Uptake of adenosine triphosphate by isolated adrenal chromaffin granules: A carrier-mediated transport

Partly purified chromaffin granules from bovine adrenal medulla were incubated in vitro with [³H]adenosine triphosphate (ATP). After several washes the particles were subjected to density gradient centrifugation. It was demonstrated that chromaffin granules had taken up radioactive material. Thin-layer chromatography revealed that more than 60% of the radioactivity was confined to [³H]ATP. Incubation experiments with a mixture of [³H]ATP and [³²P]ATP indicated that ATP was taken up by chromaffin granules as an intact molecule. The uptake of [³H]ATP was temperature dependent and linear up to 10 min. The rate of uptake depended upon the ATP concentration and levelled off at ATP concentrations above 2 mM. At this ATP concentration the rate was 0.268 nmol ATP/mg protein/min. The uptake of ATP was compared with that of catecholamines. Both processes were activated by Mg²⁺, but inhibited by N-ethylmaleimide and an uncoupler of oxidative phosphorylation. Atractyloside inhibited only the ATP-uptake, whereas reserpine only interfered with that of the catecholamines.It is suggested that this uptake mechanism for ATP is carrier-mediated and that it enables newly formed chromaffin granules to accumulate and to maintain a high nucleotide concentration, which is required for the formation of a storage complex with catecholamines.