Adenine Nucleotide Translocation of Mitochondria: Kinetics of the Adenine Nucleotide Exchange

Abstract

The kinetics of the adenine nucleotide exchange in liver mitochondria have been measured by rapid sampling techniques and analysed according to a simple exchange model. The exchange was found to follow accurately a first order reaction if related only to the portion of endogenous ADP and ATP, i. e. the endogenous AMP content is subtracted from the total adenine nucleotide pool. The first order reaction rate increases inversely to the size of the endogenous ADP + ATP pool. The translocation activity can then be obtained in μmoles × min−1× g protein−1 by multiplying the first order rate constant with the size of the ADP + ATP pool. The translocation activity with external AMP was also found to follow first order kinetics. The activity for AMP was 0.3 μmoles × min−1× g protein−1 as compared to 25 μmoles × min−1× g protein−1 for ADP and 10 μmoles × min−1× g protein−1 for ATP at 6°. The kinetics of the exchange activity with ATP remained first order under the various conditions of the energy transfer system which regulate the activity of the ATP exchange. The regulation of the ATP exchange in the energy‐rich and the energy‐depleted state was evaluated in terms of translocation activities. The temperature dependence of the translocation activity was measured for ADP and ATP in the range between 0 and 20°. In the Arrhenius plot the dependence on the temperature showed different slopes above and below 8° for both ADP and ATP. In the lower range EA for ADP was 34 kcal and above 8° 21 kcal, while the EA for ATP was 35 kcal in the lower range and 23 kcal above 8°. The concentration dependence of the adenine nucleotide exchange, as measured at low concentrations of mitochondria, gave an approximately linear relation up to 10 to 25 μM ADP and ATP in the “Eadie”‐plot. In this range Km ADP = 1.3 and KmATP = 2.5 μM; with Mg++KmADP = 4 and KmATP = 12 μM was obtained. Copyright © 1969, Wiley Blackwell. All rights reserved