Rabbit optic nerve phosphorylates glucose through a glucokinase-like enzyme: studies in normal and spontaneously hyperglycemic animals

We investigated glucose phosphorylation at various concentrations of glucose (1, 5, 10, 25, 50, 100 mmol/liter) in rabbit optic nerve. In the 3000 g supernatant of whole rabbit optic nerve homogenates from female albino rabbits (n = 10, 1.8-2.0 kg body weight, mean +/- SEM morning glycemia: 8.25 +/- 0.29 mmol/liter), the glucose phosphorylating activity (NADP reduction measured as change in optical density at 366 nm at pH 7.5) increased progressively with the increase in glucose concentration (r = 0.89; P < 0.05) and approached the maximum at a very high glucose level (100 mmol/liter), with values (mean +/- SEM) of 8.75 +/- 0.97 nanomol/min/mg protein and 11.57 +/- 1.15 at 1 and 100 mmol/liter glucose, respectively (+32.23%; P < 0.01). At a more alkaline pH (8.2; n = 5, mean +/- SEM morning glycemia: 8.83 +/- 0.07 mmol/liter) glucose phosphorylation was higher than at pH 7.5 and retained the glucose concentration dependence (r = 0.95, P < 0.01). These kinetic characteristics are reminiscent of those of the low-affinity enzyme glucokinase, which is typically present in the liver. By subtracting the activity at 1 mmol/liter glucose from that at higher glucose concentrations, we calculated the "glucokinase component," forms the "total" glucose phosphorylating activity. In five rabbits (of similar age and weight) with spontaneous hyperglycemia (mean +/- SEM: 11.71 +/- 0.60 mmol/liter), the optic nerve glucose phosphorylating activity was lower (value at 1 mmol/liter glucose: 5.42 +/- 1.31, -38.06%, P < 0.05)