Insulin Effects on Brain Energy Metabolism and the Related Hexokinase Distribution

Abstract

Abstract After 11 hours, male chicks (5 days old) injected intraperitoneally with 2 units of insulin, became hypoglycemic (2.51 ± 0.48 mm versus 14.42 ± 0.35 mm) and occasionally convulsed. The whole brain glucose, glucose 6-phosphate, fructose 1,6-diphosphate, and lactate levels were diminished, respectively, from 1.60 ± 0.18 to 0.24 ± 0.04 mm, from 0.044 ± 0.002 to 0.016 ± 0.002 mm, from 0.139 ± 0.010 to 0.038 ± 0.003 mm, and from 3.78 ± 0.18 to 1.74 ± 0.14 mm. Cerebellar metabolites showed similar decreases. Intraperitoneal injection of glucose resulted in recovery of all metabolites to near-normal levels within 10 min. Hexokinase was found in both particulate and soluble fractions. After administration of insulin, hexokinase activity increased in the particulate fraction and decreased in the soluble fraction. After injection of glucose, hexokinase activity rapidly decreased in the particulate fraction and increased in the soluble fraction, indicating solubilization of the mitochondrially bound form of the enzyme. Thus, the following equilibrium was shown in vivo: mitochondrial hexokinase ⇄ soluble hexokinase. The percentage of hexokinase in the soluble fraction correlated directly with the intracellular glucose 6-phosphate level, suggesting that glucose 6-phosphate is an especially important metabolite in regulating the distribution of hexokinase. Finally, the Ki (glucose 6-phosphate competitive with ATP) was estimated as 6 µm for the soluble form and 30 µm for the bound form. Thus, the bound form of the enzyme is the more active form and under conditions of energy stress; e.g. insulin-induced hypoglycemia, the enzyme was converted to this form but reverted to the more inhibitable form (i.e. soluble) following glucose administration.