Using rat liver mitochondria we determined that the primary biochemical target for inhibition of mitochondrial bioenergetic function by rhodamine 123 (Rh123) was FoF1-ATPase and that the amount of Rh123 associated with mitochondria is proportional to the mitochondrial membrane potential. Inhibition of coupled respiration by Rh123 in mitochondria isolated from CX-1, a Rh123-sensitive carcinoma cell type, and CV-1, a Rh123-insensitive normal epithelial cell type, was linearly related to the amount of Rh123 added (µg/mg protein) with CX-1 mitochondria exhibiting 2-fold greater inhibition compared to CV-1 mitochondria at any given amount of dye. The inhibition pattern for mitochondria isolated from MIP101, a Rh123-insensitive carcinoma cell type, was nonlinear, exhibiting greater sensitivity than CV-1 mitochondria at very low amounts of Rh123 but becoming less sensitive than either CV-1 or CX-1 at higher amounts. Rh123 inhibited FoF1-ATPase activity to a similar extent and in a concentration-dependent manner in both CV-1 and CX-1 mitochondria, but a different and complex pattern of inhibition was apparent for MIP101 mitochondria. Moreover, mitochondria from the 2 carcinoma cell types, CX-1 and MIP101, had higher membrane potentials (163 ± 7 and 158 ± 8 mV, respectively) than did mitochondria from the normal epithelial cell type, CV-1 (104 ± 9 mV). It was concluded that differences in both mitochondrial membrane potential and sensitivity of FoF1-ATPase contribute to the selective cytotoxicity exhibited by Rh123 for certain cell types in vitro.

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