November 15, 2005 (Vol. 25, No. 20)
Advances in Prostaglandin & Leukotriene Analysis
Immunoassay technology has advanced considerably since the era when all assays were performed in glass test tubes with dangerous radioactive tracers over a period of several days. The microtiter plate is ubiquitous, and its standard 96-well version is now routinely subdivided into over 1,500 assays that can fit in the palm of your hand.
Enzymatic and fluorescent tracers make it possible to perform assay work quickly and in a relatively uncontrolled environment. The solid-phase format of immunoassays can often be replaced with no-wash, homogenous assays utilizing time-resolved fluorescence (TRF), fluorescence resonance energy transfer (FRET, TR-FRET), and fluorescence polarization (FP)-based methods.
A recent assay innovation is the multiplexed assay, pioneered by Luminex (www.luminexcorp. com) as well as other vendors. Multiplexed assays allow two, three, four, or even a dozen assays to be run simultaneously on one single sample of as little as 100 microliters.
Even so, nearly all of the commercial multiplexed assays are two-site, immunometric sandwich assays that only work with large analytes. (For oligopeptides, about 2830 amino acid residues is a critical minimum size.) So while this has worked well for large substances like interleukins, small molecule analytes like steroids, prostaglandins, and leukotrienes have not been developed to the same extent.
This article briefly describes Cayman Chemical’s (www.caymanchemical.com) development of assays for prostaglandins (PGE2, PGD2), thromboxanes, (TXB2), and leukotrienes (CTC4 and LTB4) utilizing Luminex, Meso Scale Discovery (www.meso-scale.com), and fluorescence polarization
Fluorescence polarization (FP) assays are homogeneous, single-step assays suited for high throughput screening of large numbers of samples. All FP assays employ a large molecular species, or binding partner (BP), in conjunction with a small, low molecular weight fluorophore-labeled analyte (FA).
When the large BP molecule is an antibody, the assay is referred to as a fluorescence polarization immunoassay (FPIA). Fluorescence polarization is typically assessed by measuring fluorescence emission though a polarized filter in both parallel and perpendicular orientations compared to the polarized excitation filter.
FPIAs are based on the competition of FA with free (i.e., unlabeled) analyte in the samples or standards for the high-affinity binding site of an antibody. A microplate well filled with the FA:antibody complex will give a high FP reading (measured in millipolarization units or mP) due to the slow rotation of the larger complex compared to that of a smaller molecule.
As competition with the free analyte happens, some of the FA will be released from the antibody, and will resume its intrinsic, rapid rate of rotation. This will cause a detectable loss of FP in the well.
The addition of increasing amounts of analyte will result in increased reduction in mP. Plotting mP versus analyte concentration allows the construction of a standard curve with a broad dynamic range. This is similar to, but not strictly analagous to, the sigmoidal dose-response curve in a traditional solid-phase EIA.
Figure 1 depicts the standard curve and Z’-factor determination for Cayman Chemical’s recently developed FPIA for PGD2. Duplex measurement of PGE2 and PGD2 can be accomplished when fluorescein or rhodamine tracers are utilized for separate assays.
Meso Scale Discovery
The Meso Scale Discovery MSD assay platform utilizes Ruthenium (II) tris-bipyridine-(4-methylsulfone) [Ru(bpy)3] that, once conjugated to the analyte, serves as the tracer in competitive assays. The Ru(bpy)3-based tag undergoes a rapid redox reaction that emits light in the presence of an applied voltage. Only label bound in proximity of the electrode is detected (Figure 3). Assays for several eicosanoids have been adapted to the MSD platform.
Figure 4 shows the standard curves for both PGE2 and TxB2. The standard curves are characterized by low variability and a dynamic range of approximately 3 logs.
Luminex xMAP technology is based on microsphere beads that have been color-coded to create 100 distinct sets. Each microsphere set carries a different assay similar to the wells on an ELISA plate or different spots on a microarray. The xMAP microspheres are readily suspendable in liquid and exhibit favorable kinetics during an assay. This ability to suspend the microspheres in liquid combined with unique spectral addressing (i.e., color-coding) allows assays to be multiplexed.
For application to the measurement of eicosanoids in a competitive format, microspheres are coated with antibodies against a specific analyte of interest. Individual analytes are conjugated to phycoerytherin and serve as tracers in the assay. The tracer and analyte (whether in a sample or standard) compete for a limited number of antibody binding sites on the bead within each well of the plate. After sufficient equilibration time, typically two hours, the assays are read using the Luminex analyzer.
Once inside the instrument, fluidics cause the microspheres to line up single file as they pass by two lasers. A red laser classifies each color-coded microsphere, determining which assay is carried on that particle, and the green laser measures the amount of bound tracer on its surface. Since thousands of each microsphere set are used for each separate assay, many readings are made of each individual assay, yielding robust data. In a competitive assay, the amount of bound tracer is inversely proportional to the concentration of analyte in the sample. Figures 5 and 6, respectively, show a simplified picture and data from a duplex assay of PGE2 and LTC4.
In summary, FPIA is a rapid, high-throughput system ideally suited for screening libraries. MSD is a more sensitive platform that enables multiplexing as well as rapid, homogenous screening. The Luminex platform has versatile multiplex capabilities, is quite sensitive, but is not as rapid as the other platforms.
The standard 96-well EIA kits developed decades ago by Cayman Chemical for lipid signaling molecules will always be the alternative simple and cost-effective option to advanced platforms.