Factors affecting the glucose 6-phosphate inhibition of hexokinase from cerebral cortex tissue of the guinea pig

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Abstract
1. The inhibition of hexokinase by glucose 6-phosphate has been investigated in crude homogenates of guinea-pig cerebral cortex by using a sensitive radio-chemical technique for the assay of hexokinase activity. 2. It was observed that 44% of cerebral-cortex hexokinase activity did not sediment with the microsomal or mitochondrial fractions (particulate fraction), and this is termed soluble hexokinase. The sensitivities of soluble and particulate hexokinase, and hexokinase in crude homogenates, to the inhibitory actions of glucose 6-phosphate were measured; 50% inhibition was produced by 0·023, 0·046 and 0·068mm-glucose 6-phosphate for soluble, particulate and crude homogenates respectively. 3. The optimum Mg2+ concentration for the enzyme was about 10mm, and this appeared to be independent of the ATP concentration. In the presence of added glucose 6-phosphate, raising the Mg2+ concentration to 5mm increased the activity of hexokinase, but above this concentration Mg2+ potentiated the glucose 6-phosphate inhibition. When present at a concentration above 1mm, Ca2+ ions inhibited the enzyme in the presence or absence of glucose 6-phosphate. 4. When the ATP/Mg2+ ratio was 1·0 or below, variations in the ATP concentration had no effect on the glucose 6-phosphate inhibition; above this value ATP inhibited hexokinase in the presence of glucose 6-phosphate. ATP had an inhibitory effect on soluble hexokinase similar to that on a whole-homogenate hexokinase, so that the ATP inhibition could not be explained by a conversion of particulate into soluble hexokinase (which is more sensitive to inhibition by glucose 6-phosphate). It is concluded that ATP potentiates glucose 6-phosphate inhibition of cerebral-cortex hexokinase, whereas the ATP–Mg2+ complex has no effect. Inorganic phosphate and l-α-glycerophosphate relieved glucose 6-phosphate inhibition of hexokinase; these effects could not be explained by changes in the concentration of glucose 6-phosphate during the assay. 5. The inhibition of hexokinase by ADP appeared to be independent of the glucose 6-phosphate effect and was not relieved by inorganic phosphate. 6. The physiological significance of the ATP, inorganic phosphate and α-glycerophosphate effects is discussed in relation to the control of glycolysis in cerebral-cortex tissue.