The inhibition by bromothymol blue of anion translocation across the mitochondrial membrane.

1. In rat liver mitochondria bromothymol blue inhibited the exchange of [14C]succinate for succinate, malonate, L-malate and inorganic phosphate; the [14C]citrate/citrate and [14C]citrate/malate exchange reactions and the phosphate/hydroxyl exchange were also inhibited by this dye. The inhibition of the rate of succinate, citrate and phosphate uptake by bromothymol blue is found to be competitive. 2. The degree of inhibition by bromothymol blue of the ]14C]succinate/malonate exchange reaction was pH dependent. It has been shown that the inhibition increased linearly while the pH was increased from 6.0 to 8.2. However, the binding rate of bromothymol blue to the mitochondria decreased with the rising pH of the medium. It is concluded that the binding of acidic bromothymol blue was not essential for the inhibitory effect. 3. Other sulfonephthalein derivatives also inhibited [14C]succinate/malonate exchange reaction. At pH 7.2 the relative order of the strength of the inhibitory action of the sulfonephthalein compounds tested was: thymol blue greater than bronocresol green greater than bromothymol blue greater than phenol red greater than bromocresol purple. The results do not indicate any correlation between the pK values of pH values of pH indicators and their extents of inhibition. 4. It is suggested that the negatively charged bromothymol blue interacts with the positively charged centers of the anion carrier systems causing inhibition of membrane permeability for anions.