Adenine nucleotide metabolism in contracting skeletal muscle.

During steady-state muscle contractions, ATP production and utilization are well matched. When the rate of ATP hydrolysis exceeds the capacity of a given muscle fiber to phosphorylate ADP, the ADPf and AMPf concentrations rise, first leading to the deamination of adenylates and subsequently to the dephosphorylation of AMP or IMP, or both, to their respective nucleosides and bases. Several proposed roles for the purine nucleotide cycle in skeletal muscle have been reviewed and evaluated. The deaminating limb of the purine nucleotide cycle is most important; it maintains the ATP/ADP ratio and lessens adenine nucleotide degradation. Regulation of glycolytic pathway enzymes by the products of AMP deamination (IMP and NH4+) does not seem likely. During reamination there is a net production of fumarate, with the branch-chain amino acids potentially supplying a significant fraction of the amine; reamination, however, is probably not concurrent with a high rate of deamination. Evidence from some studies of AMP deaminase-deficient persons suggests that an intact purine nucleotide cycle is required for normal muscle function during intense exercise; the issue is clouded, however, by the occurrence of asymptomatic AMP deaminase deficiency. Skeletal muscle is capable of extensive adenine nucleotide degradation during severe, energy-depleting conditions. Purine nucleosides and bases not reincorporated by the salvage pathway must be synthesized de novo. The capacity for de novo synthesis differs among fiber types, being highest in muscle with the highest oxidative capacity.