ADENOSINE TRANSPORT BY ISOLATED RAT LIVER MITOCHONDRIA: 159

Abstract

Rat liver mitochondria were incubated with ³H-adenosine (ado) and then separated from the medium and quenched by centrifugation through silicone oil into perchloric acid. The radioactive compounds in the mitochondria were analyzed by thin-layer chromatography. In the presence of 2 μM ado, 6 mM pyruvate and 2 mM malate, ado was incorporated into mitochondrial nucleotides at a rate of 0.79 pmol/mg mitochondrial protein/min. 23 μM Carboxyatractyloside, 1 mM ADP, 5 mM ATP, and 0.1 mM P¹,P⁵-di(adenosine-5')-pentaphosphate decreased the incorporation by 78%, 37%, 65%, and 67%, respectively. Conversion of adenosine to AMP + ADP + ATP in the incubation medium (analyzed without separation of mitochondria from the medium) occurred at 3.8 pmol/mg protein/min. Addition of 5 mM ATP increased this to 4.8 pmol/mg protein/min in spite of the decreased incorporation seen in the mitochondria, suggesting that externally added ATP stimulates the phosphorylation of adenosine and release of nucleotides into the medium. The amount of adenosine associated with the mitochondria was constant from 0 to 10 min of incubation under all experimental conditions. It was higher than predicted from sucrose-accessible and mitochondrial matrix volumes determined under identical conditions, suggesting binding of adenosine to mitochondrial proteins. Our results suggest that adenosine is converted to ADP and ATP in the intermembrane space and that ADP or ATP or both are then transported into the matrix by the ADP/ ATP translocase located in the inner mitochondrial membrane.

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