Effect of the divalent cation ionophore A23187 on the translocation of adenine nucleotides in liver mitochondria

Abstract

20-30 ng atom magnesium [l-3], about half of which is located in the matrix compartment 123. They also contain about 10 nmol adenine nucleotides per mg protein [4], most of it in the matrix. Although the form of magnesium and adenine nucleotides present in the mitochondrial matrix is not known precisely, it seems likely that most of the nucleotides are complexed by Mg. On the other hand, it has been suggested [5-71 that free ADP and ATP, and not their magnesium complexes, are transported across the mitochondrial membrane by the specific adenine nucleotide translocase [8]. Experimental evidence is a decrease of the translocation rate by increasing external Mg2+ concentration [7]. Thus, a very low intramitochondrial concentration of free, non- complexed, ADP and ATP might be one of the factors controlling the rate of the translocation. One of the intriguing features of the translocase is its preference for external ADP over external ATP [5,9]. An explanation for this can be provided by assuming that the exchange of ATP against ADP is mostly electrogenic [5,10,1 l] and is therefore driven in one direction only by the mitochondrial trans- membrane potential which is negative inside and positive outside. An alternative explanation assumes