title: Platelet AMP deaminase. Regulation by Mg-ATP2- and inorganic phosphate and inhibition by the transition state analog coformycin.
abstract: Kinetic studies with platelet AMP deaminase, at pH 7.0 and 100 mM NaC1, gave cooperative initial velocity curves with AMP as substrate, with Mg-ATP2- as an activator, and with Pi as an inhibitor. In the absence of Mg-ATP2-, the s0.5 for AMP was 4.5 mM with a Hill coefficient approaching 2.0. In the presence of saturating Mg-ATP2-, the s0.5 for AMP was reduced to 0.18 mM, the maximum velocity was increased by about 35%, and the Hill coefficient was 1.0. The half-activation constant for Mg-ATP2- varied from 0.7 to 0.07 mM as the concentration of AMP was varied from 0.1 to 5.0 mM and the Hill coefficient for Mg-ATP activation changed from 2.0 to 1.0 over the same range. Phosphate inhibition was competitive with AMP and with Mg-ATP2- (Ki = 2.0 mM) and reversed the activation by Mg-ATP2-. Coformycin inhibited the Mg-ATP-activated enzyme with a Ki less than 0.25 microM. Coformycin inhibition was slow, with a second order rate constant of 6.0 X 10(4) M-1 min-1, suggesting that the compound acts as a transition state analog according to Frieden, C., Kurz, L. C., and Gilbert, H. R. (1980) Biochemistry 19, 5303-5309. The kinetic properties of the enzyme indicate that substantial regulation can occur through changes in AMP concentration acting synergistically to enhance Mg-ATP2- binding and displace Pi from a single type of regulatory site.
mesh_terms: D000659:AMP Deaminase / Q000097:blood*; D000249:Adenosine Monophosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D001792:Blood Platelets / Q000201:enzymology*; D003070:Coformycin / Q000494:pharmacology*; D004789:Enzyme Activation; D006801:Humans; D007700:Kinetics; D009709:Nucleotide Deaminases / Q000097:blood*; D010710:Phosphates / Q000494:pharmacology*; D012263:Ribonucleosides / Q000494:pharmacology*
chemical_list: D010710:Phosphates; D012263:Ribonucleosides; D003070:Coformycin; D000249:Adenosine Monophosphate; D000255:Adenosine Triphosphate; D009709:Nucleotide Deaminases; D000659:AMP Deaminase