The temperature dependence of creatine kinase fluxes in the rat heart

A 31P nuclear magnetic resonance saturation transfer method was used to measure the temperature dependence of creatine kinase-catalysed fluxes in Langendorff-perfused rat hearts. A decrease in temperature from 37 to 4 degrees C lowered the observed steady-state fluxes by about 80%. These data were used in conjunction with calculated changes in substrate concentrations with temperature to estimate the activation energy for creatine kinase in situ. The apparent activation energy of 42 kJ/mol agrees reasonably well with the range of literature values for the enzyme in vitro. This demonstrates that the reaction is not diffusion-limited in situ and that extraction and dilution of the enzyme for study in vitro does not alter fundamental kinetic properties of the enzyme exhibited in the intact tissue.