Fructokinase activity in rat liver, ileum, parotid gland, pancreas, pancreatic islet, B and non-B islet cell homogenates

The presence of fructokinase (ketohexokinase) in rat pancreatic islet homogenates was previously documented. However, no information was so far available on the activity of this enzyme in islets relative to that in other tissues and on the respective contribution of insulin-producing B cells and non-B islet cells. The present study provides such an information. The activity of fructokinase, as assessed by the phosphorylation of 1.0 mM D-fructose, was compared to that of hexokinase isoenzyme(s), as measured in the presence of 1.0 mM D-glucose, and further characterized by its heat-resistance, K+ dependency and resistance to the inhibitory action of D-mannoheptulose. As judged from the results obtained in heated homogenates, the activity of fructokinase, expressed relative to protein content (nmol/min per mg protein) was highest in liver (21.5±2.5; n=11) and lowest in parotid gland (0.16±0.09; n=3), with in-between values in ileum (2.45±0.53; n=3), pancreas (0.82±0.11; n=11) and pancreatic islets (0.46±0.07; n=6). The paired ratio between fructokinase and hexokinase isoenzyme activity was also highest in liver (548±45%; n=8) and lowest in parotid gland (0.93±0.52%; n=3). Such a ratio was not significantly different in pancreas, islets and purified B or non-B islet cells, with an overall mean value of 2.57±0.46% (n=12). The present findings thus unambiguously document the presence of fructokinase activity in all cell types under consideration, except possibly parotid cells, with the following hierarchy: liver > ileum > pancreas. Relative to paired hexokinase activity, no obvious difference was found for fructokinase activity in B versus non-B islet cells.