ADENOSINE-AMP EXCHANGE ACTIVITY IS AN INTEGRAL-PART OF THE MAMMALIAN ADENOSINE KINASE

Purified adenosine kinase (AK) from Syrian hamster and bovine liver wa s examined for the presence of adenosine (Ad)-AMP exchange activity. T he enzyme from both sources, in addition to catalyzing the conventiona l ATP-dependent phosphorylation of adenosine, supported an Ad-AMP exch ange reaction that required ADP. Under optimal conditions both these r eactions were found to occur at comparable rates. Several observations strongly indicate that the Ad-AMP exchange activity is an integral pa rt of AK and it is likely associated with its catalytic mechanism. The se observations include: (i) Both AK and Ad-AMP exchange activities sh ow a nearly complete dependence upon the presence of pentavaient ions such as phosphate, arsenate or vanadate for catalysis; (ii) Both activ ities show similar heat-lability and inhibition by 5-iodotubercidin (5 -ITu); (iii) In a Chinese hamster cell mutant resistant to adenosine a nalogs that lacked AK activity, the Ad-AMP activity was also found to be absent. The presence of a phosphoryl-enzyme intermediate, or any ex change between free P-32(i) and any of the reactants, however, was not detected under the reaction conditions. Some implications of these ob servations regarding the catalytic mechanism of AK are discussed.