Can You Really Use the Creatine Equilibrium to Calculate Free ADP Concentrations?

Christine Nabuurs¹, Cees Hilbers², Arend Heerschap¹

31P saturation transfer experiments in MAK=/= and WT muscle demonstrated a neglegible effect of CK and AK mediated enzymatic phosphoryl exchanges between -ADP and -ATP. These results are in conflict with the expected 65% reduction of -ATP upon saturation of the (ATP/)-ADP resonance. Hence, the major ADP pool which is available to the CK reaction cannot be saturated. We propose a solid-state like ADP pool, which is in exchange with a transient ADP pool that associates with CK. The inability to saturate the ADP spin system challenges the validity of calculating the free ADP concentration from the CK equilibrium.