ADP and ATP transport in mitochondria evidence for a metal-ion involved in transport catalysis by use of metallochromic indicators

Abstract

This study introduces a new class of active-site directed probes with respect to ADP and ATP transport catalysis in rat liver mitochondria. The anionic monoazo dyes, e.g., p-(2- hydroxy-1- naphthylazo) naphtholsulfonic acid , are competitive inhibitors of carrier-mediated ADP uptake ( K i 20–30 μM). The azo dyes also can displace the same amount of carrier-specific bound ADP as does carboxyatractyloside. Two essential substructures could be derived from a structure-activity study. Firstly, a sulfonic acid group in the para position relative to the azo bridge which becomes neutralized upon binding by a specifically located positive charge of the carrier protein. This electrostatic binding component, which presumably is represented by a strategic arginyl residue, seems to be essential for substrate binding as well as inhibitor binding. The second structural requirement for effective inhibition was found to be the o- hydroxy or o,o′- dihydroxyazo system, which is known to form stable complexes with metal ions by chelation. Experiments on prevention and reversal of dye-mediated inhibition revealed that the metal-chelating properties are responsible for the effects observed. In addition, using bovine serum albumin or the synthetic polymer Kollidone, inhibition could be prevented as well as abolished. It is postulated that a metal ion, possibly Mg 2+ , which is bound to the carrier protein plays an essential role for transport catalysis. The metal ion is assumed to form a functional ternary complex, i.e., a metal bridge complex between the carrier protein and its substrate.