31P Saturation Transfer Spectroscopy Predicts Differential Intracellular Macromolecular Association of ATP and ADP in Skeletal Muscle

The kinetics of phosphoryl exchange involving ATP and ADP have been investigated successfully by in vivo P-31 magnetic resonance spectroscopy using magnetization transfer. However, magnetization transfer effects seen on the signals of ATP also could arise from intramolecular cross-relaxation. This relaxation process carries information on the association state of ATP in the cell. To disentangle contributions of chemical exchange and cross-relaxation to magnetization transfer effects seen in 31P magnetic resonance spectroscopy of skeletal muscle, we performed saturation transfer experiments on wild type and double-mutant mice lacking the cytosolic muscle creatine kinase and adenylate kinase isoforms. We find that cross-relaxation, observed as nuclear Overhauser effects (NOEs), is responsible for magnetization transfer between ATP phosphates both in wild type and in mutant mice. Analysis of P-31 relaxation properties identifies these effects as transferred NOEs, i.e. underlying this process is an exchange between free cellular ATP and ATP bound to slowly rotating macromolecules. This explains the beta-ATP signal decrease upon saturation of the gamma-ATP resonance. Although this usually is attributed to beta-ADP <-> beta-ATP phosphoryl exchange, we did not detect an effect of this exchange on the beta-ATP signal as expected for free [ADP], derived from the creatine kinase equilibrium reaction. This indicates that in resting muscle, conditions prevail that prevent saturation of beta-ADP spins and puts into question the derivation of free [ADP] from the creatine kinase equilibrium. We present a model, matching the experimental result, for ADP <-> ATP exchange, in which ADP is only transiently present in the cytosol.