XTT Assay: A Colorimetric Assay to Assess Cell Viability and Mitochondrial Activity

- Low-frequency ultrasound waves lead to the selective lysis of human leukemia cells and the XTT assay helps determine the degree of this damage. The assay works on the principle that the mitochondrial enzymes of healthy cells convert a yellow XTT dye into an orange compound called formazan. To begin the assay, treat the cells with methyl beta-cyclodextrin, a cholesterol-depleting agent, and subsequently sonicate the cells at low power to induce cell damage.

Centrifuge to pelletize the cells and add fresh cell growth media to the pellet. Seed the cells at the desired concentration into a multi-well microplate and incubate it overnight. Now, add the activated XTT solution to the cells and incubate until the orange color develops in the microplate's wells. Place the plate in a spectrophotometer. Measure the absorbance at 450 to 500 nanometers and then at 630 to 690 nanometers. Subtract the two values to get precise results. The higher value of absorbance in the XTT assay indicates the higher activity of mitochondrial enzymes and higher cell viability.

In the following video, we will demonstrate the use of XTT assay for determining the efficacy of low-frequency ultrasound waves to cause cell damage in methyl beta cyclodextrin treated leukemia cells.

- To assess the viability of sonicated cells by XTT assay, sonicate the cells using a range of one to three one-second pulses, based one second apart, to develop a range of damage for the XTT kit to assess. Then spin down the cells and re-suspend the pellets to 1 times 10 to the fifth cells per milliliter in freshly prepared growth medium. Next, seed 100 microliters of cells per well into individual flat bottom 96 well microtiter plates in triplicate, including three control wells containing 100 microliters of complete growth medium alone for blank absorbance readings. Incubate the cells for 24 hours.

On the morning of the assay, gently swirl aliquots of 37 degrees celsius-warmed XTT and activation reagents until the solutions are clear. Then add 0.1 milliliters of the activation reagent to a 5 milliliter aliquot of the XTT reagent. Next, add 50 microliters of the activated XTT solution to each well of the U937 and THP1 inoculated plates and return the cells to the cell culture incubator. After two hours, shake the plates gently to evenly distribute the orange dye in the positive wells and measure the absorbance on a microtiter plate reader between 450 and 500 nanometers. Finally, measure the absorbance of all the assay wells again between 630 and 690 nanometers.