle and the liver in the triple therapy group, whereas no change was observed in the control group.

CONCLUSIONS

We conclude that treatment of the three major pathophysiological defects in type 2 diabetic subjects by triple therapy significantly improved glucose metabolism in obese type 2 diabetic subjects.

Quantitative investigation of the affinity properties of different recombinant forms of protein G by means of high-performance monolithic chromatography

The recombinantly produced different forms of protein G, namely monofunctional immunoglobulin G (IgG) binding, monofunctional serum albumin (SA) binding and bifunctional IgG/SA binding proteins G, are compared with respect to their specific affinities to blood IgG and SA. The affinity mode of the recently developed high-performance monolithic disk chromatography has been used for fast quantitative investigations. Using single affinity disks as well as two discs stacked into one separation unit, one order of magnitude in adsorption capacities for IgG and SA were found both for monofunctional and bifunctional protein G forms used as specific affinity ligands. However, despite the adsorption difference observed, the measured dissociation constants of the affinity complexes seemed to be very close. The analytical procedure developed can be realized within a couple of minutes. Up-scaling of the developed technology was carried out using another type of monolithic materials, i.e. CIM affinity tubes.

Detection of Aspergillus galactomannan: comparison of an enzyme-linked immunoassay and a europium-linked time-resolved fluoroimmunoassay

With a view to improving the sensitivity of serological detection of Aspergillus galactomannan (GM), a europium-linked time-resolved fluoroimmunoassay was developed. This method was compared to an enzyme-linked immunosorbent assay using a peroxidase-conjugated detector antibody. No increase in the sensitivity of the detection of GM standards was seen with the europium-based fluoroimmunoassay.

Simultaneous quantitation of diphtheria and tetanus antibodies by double antigen, time-resolved fluorescence immunoassay

A dual, double antigen, time-resolved fluorescence immunoassay (DELFIA) for the simultaneous detection and quantitation of diphtheria (D) and tetanus (T) antibodies in sera has been developed. In the double antigen format one arm of the antibody binds to antigen coated microtitre wells and the other arm binds to labelled antigen to provide a fluorescent signal. This assay was found to be functionally specific for IgG antibodies and showed a good correlation with established toxin neutralization assays. Furthermore, the double antigen set-up was species independent, permitting the direct use of existing international references of animal origin to measure protective antibody levels in humans in international units (IU/ml). The detection limit corresponded to 0.0003 IU/ml with Eu(3+)-labelled toxoids and to 0.0035 IU/ml using Sm(3+)-labelled toxoids. The assay was fast with a high capacity making it a suitable method for serological surveillance studies.

The use of an indirect ELISA with protein G-peroxidase conjugate and a blocking ELISA to demonstrate recent bluetongue infection in sheep

The humoral immune response of sheep infected with bluetongue virus serotypes 3, 9 and 16 was monitored by plaque inhibition (PI), blocking ELISA (BELISA) and indirect ELISA over a period of 63 days post-infection. Results indicated that testing of a single plasma or serum sample by both a BELISA and an indirect ELISA using a recombinant streptococcal protein G (PrG) peroxidase conjugate enabled an assessment of the proximity of a recent infection based on the failure of PrG to bind ovine IgM class antibodies. When BELISA and indirect ELISA results were expressed as a ratio, values indicative of recent infection (> or = 5) were observed for an average duration of 16.5 days (range 8 to 23 days) following the initial detection of antibody by BELISA. This approach has potential to improve diagnosis of a wide range of virus infections by providing an indicator for the relationship of serological status with a recent infection. However, where reinfection may occur, as with bluetongue virus, alternative methods may be required for definitive diagnosis.