Abnormal regulation of hexokinase in insulin-resistant skeletal muscle.
The present study has assessed the potential involvement of hexokinase in the control of insulin-mediated glucose metabolism in insulin-sensitive and -resistant skeletal muscle. Soleus muscle strips from lean (insulin-sensitive) and obese (insulin-resistant) Zucker rats were incubated with 10 or 10,000 microU insulin.ml-1 and then homogenized using a protocol to maintain the location of hexokinase in situ. Hexokinase is inhibited by glucose 6-phosphate, a metabolic intermediate which may have a central role in the regulation of glycogen synthesis. Two separate measurements of hexokinase activity were made on each muscle homogenate: the total hexokinase activity (glucose 6-phosphate was metabolized immediately by glucose 6-phosphate dehydrogenase) and the fractional hexokinase activity (glucose 6-phosphate accumulated so as to regulate the enzyme as in vivo). The total hexokinase activity was equal in insulin-sensitive and -resistant muscle and was unaffected by the extracellular insulin concentration. The fractional hexokinase activity was significantly increased by insulin (10,000 microU.ml-1) in all muscles (lean, 82%; obese, 52%; P < 0.05) although the stimulated fractional hexokinase activity was lower in the muscle from obese Zucker rats compared to lean (P < 0.05). These results provide evidence that insulin decreases the inhibition of hexokinase by glucose 6-phosphate in insulin-sensitive but not in insulin-resistant muscle. This study has revealed short-term regulation of hexokinase by insulin which is defective in insulin-resistant skeletal muscle. Thus, the study has identified hexokinase as a potential regulatory site of insulin action that is abnormal in insulin resistance. The altered regulation of hexokinase may be a major contributing factor to the reduced insulin-mediated glucose fluxes in insulin-resistant skeletal muscle.
Department of Biochemistry, University of Oxford, United Kingdom.
A L
AL
Sanderson
G K
GK
Radda
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B
Leighton
D000818:Animals; D019298:Glucose-6-Phosphate / Q000378:metabolism; D005954:Glucosephosphate Dehydrogenase / Q000378:metabolism; D006593:Hexokinase / Q000378:metabolism*; D007333:Insulin Resistance; D018482:Muscle, Skeletal / Q000201:enzymology*; D009765:Obesity / Q000201:enzymology; D051381:Rats; D011924:Rats, Zucker
D019298:Glucose-6-Phosphate; D005954:Glucosephosphate Dehydrogenase; D006593:Hexokinase