Passive Transport of 5,5-Dimethyl-2,4-Oxazolidinedione into Beef Heart Mitochondria

5,5-Dimethyl-2,4-oxazolidinedione is used to estimate intracellular pH. The mechanism of transport of this compound into beef heart mitochondria was investigated to assess the validity of its use for the measurement of intramitochondrial pH. This compound and (14)C-labeled carboxy dextran were simultaneously used to determine the intra- and extramitochondrial compartments and the distribution of the compound, which was passively transported into resting, respiring, and simultaneously respiring and phosphorylating mitochondria. The transport of the compound was neither associated with electron transport nor with oxidative phosphorylation.