This method allows large-scale screening of conditions in
96- or 384-multiwell plates. Nevertheless, some caution is needed
because LDH activity tends to decrease with time due to instability
and proteolytic degradation, and it can be affected by several variables,
including pH and the presence of specific components in the
culture medium, such as pyruvate [29]. LDH activity in the culture
medium can be measured by the CytoTox 96 Assay (Promega),
which is based on a two-step reaction. In the first step, LDH catalyzes
the coupled redox conversion of NAD+ to NADH and H+ and
lactate to pyruvate. In the second step, diaphorase uses the newly-
formed NADH and H+ to catalyze the reduction of a tetrazolium
salt [2-p-(iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium
chloride, INT] to highly-colored formazan, which absorbs at 490–
520 nm. The amount of formazan formed is proportional to the
amount of LDH released into the culture medium. The major
advantage of measuring LDH release is that it is an endpoint assay