Mapungwana, Sibusisiwe Mavis (1982)
Regulation of pyruvate kinase in isolated hepatocytes by metabolites arising from the glycolysis of fructose and other related substrates.
Full text access: Open
It has been reported that fructose is rapidly converted to lactate by the liver. The only regulatory enzyme involved in this conversion is L-type pyruvate kinase. Thus the effects of fructose and other related substrates which enter the glycolytic sequence at the triose phosphate level are of physiological importance. Incubation of isolated hepatocytes with fructose at high concentrations and with glycerol caused an apparent inhibition of pyruvate kinase. This inhibition was correlated to the depletion of ATP caused by these substrates since dihydroxyacetone and D-glyceraldehyde did not affect the enzyme activity. When hepatocytes were washed with fresh medium after the incubation, but before the extraction of enzyme, fructose, dihydroxyacetone and D-glyceraldehyde all caused stimulation whereas glycerol was without effect. This stimulation correlated closely with the increase in concentrations of fructose-1,6-bisphosphate and/or fructose-1-phosphate within the hepatocytes. The accumulation of inhibitor(s) in the extrahepatocyte medium in response to fructose, prevented the stimulation of the enzyme by fructose-1,6-bisphosphate which occurs despite the large dilution of the cellular contents during the extraction and assay procedure. It was found that the accumulation of two inhibitors of pyruvate kinase could explain the effect of fructose on hepatocyte pyruvate kinase activity. These are alanine, a known inhibitor of the enzyme, and allantoin.The regulation of the intracellular pyruvate kinase was examined by estimating glycolytic flux from the accumulation of lactate and pyruvate. The glycolytic flux from fructose exceeded that from other substrates despite the inhibition of pyruvate kinase described above. 5-Glycolysis from dihydroxyacetone although initially rapid, was much slower than from fructose whereas glycerol and other reduced substrates caused an inhibition of glycolysis probably as a result of an increase in the (NADH)/(NAD+) ratio. The rapid glycolysis from fructose can be explained by the accumulation of fructose-1,6-bisphosphate and fructose-l-phosphate, causing a stimulation of pyruvate kinase and a depletion of ATP, relieving the inhibition of the enzyme. Changes in hepatocyte phosphoenolpyruvate concentrations correlate well with the rates of glycolysis observed.
This is a Accepted version
This version's date is:
is not peer reviewed
Deposited by () on
in Royal Holloway Research Online.Last modified on 01-Feb-2017
Digitised in partnership with ProQuest, 2015-2016.
Institution: University of London, Royal Holloway College (United Kingdom).