title: Phosphorylation and dephosphorylation kinetics of potassium-stimulated ATP phosphohydrolase from hog gastric mucosa.
abstract: Partial reactions of potassium-stimulated ATP phosphohydrolase from hog gastric mucosa were studied by means of a rapid-mixing apparatus. At 21 degrees C, in the presence of 2 mM MgCl2 and 5 microM [gamma-32P]ATP there was a rapid phosphorylation of the enzyme with a pseudofirst order rate constant of 1400 min-1. Addition of the ATP about 120 ms before the MgCl2 increased this rate constant to 4400 min-1. In the absence of MgCl2 there was no phosphorylation. Addition of 4 or 10 mM KCl to the phosphoenzyme which had been formed in the absence of KCl produced a rapid initial rate of dephosphorylation (k = 2600 and 3200 min-1 respectively). An additional slow component of dephosphorylation was observed when unlabeled ATP was added together with the KCl (k = 700 to 900 min-1). At a 4 mM concentration, KCl stimulated the ATPase activity about 9-fold. At higher concentrations, the activity was reduced in parallel with a reduction of the steady state level of phosphoenzyme. Addition of KCl to the enzyme before the addition of ATP plus MgCl2 resulted in a low rate and extent of phosphorylation. KCl appeared to inhibit the phosphorylation at a level preceeding the E.ATP complex.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D005753:Gastric Mucosa / Q000201:enzymology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism; D013552:Swine
chemical_list: D010042:Ouabain; D000251:Adenosine Triphosphatases; D008274:Magnesium; D011188:Potassium