A glucokinase-like enzyme carries out glucose phosphorylation in capillaries of normal and spontaneously hyperglycemic rabbits

We have studied glucose phosphorylation at increasing glucose concentrations (1, 5, 10, 25, 50, and 100 mmol/liter) in capillaries of the choroidocapillary lamina from the eye of normal female albino rabbits (n = 10; body wt 1800-2000 g; mean +/- SEM morning glycemia: 147.77 +/- 4.02 mg/dl) and from the eye of spontaneously hyperglycemic rabbits (n = 5, body wt 1800-2000 g, mean +/- SEM morning glycemia; 211.00 +/- 10.76 mg/dl). In the 3000g supernatant of capillary homogenates, the glucose phosphorylating activity (NADP reduction measured as optical density change at 366 nm at pH 7.5) increased progressively with the rise of glucose concentration (r = 0.36; P < 0.05), approaching the peak at high glucose level (25 mmol/liter), with values ranging from 5.32 +/- 0.46 (SEM) nmol/min/mg protein to 7.14 +/- 0.74 (+34.21%, P < 0.01). When measured at a more alkaline pH (8.2) the glucose phosphorylation was higher than at pH 7.5 and retained the responsiveness to increasing glucose concentrations. These kinetic characteristics differ from those seen in most tissues and are somewhat reminiscent of those shown by hepatic glucokinase. Indeed, by subtracting the activity at 1 mmol/liter glucose from that at higher glucose concentrations, we calculated the "glucokinase component" which together with the "hexokinase component" form the total glucose phosphorylating activity. Glucose phosphorylation in capillaries from spontaneously hyperglycemic rabbits was lower than normal (values: 3.66 +/- 0.31 vs 5.32 +/- 0.46 of the normal rabbits; -31.20%; P < 0.05). This could contribute to the hyperglycemia by reducing glucose utilization. However, in these animals the enzyme activity retained the responsivity to increasing glucose concentrations (r = 0.41, P < 0.05). Therefore, the actual capillary glucose phosphorylation in these animals would depend upon both the enzyme level (which is reduced) and the glucose concentration (which is increased). Due to the in vivo inhibition of the hexokinase component, the glucokinase component may be predominant in vivo, making the stimulating effects of hyperglycemia much more pronounced than it would appear from our data in vitro. This may lead to glucose overutilization. These kinetic characteristics of glucose phosphorylation in capillaries might be relevant to the mechanisms leading to diabetic microangiopathy.