Effects of Energy Metabolism on in Vivo Distribution of Hexokinase in Brain

Abstract

Abstract Hexokinase partitioning between the soluble and mitochondrial fractions of brain has been observed to vary according to the energy status of the brain. Ischemia, between 0.5 and 2.5 min, in chick brain led to an increased hexokinase activity in the mitochondrial fraction of cerebellar homogenates from 4.4 ± 0.84 to 7.6 ± 0.66 µmoles per min per g fresh weight; correspondingly, the activity in the soluble fraction decreased from 3.9 ± 0.60 to 1.6 ± 0.28 µmoles per min per g fresh weight. Feeding galactose to chicks, which is known to decrease adenine nucleotide energy charge and the levels of several glycolytic intermediates, resulted in a redistribution of hexokinase in both cerebellum and cerebrum, mimicking the changes that occur during ischemia. Intraperitoneal injection of glucose into chicks fed galactose caused increases in brain ATP and glucose 6-phosphate to those levels observed in control animals. Concomitantly, soluble and mitochondrial hexokinase activities readjusted toward those values observed in control chicks. A regional difference was observed in hexokinase distribution in control animals. The percentage of soluble hexokinase in the cerebellum was 47% as compared with 53% in the mitochondrial form, whereas in the cerebrum, the corresponding hexokinase activities were 31 and 69%. These distributions correlate with differences in ATP content. Cerebellar levels of ATP were 2.51 ± 0.39 µmoles per g, while cerebral levels were 1.91 ± 0.17. In vitro studies indicated that hexokinase solubilization was most sensitive at these concentrations of ATP. These observations indicate an in vivo soluble ⇄ mitochondrial equilibrium responsive to the energy posture of the brain and are consistent with the involvement of such an equilibrium in regulation of carbohydrate metabolism.