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1 Department of Biochemistry, University of Cambridge, Cambridge CB2 1QW, United Kingdom; and 2 Division of Biochemistry, University of Tasmania, Hobart 7001, Tasmania, Australia
Proton pumping
across the mitochondrial inner membrane and proton leak back through
the natural proton conductance pathway make up a futile cycle that
dissipates redox energy. We measured respiration and average
mitochondrial membrane potential in perfused rat hindquarter with
maximal tetanic contraction of the left gastrocnemius-soleus-plantaris muscle group, and we estimate that the mitochondrial proton cycle accounted for 34% of the respiration rate of the preparation. Similar
measurements in rat hepatocytes given substrates to cause a high rate
of gluconeogenesis and ureagenesis showed that the proton cycle
accounted for 22% of the respiration rate of these cells. Combining
these in vitro values with literature values for the contribution of
skeletal muscle and liver to standard metabolic rate (SMR), we
calculate that the proton cycle in working muscle and liver may account
for 15% of SMR in vivo. Although this value is less than the 20% of
SMR we calculated previously using data from resting skeletal muscle
and hepatocytes, it is still large, and we conclude that the futile
proton cycle is a major contributor to SMR.
standard metabolic rate; proton pumping; redox energy; contracting skeletal muscle; working liver cells
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