Human placental adenosine kinase. Kinetic mechanism and inhibition.
The kinetic properties of human placental adenosine kinase, purified 3600-fold, were studied. The reaction velocity had an absolute requirement for magnesium and varied with the pH. Maximal activity was observed at pH 6.5 with a Mg2+:ATP ranging from 1:1 to 2:1. High concentrations of Mg2+ or free ATP were inhibitory. Double reciprocal plots of initial velocity studies yielded intersecting lines for both adenosine and MgATP2-. The Michaelis constant was 0.4 micro M for adenosine and 75 micro M for MgATP2-. Inhibition by adenosine was observed at concentrations greater than 2.5 micro M. AMP was a competitive inhibitor with respect to adenosine and a noncompetitive inhibitor with respect to ATP. ADP was a noncompetitive inhibitor with respect to adenosine and ATP. Hyperbolic inhibition was observed during noncompetitive inhibition of adenosine kinase by AMP and ADP. Other purine and pyrimidine nucleoside mono-, di-, and triphosphates were poor inhibitors in general. S-Adenosylhomocysteine and 2'-deoxyadenosine inhibited adenosine kinase. The data suggest that (a) MgATP2- is the true substrate of adenosine kinase, and both pH and [Mg2+] may regulate its activity; (b) the kinetic mechanisms of adenosine kinase is Ordered Bi Bi; and (c) adenosine kinase may be regulated by the concentrations of its products, AMP and ADP, but is relatively insensitive to other purine and pyrimidine nucleotides.
D000248:Adenosine Kinase / Q000378:metabolism*; D005260:Female; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009705:Nucleosides / Q000494:pharmacology; D009711:Nucleotides / Q000494:pharmacology; D010770:Phosphotransferases / Q000378:metabolism*; D010920:Placenta / Q000201:enzymology*; D011247:Pregnancy
D009705:Nucleosides; D009711:Nucleotides; D010770:Phosphotransferases; D000248:Adenosine Kinase; D008274:Magnesium