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ELISA part2. Detection systems. For all enzyme-linked immunoassays, the final stage is the addition of the enzyme substrate. The substrate is chosen for its quantitative yield of a colored, fluorescent or luminescent reaction product.
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Detection systems • For all enzyme-linked immunoassays, the final stage is the addition of the enzyme substrate. The substrate is chosen for its quantitative yield of a colored, fluorescent or luminescent reaction product. • The decision as to which substrate is the best for any type of assay depends on the sensitivity desired, the timing requirements, and the detection device to be used. • Types of detection systems: • Colorimetric Assays • Fluorescent Assays • Luminescent Assays
ELISA assay variations • Colorimetric assays result in a colored reaction product that absorbs light in the visible range. • Fluorescent immunoassays (ELFIA) are simply a variation of colorimetric ELISA. An enzyme converts a substrate to a reaction product that fluoresces when excited by light of a particular wavelength. • In comparison to the colorimetric ELISA, fluorescent immunoassays are only slightly more sensitive.
Luminescent immunoassays, like fluorescent immunoassays, are variations of the standard ELISA. An enzyme converts a substrate to a reaction product that emits photons of light instead of developing a visible color. • It is believed that luminescence is the most sensitive detection method currently in use due to the ability of signal multiplication and amplification
Microplate Reader • The microplate reader also known as “Photometric micro-plate reader or ELISA reader” is a specialized spectrophotometer designed to read results of the ELISA test • This instrument works on the principle of "Photoelectric Colorimetry" • Unlike the conventional spectrophotometer which facilitates readings on a wide range of wavelengths, the microplate reader has filters or diffraction gratings that limit the wavelength range to that used in ELISA, generally between 400 to 750 nm (nanometres). • Some readers operate in the UV range and carry out analyses between 340 to 700 nm.
The optical system uses optic fibers to supply light to the microplate wells containing the samples. • A detection system (sensors) detects the light coming from the sample, amplifies the signal and determines the sample’s absorbance. • The location of the optical sensors of the microplate reader varies depending on the manufacturers: these can be located above the sample plate, or directly underneath the plate’s wells.
Structure of antibody-coated microplate and treatment • 12 strips of 8 wells connected each other are set in a microplate frame. • The surface of the plate is covered with a seal to avoid dryness. Remove the seal after the plate gets to the room temperature. • Remove the seal just before starting assay. • If you want to use only a part of the wells, cut the part of the seal, and take out the strips and transfer them to another frame, and remove the seal. • store the rest of the wells at 2-8C.
Washing • The first washing after the first reaction with standards or samples: Shake off the reaction mixture from the plate onto a sink. • Then add washing solution using a multi-delivery pipette set at 250µl to avoid carry-over caused by flowing out the buffer to other wells. Then shake off the buffer. • Add new washing buffer from the washing bottle with nozzle, and shake off. • Repeat filling and shaking off as many times as indicated.
Washing solutions are usually concentrated and need to be diluted. 20X : (1:20)= 1 part of washing concentrate with 19 part of D.W, using volumes equivalent to number of wells to be washed. • For 8 wells, you require (8*350ul = approx. 3ml) • Take 3.8ml of DW with 200 ul of washing concentrate. (total= 4ml).
After final shaking off the buffer from the plate, strike the plate on to some sheets of paper towel for several times to remove the residual buffer • Automatic plate washers are also commercially available
Shaking of the well-plate for mixing • After addition of a reagent solution, the microplate should be shaken to mix the solutions. A short shaking is enough because of the small volume of the reaction mixture. microplate mixer (microplate shaker)
Careful addition of reaction stopping solution • As the reaction stopper is strongly acidic, please, be careful in treatment of the solution. • After addition of the stopping solution the color of the reaction mixture in the well will change from blue to orange-yellow.
Measurement of absorption using a plate reader • After stopping the enzyme action, absorbance is read using a densitometer for microplates (microplate reader). • If possible measure absorbance at both 450nm and 620nm, and calculate the difference between absorbance at 450nm and 620nm. • By doing this, we can compensate influences of some non-specific factors of wells like small scar on the bottom of well, and the variation in the structure of wells, e.g., thickness of the bottom. • The difference is expressed as Abs.450(∆620)nm.
Preparation of standard curve and calculation of assay values • In manual calculation, prepare a standard curve using bi-logarithmic section paper by plotting Abs.450(∆620)nm on Y-axis against standard concentration (ng/ml) on X-axis. • Because the coefficients of variation (CV) in absorbance do not change so much throughout all the assay range, it is proper to choose logarithmic expression in both concentration and absorbance. This expression allows easy reading of lower concentration area.
Read the concentrations of the substance to measure in samples from their Abs.450(∆620)nm, and multiply the assay value by sample dilution rate if samples have been diluted.
Important points in performing ELISA and improvement of assay performance • Sampling and treatments of samples • Serum or plasma? • serum samples is most recommended • In getting plasma: • heparin is most often used as an anticoagulant • Use of fluoride must be avoided because fluoride ion is a potent inhibitor of peroxidase. • An important phenomenon with frozen plasma is that an insoluble substance (fibrin) will be formed when thawed. • In this case, the sample must be vortexed and centrifuged, then the insoluble cluster flowing in the plasma should be taken out by a thin wire needle sharply bent at an end. If such fibrin remains in the sample, it may clog the tip of a pipette and influences assay variability.
