Identification and significance of glucokinase in transplantable insulinomas

Characteristics of glucokinase of the Kirkman insulinoma

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.

Metabolic effects of radiation induced rat insulinoma at pancreatic, hepatic and subscapular transplantation sites

Metabolic effects of 3 different sites of transplantation of cultured tumour cells from a radiation induced insulinoma (28 X 10(6) viable cells per rat) were examined in 15-18 weeks old male NEDH rats. Subscapular implantation consistently produced a highly vascularised encapsulated tumour associated with hyperinsulinaemia, hyperphagia and hypoglycaemia by 21 days, which progressed to fatal neuroglycopaenic coma at 37 +/- 3 days (mean +/- SEM). Implantation of tumour cells into the hepatic portal vein resulted in a multilobular hepatic tumour in two out of nine rats, with hyperinsulinaemia and fatal hypoglycaemia by 49-54 days. Irregularities of glucose homeostasis were observed in a further three rats by 62 days. Intrapancreatic implantation consistently produced a similar tumour to that observed at the subscapular site. Implantation into the pancreas produced the most rapid onset of hyperinsulinaemia, hyperphagia and hypoglycaemia, with survival for only 28 +/- 3 days. The results demonstrate an important effect of transplantation site on the function and metabolic consequences of the NEDH rat insulinoma.

New perspectives on pancreatic islet glucokinase

Control of blood sugar involves the complex interaction of the pancreatic glucose-sensing beta-cells with the liver, which serves as the primary site of glucose disposal after a meal. Glucokinase occupies an important role in controlling glucose phosphorylation and metabolism both in the liver and in pancreatic islets. In the beta-cells, glucokinase functions as pacemaker of glycolysis at physiological glucose levels. It determines the unique characteristics of islet hexose usage, that is, the rate, affinity, cooperativity, and anomeric discrimination of glucose metabolism. Because glycolysis controls hexose-induced insulin release, glucokinase is considered the best-qualified candidate for the elusive glucose sensor of beta-cells. A deficiency of glucokinase would disturb glucose homeostasis. Decreased islet glucokinase would diminish islet glycolysis and would result in a higher set point of beta-cells for glucose-induced insulin release. Decreased liver glucokinase would cause less efficient hepatic glucose disposal. Human maturity-onset diabetes (type II diabetes) has these characteristics. It is thus conceivable that certain forms of type II diabetes are due to a glucokinase deficiency.