The succinate dehydrogenase inhibition (SDI) test and the adenosine triphosphate (ATP) assay, are both used for in vitro human tumor chemosensitivity testing. We exposed HeLa cells to various concentrations of mitomycin C for 1, 2 or 3 days and found that the decrease in number of viable cells correlated with that of succinate dehydrogenase (EC 22.214.171.124) activity and that of intracellular ATP level of the viable cells. In the dead cells, the ATP level was extensively decreased, but the succinate dehydrogenase activity remained at a level of 24% of that of mitomycin C-untreated viable cells, even on day 3. Thus, the ATP level better reflected the cell viability. In clinical situations, the succinate dehydrogenase activity and the ATP level are assayed in whole cells following exposure to anticancer drugs, therefore the activity remaining the dead cells must be taken into consideration for the chemosensitive prediction with the SDI test, but not with the ATP assay. This higher sensitivity of the ATP assay will enable a more accurate prediction of cell viability.
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