Suramin Inhibits Mitochondrial ADP/ATP Carrier, Not Only from the Cytosolic Side But Also from the Matrix Side, of the Mitochondrial Inner Membrane

Abstract

Suramin was earlier reported to show inhibitory effects on the mitochondrial ADP/ATP carrier. However, two important questions, i) whether it shows a specific inhibition of the ADP/ATP carrier when applied to isolated mitochondria, and ii) whether it inhibits the mitochondrial ADP/ATP carrier only from the cytosolic side or from the matrix side, as has been observed with its canonical inhibitors of carboxyatractyloside or bongkrekic acid, remain to be answered. In the present study, we sought exact answers to these questions. As for the first question, suramin showed certain inhibitory effects on the mitochondrial respiratory chain; and at a concentration of 25 μM it showed strong inhibition of the mitochondrial ADP/ATP carrier. This property was due to its weaker inhibitory effects on the mitochondrial ADP/ATP carrier than those of carboxyatractyloside or bongkrekic acid. As for the second question, suramin inhibited the ADP/ATP carrier from both sides of the mitochondrial inner membrane. Thus, suramin was concluded to be utilizable as a new type of inhibitor for the ADP/ATP carrier; but we must pay attention to its side-effects, especially when it is applied to whole mitochondria.

KH‐17, a simplified derivative of bongkrekic acid, weakly inhibits the mitochondrial ADP/ATP carrier from both sides of the inner mitochondrial membrane

“…These features may be attributable to their selective affinities for individual conformations of the ADP/ATP carrier: a high affinity for one side of the carrier may make an inhibitor ineffective from the other side. This interpretation is reasonable, because suramin was found to weakly inhibit the ADP/ATP carrier from both sides of the inner mitochondrial membrane (Fujiwara et al, 2021).…”

“…Mitochondria were prepared from liver of male Wistar rats or from fresh bovine heart, as described previously (Fujiwara et al, 2021). When mitochondria were prepared from rat liver, after determination of their protein concentration by the Biuret method using bovine serum albumin solutions as standards, they were promptly used for the [ 3 H]ADP uptake experiments.…”

“…When mitochondria were prepared from bovine heart, after determination of their protein concentration, they were stored at −80°C until use. Submitochondrial particles were prepared from bovine heart mitochondria, as described previously (Fujiwara et al, 2021). Briefly, bovine heart mitochondria that had been stored at −80°C were thawed and suspended at 15 mg protein/ml in medium consisting of 250 mM sucrose, 5 mM ATP, 15 mM MgCl 2 and 3 mM Tris–HCl buffer, pH 7.4.…”

“…These inhibitors specifically block the ADP/ATP carrier from either the cytosolic side or the matrix side, respectively. To understand how the ADP/ATP carrier interacts with its inhibitors or substrates, several strategies have been used: (i) purification and crystallization of the carrier complexed with its transport substrates or with its inhibitors, followed by X‐ray crystallography and structural characterization (Nury et al, 2005; Pebay‐Peyroula et al, 2003; Ruprecht et al, 2014, 2019); (ii) analyses of the structure and function of the ADP/ATP carrier using mutants or revertants (Babot et al, 2012; Zeman et al, 2003); and (iii) analyses of the structure and function of the ADP/ATP carrier using a chemically modified transport substrate or inhibitor (Fujita et al, 2018, 2021; Jaiquel et al, 2021; Todisco et al, 2016; Yamamoto et al, 2015).…”