Capillaries phosphorylate glucose in a concentration-dependent manner through a glucokinase-like enzyme: a study in the eel

Glucose phosphorylation was studied in a pure capillary preparation obtained from the rete mirabile of the eel swimbladder. In the 3000g supernatant of capillary homogenates, the glucose phosphorylating activity did not reach the maximum at low glucose concentration (1 mmole/liter), as it occurs in most tissues, but increased with the increase in glucose concentration and approached the maximum at very high (300 mmole/liter) glucose levels, with values (mean +/- SEM, n = 10) of 5.85 +/- 0.94 nmole.min-1.mg-1 protein and 19.97 +/- 1.89 at 1 and 300 mmole/liter glucose, respectively. The apparent Km value for glucose was about 50 mmole/liter, i.e., at supraphysiological glucose concentration, like the enzyme glucokinase, typically present in the liver but absent from most other tissues. This new enzyme did not phosphorylate fructose (similar to glucokinase from liver, which is rather specific for glucose) but was not inhibited by N-acetyl-glucosamine (in contrast to hepatic glucokinase). Thus, capillaries phosphorylate glucose in a concentration-dependent manner, which suggests that they are equipped with a glucokinase-like enzyme. This may explain the reported increase in glucose uptake during capillary exposure to high glucose concentrations and would suggest that the hyperglycemia of the diabetic state may be associated with increased glucose utilization, which may play a role in the development of microangiopathy