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Adenosine kinase from bovine adrenal medulla was purified 1600-fold by using ammonium sulfate precipitation, gel filtration and affinity chromatography. Gel filtration yielded a relative molecular mass around 42 000 and Michaelis constants were 0.2 pM for adenosine and 20 pM for MgATP. The enzyme showed a broad specificity for purine nucleoside triphosphate as phosphate donors. Both free MgZ + and ATP were inhibitors. AMP was a competitive inhibitor with regard to adenosine and a non-competitive inhibitor versus MgATP, while ADP was a uncompetitive inhibitor with regard to adenosine and a non-competitive inhibitor versus MgATP.Adenosine kinase was strongly inhibited by the bis(adenyly1) polyphosphates Ap,A and Ap,A. These compounds inhibited the enzyme competitively versus MgATP (Ki = 0.06 pM for Ap4A and 0.4 pM for Ap,A) and uncompetitively with regard to adenosine. The results of the kinetic analysis suggest an ordered bi-bi mechanism, adenosine being the first substrate. The phosphorylation of adenosine was unaffected in the presence of vanadate ions.The catalysis of adenosine phosphorylation to AMP by adenosine kinase was first described in yeast [l] but its presence has been firmly established in a great variety of mammaiian cells [2 -51. The use of affinity Chromatography on AMP linked to agarose or Sepharose beads allowed highly purified mammalian adenosine kinase preparations, including human, to be obtained [6 -Adenosine kinase plays a key role in the regulation of intracellular levels of adenosine [12]. This enzyme can use a number of adenosine analogues as substrates and this property has been exploited in the field of antineoplasic chemotherapy [4, 131; in fact, the observed resistance of some tumoural cell types to these drugs are correlated with decreased levels of adenosine kinase [14, 151. Nevertheless, other adenosine analogues, such as those found in some marine organisms [16] or chemically synthesized [17] are potent inhibitors of kinase activity. All these compounds might be useful tools in furthering our understanding of the roles of adenosine and adenosine kinase in normal and certain pathological states, as well as in the therapeutic approach.The chromaffin cells from adrenal medulla store, in their secretory granules, catecholamines and ATP at high concen- catecholamines and the remaining soluble intragranular content in response to cholinergic stimulation of the adrenal medulla. Evidence has been obtained that intragranular ATP may be labelled by exogenous radioactive adenosine in experiments of perfusion of whole adrenal gland [21, 221. On the other hand, isolated or cultured chromaffin cells readily incorporate extracellular adenosine into their nucleotide pools [23, 241. These findings suggest the involvement of adenosine kinase in the mechanisms of purine ring replenishment in the chromaffin cells, but to our knowledge no studies on adenosine kinase have been performed in the adrenal medulla. In this paper, purification and properties of adenosine kinase from bovine adrenal medull...

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We investigated the extracellular degradation of diadenosine polyphosphates (ApnA) by cultured adrenomedullary endothelial cells using fluorogenic analogs of ApnA, the di(1,N6-ethenoadenosine) 5',5"'-P1,Pn-polyphosphates [epsilon-(ApnA)]. Kinetic parameters of epsilon-(ApnA) cleavage and effects of pH, ions, and inhibitors were determined by continuous fluorometric assays, using suspensions of endothelial cells grown on Cytodex-1 microspheres. Ecto-enzyme kinetic parameters for epsilon-(Ap3A), epsilon-(Ap4A), and epsilon-(Ap5A) hydrolysis are as follows: Michaelis-Menten constants of 0.39 +/- 0.07, 0.42 +/- 0.09, and 0.37 +/- 0.05 microM respectively, and maximal velocities of 26.1 +/- 6.8, 74.2 +/- 16.4, and 24.4 +/- 3.4 pmol.min-1.10(6) cells-1, respectively. ApnA and guanosine 5',5"'-P1,P4-tetraphosphate behave as competitor substrates of epsilon-(Ap4A) hydrolysis. The ectoenzyme is activated by Mg2+ and Mn2+ and inhibited by Ca2+, F-, adenosine 5'-tetraphosphate, adenosine 5'-O-(3-thiotriphosphate), and suramin. Optimum pH is around 9.0. High-performance liquid chromatography analysis reveals that the ecto-enzyme hydrolyzes epsilon-(ApnA) to give epsilon-adenosine-5'(n-1)-phosphate and epsilon-AMP, which are then further catabolized up to epsilon-adenosine via the membrane-bound nucleotidase system ecto-ATPase, ecto-ADPase (or apyrase), and ecto-5'-nucleotidase. The endothelial ecto-diadenosine polyphosphate hydrolase studied here exhibits different kinetic parameters and sensitivity to ions with respect to the enzyme from the tissue-related neurochromaffin cells. These different properties may be important in the extracellular signaling by ApnA.

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