IMP degradative capacity in rat skeletal muscle fiber types.
During contractions, when the rate of ATP hydrolysis exceeds that of ADP phosphorylation, inosine 5'-monophosphate (IMP) accumulates in skeletal muscle. If the cellular energy balance is not promptly restored, subsequent purine degradation to inosine via 5'-nucleotidase can occur, a process that is most robust in the slow-twitch red, as compared to fast-twitch, skeletal muscle. We measured the distribution of 5'-nucleotidase activity among membrane-bound and soluble fractions of fiber specific skeletal muscle sections and found most (80-90%) of the total 5'-nucleotidase activity to be membrane-bound. The 5' IMP nucleotidase activity present in the soluble fraction of muscle extracts differs among fiber types with slow-twitch red > fast-twitch red > mixed fibered > fast-twitch white. Experiments testing the substrate dependence of IMP and AMP dephosphorylation by the soluble fraction of muscle extracts revealed a lower Km toward IMP (approximately 0.7-1.5 mM) than AMP (1.9-2.8 mM). Among skeletal muscle fiber sections, the soluble 5'-nucleotidase activity present in slow-twitch red muscle extracts had the highest substrate affinity, the highest activity with IMP as substrate, and an estimated catalytic efficiency (Vmax/Km) that was > 3-fold higher than calculated for fast-twitch muscle extracts. This is likely due to the Mg2+ dependent cytosolic 5' IMP nucleotidase isoform, since immunoprecipitation experiments revealed 3-4 times more activity in slow-twitch red than in fast-twitch red or fast-twitch white fibers, respectively. These finding are consistent with the previously recognized in vivo pattern of nucleoside formation by muscle where the soleus demonstrated extensive inosine formation at a much lower IMP content than fast-twitch red or fast-twitch white muscle fiber sections.
Tullson
P C
PC
Department of Physiology, State University of New York, Health Science Center at Syracuse, USA.
Terjung
R L
RL
D000249:Adenosine Monophosphate / Q000378:metabolism; D000818:Animals; D004734:Energy Metabolism; D007291:Inosine Monophosphate / Q000378:metabolism*; D007527:Isoenzymes; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism; D018657:Muscle Fibers, Slow-Twitch / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism*; D009708:Nucleotidases / Q000378:metabolism*; D011233:Precipitin Tests; D051381:Rats; D017207:Rats, Sprague-Dawley; D012995:Solubility; D013379:Substrate Specificity
D007527:Isoenzymes; D007291:Inosine Monophosphate; D000249:Adenosine Monophosphate; D009708:Nucleotidases; C030287:nucleotidase; D008274:Magnesium