Effect of 6‐Phosphogluconate on Phosphoglucose Isomerase in Rat Brain In Vitro and In Vivo

The activity of phosphoglucose isomerase, its kinetic properties, and the effect of 6‐phosphogluconate on its activity in the forward (glucose 6‐phosphate ± fructose 6‐phosphate) and the reverse (fructose 6‐phosphate ± glucose 6‐phosphate) reactions were determined in adult rat brain in vitro. The activity of phosphoglucose isomerase (in nmol/min/mg of whole brain protein) was 1,865 ± 20 in the forward reaction and 1,756 ± 32 in the reverse reaction at pH 7.5. It was 1,992 ± 28 and 2,620 ± 46, respectively, at pH 8.5. The apparent K m and V max of phosphoglucose isomerase were 0.593 ± 0.031 m M and 2,291 ± 61 nmol/min/mg of protein, respectively, for glucose 6‐phosphate and 0.095 ± 0.013 m M and 2,035 ± 98 nmol/min/mg of protein, respectively, for fructose 6‐phosphate. The activity of phosphoglucose isomerase was inhibited intensely and competitively by 6‐phosphogluconate, with an apparent K i of 0.048 ± 0.005 m M for glucose 6‐phosphate and 0.042 ± 0.004 m M for fructose 6‐phosphate as the substrate. With glucose 6‐phosphate as the substrate, at concentrations from 0.05 to 0.5 m M , the activity of the enzyme was inhibited completely in the presence of 0.5–2.0 n M 6‐phosphogluconate. With 0.05–0.2 n M fructose 6‐phosphate as the substrate, it was inhibited ∼85% at the same concentrations of the inhibitor. No significant changes were observed in the values of K m , V max , and K i ; for phosphoglucose isomerase in the brain of 6‐aminonicotinamide‐treated rats. These findings in vitro and the results of previous experiments, showing an increase from negligible levels to 1.5 m M 6‐phosphogluconate in the brain after administration of 6‐aminonicotinamide to rats, were discussed in support of the view that the two pathways of glucose metabolism—the Embden‐Meyerhof pathway and the hexose monophosphate shunt—occur in distinct compartments in the brain in vivo.