Protecting the cellular energy state during contractions: role of AMP deaminase.

AMP deaminase activity (AMP->IMP+NH3) is the entry reaction to the purine nucleotide cycle. In skeletal muscle, excessive energy demands during contractions leads to a net production of ADP, because ATP hydrolysis exceeds ADP rephosphorylation. Elevations in ADP increase AMP, via the myokinase reaction. This accumulation of ATP hydrolysis products should lead to a catastrophic reduction in the energy state of the myocyte. The removal of AMP to IMP in times of excessively high energy demands have been hypothesized as essential to protect the energy state of the cell. While AMP deamination leads to a net loss of adenine nucleotides (principally, as ATP), the viability of the myocyte is preserved. Following these demanding contraction conditions, the concentration of IMP of fast-twitch muscle is rapidly reduced, typically with the return of the muscle adenine nucleotide content (ATP + ADP + AMP) to pre-contraction levels. While these observations are generally observed for fast-twitch skeletal muscle and consistent with the hypothesis, there has been no direct experimental evaluation. In the AK1 (-/-) mouse, there is a markedly reduced accumulation of AMP, during conditions of excessive contractile activity. Rather, there is a high ADP concentration, approaching 1.5 mM, that remains unbound 'free' within the muscle. This contributes to an inordinate reduction in the ATP/ADP ratio. At the same time, PCr hydrolysis is nearly complete leading to a large increase in orthophosphate. In combination, this leads to an exceptional decline in the free energy of ATP hydrolysis. This is projected to impair Ca(2+) handling by the sarcoplasmic reticulum and slow cross-bridge cycling rate. The outcome should be slowed contraction characteristics and possible contracture. While some contractile changes were observed, there was a remarkable ability of the muscle to function under these challenging energetic conditions. Thus, it is not essential that the AMP deaminase reaction be operating during intense contraction conditions. This helps explain why patients deficient in AMP deaminase do not always exhibit an impaired muscle function.