Temperature dependence of the creatine kinase reaction measured in rat brain in vivo by P-31 NMR saturation transfer

Creatine kinase (CK) catalyzes the reversible phosphorylation of MgADP by p hosphocreatine and thus regulates cellular concentrations of ADP and ATP. T he temperature dependence of this reaction has been determined in rat brain in vivo between 30 and 400 degrees C using P-31 NMR saturation transfer me asurements. The pseudo-first-order rate constant for the forward CK reactio n, kf, varies little with temperature over this range, with an apparent act ivation energy E-a = 14.2 +/- 4.9 kJ/mol. This is considerably lower than t he values of E-a for isolated CK enzymes. However, when changes in [MgADP] and [HC] with temperature are considered, a substrate concentration-indepen dent value of E-a = 65.3 +/- 9.7 kJ/mol is obtained for the maximum forward reaction velocity V-max. This agrees well with literature values for the i solated brain-type isoform of CK.