Kinetics of ADP, ATP transport in mitochondria as studied by the quench-flow method

The kinetics in the millisecond range of ADP, ATP counterexchange in rat liver mitochondria were investigated using a quench-flow apparatus. The exchange was stopped with atractylate and the mitochondria were separated by centrifugation. 1. After correcting for leakage due to flow stress, apparent biphasic exchange kinetics were observed, with a more rapid phase within 100--200 ms, which extends to 10% of the total exchange. In general, the extent of the rapid phase increased when the translocation rate was changed under various conditions, in agreement with the model of the quench mechanism by atractylate. 2. The nature of the "rapid" phase was analyzed in the "steady" and "transient" state of the translocation and was shown to be caused by a delayed binding of atractylate due to competition with ADP. This quench delay results in a residual exchange which could explain the rapid part of the kinetics. 3. Deenergization of mitochondria by valinomycin or by uncoupler largely abolishes the rapid kinetic phase. This is explained by an increased availability of carrier sites at the outer face of the membrane to atractylate in the deenergized state, resulting in a more rapid quench. 4. The interpretation of the rapid exchange phase as a function of carrier sites accessible to atractylate quenching at the outer membrane face was simulated by a computer program based on the reorientating carrier model. With a set of rate constants, an approximate fit for the extent of quench delay with the experimental data is obtained.