1. The mechanism of adenine nucleotide translocation in mitochondria isolated from rat liver was further examined by using the local anaesthetics procaine, butacaine, nupercaine and tetracaine as perturbators of lipid–protein interactions. Each of these compounds inhibited translocation of ADP and of ATP; butacaine was the most effective with 50% inhibition occurring at 30μm for 200μm-ATP and at 10μm for 200μm-ADP. The degree of inhibition by butacaine of both adenine nucleotides was dependent on the concentration of adenine nucleotide present; with low concentrations of adenine nucleotide, low concentrations of butacaine-stimulated translocation, but at high concentrations (greater than 50μm) low concentrations of butacaine inhibited translocation. Butacaine increased the affinity of the translocase for ATP to a value which approached that of ADP. 2. Higher concentrations of nupercaine and of tetracaine were required to inhibit translocation of both nucleotides; 50% inhibition of ATP translocation occurred at concentrations of 0.5mm and 0.8mm of these compounds respectively. The pattern of inhibition of ADP translocation by nupercaine and tetracaine was more complex than that of ATP; at very low concentrations (less than 250μm) inhibition ensued, followed by a return to almost original rates at 1mm. At higher concentrations inhibition of ADP translocation resulted. 3. That portion of ATP translocation stimulated by Ca2+ was preferentially inhibited by each of the local anaesthetics tested. In contrast, inhibition by the anaesthetics of ADP translocation was prevented by low concentrations of Ca2+. 4. The data provide further support for our hypothesis that lipid–protein interactions are important determinants in the activity of the adenine nucleotide translocase in mitochondria.

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