Storage temperature and freezing-thawing • Sample storage temperature is better to be lower than -35C. • Ultra-low temperature such as -80C is recommended for a long-term storage. • A long storage in a home freezer is not recommended because the sample is frozen very slowly and the solute may separate from solvent, and is concentrated at to the bottom of the storage tube • Repeated freezing and thawing is also harmful to the protein, and may cause inactivation.
When samples are taken out from the freezer and thawed, never forget to vortex these samples because the solution after thawing is not homogeneous, and the bottom area contains more solute. • Stability of assay samples • In assay, the problem of sample stability, i.e. how long the substance to be measured can keep its immunoreactivity, in serum or plasma, is very important. Blood samples also contain enzymes to destroy peptides or proteins, and stability against those enzymes differs from substance to substance
Most Kits have their own stability ranges; most of them have the following ranges: • Stability at room temperature: 2 hours • Stability at refrigerator temperature: 24 hours • Stability against freezing-thawing: 3 times • Stability for long period: at –20℃ or-80℃
Influence of humidity and air stream As ELISA is carried out in small volume of reaction mixture in shallow wells, the ratio of surface area to the liquid volume is large. This means that evaporation of water from the surface during incubation cannot be ignored. During all the incubation process, the well-plate should be covered using the attached plate cover.
Pipettes • Selection of pipettes • Pipettes for samples and standard solutions are especially important because their precision directly influence the assay precision. When you add samples or standards at a volume of 5 or 10 µl, for example, use a pipette with the maximal setting volume of 5 or 10µl, and never use with the maximum setting volume of 100µl or more. In other words, use volume-matched pipette
Trouble shooting in ELISA 1st , Coloration
Advantages of ELISA • High sensitivity • High specificity • relatively cheap and require small amounts of reagents • rapid and can give both qualitative and quantitative results • Detection is easy: results can be detected visually or using special readers • Results are reproducible • Automated, high throughput and manual methods are available, • Very versatile • No problems with radiation or disposal of waste • Can use monoclonal or polyclonal antibodies.
Remember! • It has been found that growth hormone causes the liver (and, to a much less extent, other tissues) to form several small proteins called somatomedins that have the potent effect of increasing all aspects of bone growth. • Many of the somatomedin effects on growth are similar to the effects of insulin on growth. Therefore, the somatomedins are also called insulin-like growth factors (IGFs). At least four somatomedins have been isolated, but by far the most important of these is somatomedin C (also called IGF-1).
Somatomedin C (IGF-1) • The molecular weight of somatomedin C is about 7500, and its concentration in the plasma closely follows the rate of growth hormone secretion. • Some aspects of the somatomedin hypothesis are still questionable. One possibility is that growth hormone can cause the formation of enough somatomedin C in the local tissue to cause local growth. • It is also possible that growth hormone itself is directly responsible for increased growth in some tissues and that the somatomedin mechanism is an alternative means of increasing growth
Measurement of serum IGF-I concentration is a sensitive screening test for acromegaly. • IGF-1 measurement before and after growth hormone therapy. • IGF1 levels are dependent on the amount of growth hormones in the blood • Approximately 98% of IGF-1 is always bound to one of six binding proteins (IGF-BP). • IGFBP-3, the most abundant protein, accounts for 80% of all IGF binding. • Binding of these protein prolongs the half-life of the IGFs and alters their interaction with cell surface receptors.
Clinical Significance of IGFBP • They are used to diagnose and assess the degree of the human growth hormone (hGH)-deficient state, and to measure responsiveness and predict the outcome of hGH therapy of a patient. • They are also used in the differential diagnosis of growth disorders in order to elucidate the etiology of the disease. • In addition, IGF-1 and IGFBP-3, i.e. their ratio, can serve as safety biomarkers of hGH therapy. • The IGF-1/IGFBP-3 ratio has sometimes been used as an index of IGF bioavailability in the human circulation, but this ignores IGF-1 binding to other IGFBPs (so the ratio is affected by the concentrations of all six IGFBPs)
IGFBP-1 • The most important regulator of circulating IGFBP-1 is insulin. Fasting insulin and IGFBP-1 concentrations are inversely correlated. • In insulin dependent diabetes(IDDM), serum IGFBP-1 levels are elevated. In non-insulin dependent diabetes, in which insulin levels are high, serum IGFBP-1 is decreased. • Low levels of IGFBP-1 have also been observed in the following cases: Acromegaly, Cushing’s syndrome and polycystic ovarian syndrome (PCO).
The concentration of IGFBP-1 is correlated with glycemic conditions. • IGFBP-1 may be used for: • monitoring insulin responsiveness, • as a marker for insulin-producing tumors, • monitoring acute fluctuations in insulin action • and determining if poor glycemic control is due to inadequate insulin treatment or poor control of dietary intake.
