It was investigated whether the well-known transplantable
insulinoma of the hamster (the Kirkman
tumor) contains
glucokinase and if so, what its kinetic characteristics are, and whether its cellular levels might be regulated in a manner typical for islet tissue. The supernatant of
tumor homogenates contained a low-affinity component (Km 9.7 mmol/L) of
glucose phosphorylating activity, apparently
glucokinase. Partially purified
insulinoma glucokinase exhibited similar kinetic characteristics to liver
glucokinase (Km for
glucose 5.0 and 5.3 mmol/L, half-maximal saturation 6.9 and 6.3 mmol/L, Hill coefficient 1.63 and 1.62, Ki for
mannoheptulose 0.9 and 0.6 mmol/L in hamster
insulinoma glucokinase and hamster liver
glucokinase, respectively).
Insulinoma glucokinase activity was not affected by the age of the
tumor.
Tumor-bearing hamsters without further treatment stayed normoglycemic (172 +/- 9.5 mg/dL) for the duration of the experiment. Fasting caused
hypoglycemia (49 +/- 5.0 mg/dL), and pretreatment with
streptozotocin prior to
tumor transplantation caused
hyperglycemia (393 +/- 20.6 mg/dL) in the
tumor-bearing hamsters.
Blood glucose levels of the host hamsters did not affect the content of the
insulinoma glucokinase (83 +/- 3.5 mU/g in
hypoglycemic group, 88 +/- 9.0 mU/g in hyperglycemic group, and 86 +/- 3.5 mU/g in normoglycemic group). Thus, biosynthesis and degradation of
insulinoma glucokinase does not seem to be regulated by
glucose as found to be true for islet
glucokinase. Since
glucokinase is constitutively present, the stable transplantable Kirkman
tumor could serve as a useful model for studying the pancreatic B-cell glycolysis system which is characterized by the presence of both
hexokinase and
glucokinase.