Purification and Characterization of the Reconstitutively Active Adenine Nucleotide Carrier from Mitochondria of Jerusalem Artichoke (Helianthus Tuberosus L.) Tubers

Abstract

The adenine nucleotide carrier from Jerusalem artichoke (Helianthus Tuberosus L.) tubers mitochondria was solubilized with Triton X-100 and purified by sequential chromatography on hydroxapatite and Matrex Gel Blue B in the presence of cardiolipin and asolectin. SDS gel electrophoresis of the purified fraction showed a single polypeptide band with an apparent molecular mass of 33 kDa. When reconstituted in liposomes, the adenine nucleotide carrier catalyzed a pyridoxal 5′-phosphate-sensitive ATP/ATP exchange. It was purified 75-fold with a recovery of 15% and a protein yield of 0.18% with respect to the mitochondrial extract. Among the various substrates and inhibitors tested, the reconstituted protein transported only ATP, ADP, and GTP and was inhibited by bongkrekate, phenylisothiocyanate, pyridoxal 5′-phosphate, mersalyl and p-hydroxymercuribenzoate (but not N-ethylmaleimide). Atractyloside and carboxyatractyloside (at concentrations normally inhibitory in animal and plant mitochondria) were without effect in Jerusalem artichoke tubers mitochondria. V _max of the reconstituted ATP/ATP exchange was determined to be 0.53 μmol/min per mg protein at 25°C. The half-saturation constant K _m and the corresponding inhibition constant K _i were 20.4 μM for ATP and 45 μM for ADP. The activation energy of the ATP/ATP exchange was 28 KJ/mol between 5 and 30°C. The N-terminal amino acid partial sequence of the purified protein showed a partial homology with the ANT protein purified from mitochondria of maize shoots.