Evaluation of hepatic subcellular fractions for Alamar blue and MTT reductase activity

Alamar blue and MTT are indicators used to measure cytotoxicity of various chemicals in cultured cells. Both Alamar blue and MTT are reduced by mitochondrial enzymes. We observed enhanced fluorescence of Alamar blue when kidney epithelial cells were co-incubated with hepatic post-mitochondrial supernatant (S9) fractions as compared with cells incubated in the absence of S9 fractions. The present studies were carried out to determine whether hepatic cytosolic and/or microsomal enzymes were capable of metabolizing Alamar blue and/or MTT to their reduced products. Livers from female Sprague-Dawley rats were used to prepare S9 fraction, and mitochondrial, microsomal and cytosolic fractions. Fractions containing 1 or 5 mg protein/ml were incubated with Alamar blue or MTT for up to 4 h. Fluorescence (Alamar blue) or absorbance (MTT) were determined and expressed as differences between treated wells and controls. Hepatic fractions (S9, mitochondria, microsomes and cytosol) caused concentration- and time-dependent increases in Alamar blue fluorescence and MTT absorbance. Reduction of Alamar blue and MTT by hepatic S9 fraction was abolished by heating. Reduction of Alamar blue by hepatic mitochondria was approximately equivalent to that catalyzed by hepatic S9 fraction or cytosol. Reduction of MTT by hepatic mitochondria was approximately equivalent to that catalyzed by hepatic S9 fraction or microsomes. These data indicate that mitochondrial, cytosolic and microsomal enzymes reduce Alamar blue and MTT. Therefore, caution should be exercised in ascribing decreases in viability as due solely to mitochondrial damage when using either of these dyes.