Radioactivity immunoassay

Martin J, Brett G, Thompson W. Measurements of cortisol with a non-radioactive immunoassay based on enhanced luminescence. Clin Chem 1988 34 1217. 

Martin, J. K., Brett, G., Thompson, W., Summers, M., and Bacarese-Hamilton, T., Measurement of cortisol with a non-radioactive immunoassay based on enhanced luminescence. Clin. Chem. (Winston-Salem, N.C.) 34, 1217 (abstr.) (1988). 

These experiments indicate that the anti-DES antibody was highly discerning for the DES molecule and that the concentration of DES can be determined using a non-radioactive immunoassay. A unique application of this ELISA has been in the determination of the effects of DES on gametogenesis of the free-living nematode Caenorhabditis elegans. 

Pregnancy-specific j3i-glycoprotein has been measured using an enzyme-linked immunoassay, and a radioactive immunoassay system. 

In fact, most RIAs and many nonisotopic immunoassays use a competitive binding format (see Fig. 2). In this approach, the analyte in the sample to be measured competes with a known amount of added analyte that has been labeled with an indicator that binds to the immobilized antibody. After reaction, the free analyte—analyte-indicator solution is washed away from the soHd phase. The analyte-indicator on the soHd phase or remaining in the wash solution is then used to quantify the amount of analyte present in the sample as measured against a control assay using only an analyte-indicator. This is done by quantifying the analyte-indicator using the method appropriate for the assay, for example, enzyme activity, fluorescence, radioactivity, etc. 

ImmunO lSS iy. Chemiluminescence compounds (eg, acridinium esters and sulfonamides, isoluminol), luciferases (eg, firefly, marine bacterial, Benilla and Varela luciferase), photoproteins (eg, aequorin, Benilld), and components of bioluminescence reactions have been tested as replacements for radioactive labels in both competitive and sandwich-type immunoassays. Acridinium ester labels are used extensively in routine clinical immunoassay analysis designed to detect a wide range of hormones, cancer markers, specific antibodies, specific proteins, and therapeutic dmgs. An acridinium ester label produces a flash of light when it reacts with an alkaline solution of hydrogen peroxide. The detection limit for the label is 0.5 amol. 

There should be specific, saturable binding to the receptor, accompanied by pharmacological characteristics appropriate to the functional effects, demonstrable using a radioactive, eg, tritium or iodine-125, ligand to label the receptor. Radioligand binding assays (1,6) have become a significant means by which to identify and characterize receptors and enzymes (see Immunoassays Radioactive tracers). Isolation of the receptor or expression of the receptor in another cell, eg, an oocyte can be used to confirm the existence of a discrete entity. 

The biological and medical sciences are ripe for iastmmentation advances. Whereas most immunoassays (qv) use radioactive materials, the implementation of chemiluminescent methods, enzyme techniques, and electrochemical methods is expected to become more important. New and better noninvasive methods of iavestigation are expected to become more routine. In addition, real-time measurements, whereby analyses of a number of... 

See Immunoassay Medical diagnostic reagents Radioactive elements. 

Radioisotopes have become very important ia the practice of modem medicine, for both diagnosis and treatment. Some diagnoses are done by injecting a radionucHde ia a biochemical form such that it goes to a particular organ, and the measured radiation then allows the functional level of that organ to be determined. A common treatment is to expose a portion of the body, for example a tumor, to radiation from a radioisotope with the source either internal or external to the body. Another usage iavolves radioactively labeled antibodies (see Immunoassay). 

Several types of labels have been used in immunoassays, including radioactivity, enzymes, fluorescence, luminescence and phosphorescence. Each of these labels has advantages, but the most common label for clinical and environmental analysis is the use of enzymes and colorimetric substrates. 

Ishikawa, E., Imagawa, M., and Hashida, S. (1983b) Ultra sensitive enzyme immunoassay using fluoro-genic, luminogenic, radioactive and related substances and factors to limit the sensitivity. Proceedings of the 2nd International Symposium on Immunoenzymatic Technology. 

A prerequisite to pharmacokinetic/pharmacodynamic studies is the availability of a sufficiently selective and sensitive assay. The assay must be capable of detecting and accurately quantifying the therapeutic protein in the presence of a complex soup of contaminant molecules characteristic of tissue extracts/body fluids. As described in Chapter 7, specific proteins are usually detected and quantified either via immunoassay or bioassay. Additional analytical approaches occasionally used include liquid chromatography (e.g. HPLC) or the use of radioactively labelled protein. 

Antibody molecules have no inherent characteristic that facilitates their direct detection in immunoassays. A second important step in developing a successful immunoassay, therefore, involves the incorporation of a suitable marker . The marker serves to facilitate the rapid detection and quantification of antibody-antigen binding. Earlier immunoassay systems used radioactive labels as a marker (radioimmunoassay RIA) although immunoassay systems using enzymes (enzyme immunoassays EIA) subsequently have come to the fore. Yet additional immunoassay systems use alternative markers including fluorescent or chemiluminescent tags. 

Immunoassay is the sterile measurement of a drug molecule as an antigen using a specific antibody. Detection is performed by UV light absorption, radioactivity, or fluorescence polarization. 

In addition to these research applications of fluorescence, there is a continuing use of fluorescence detection to replace analytical methods based on radioactivity, as can be judged from the recent books and conferences on fluorescence sensing methods. (7 n) These emerging applications of fluorescence can be seen by the growth and introduction of improved methods for immunoassays, enzyme-linked immunoassays... 

Other immunoassays are based on the same antibody-antigen binding reaction but use a different labeling system for detection. Instead of an enzyme label, there are radioactive isotopes, and fluorescent and luminescent labels. Some important immunoassays are defined below ... 

RIA Radioimmuno assay uses a radioactive label on the antibody or antigen. These were among some of the first types of immunoassays. 

As indicator enzymes horseradish peroxidase (HRP or HRPO), alkaline phosphatase (AP), or /i-galactosidase, are favored, since they are relatively robust, have a high product-forming rate, are easy to purify, and are cheap. The most used colloids are from gold, silver, and iron, and iodine isotopes are mostly taken as radioactive labels in immunoassays. 

Because of the requirement to use radioactive substances, RIAs are frequently being replaced by other immunologic assays, such as ELISA and fluorescence polarization immunoassays (FPIA) (Niemann et al. 1985). These have similar degrees of sensitivity. FPIAs are highly quantitative, as are RIAs, and ELISAs can be designed to be quantitative. 

Rosalyn Yalow received the Nobel Prize in medicine in 1977 for developing immunoassay techniques in the 1950s, using proteins labeled with radioactive, 3,l to enable their detection.13 Yalow. a physicist, worked with Solomon Berson, a medical doctor, in this pioneering effort. 

Variations, which avoid the use of radioisotopes, are replacing RIA. Some utilize stable isotopes. However, 14C at such low levels that there is no radioactive waste can be coupled with accelerator mass spectrometry to provide very sensitive immunoassays.1 A great variety of other procedures are available. Some involve coupling to antibodies that carry fluorescent labels. Many are now automated. Often protein A from Staphylococcus aureus is utilized in various ways that take advantage of its ability to bind to the Fc portion of IgG from virtually all mammals. For example, it may fix antibodies to a surface or to a label)... 

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