Purification and characterization of myo-inositol hexaphosphate-adenosine diphosphate phosphotransferase from Phaseolus aureus

myo-Inositol hexaphosphate adenosine diphosphate phosphotransferase transfers phosphate from myo-inositol hexaphosphate to adenosine diphosphate to synthesize adenosine triphosphate. This enzyme has been isolated and purified from ungerminated mungbean seeds and found to be different from guanosine diphosphate phosphotransferase. A purification of about 200-fold with 15% recovery has been obtained. The optimal pH of the reaction is 7.0 and is dependent on the presence of a divalent cation, i.e., Mg<SUP>2+</SUP> and Mn<SUP>2+</SUP>. The K<SUB>m</SUB> value for myo-inositol hexaphosphate has been found to be 0.41 &#215; 10<SUP>-4</SUP><SUB>M</SUB> and V is 90.0 nmol of P<SUB>i</SUB> transferred per milligram of protein per 20 min. K<SUB>m</SUB> for ADP is 0.88 &#215; 10<SUP>-4</SUP> <SUB>M</SUB> and V is 83.3 nmol of phosphorus transferred to ADP per milligram of protein per 20 min. The ADP phosphotransferase reaction is reversible to the extent of about 50% of the forward reaction. dADP is partly effective as an acceptor but other ribonucleoside mono- and diphosphates cannot substitute for ADP. The products ATP and myo-inositol pentaphosphate have been confirmed by several criteria. It has also been shown that this enzyme transfers phosphate only from a specific phosphoryl group (C-2 position) of myo-inositol hexaphosphate for the synthesis of ATP and 1,3,4,5,6-myo-inositol pentaphosphate or pentakis (dihydrogen phosphate)