Determination of the deoxyglucose and glucose phosphorylation ratio and the lumped constant in rat brain and a transplantable rat glioma.

Mitochondrially bound hexokinase (ATP-D-hexose-6-phosphotransferase; EC 2.7.1.1) was dissociatively extracted from normal rat brains and intracerebral and subcutaneous implants of the 36B-10 glioma. At least 70% of the total hexokinase enzyme activity in normal and glioma tissue was associated with the mitochondrial fraction. Purification of the crude tissue extracts by ion-exchange and affinity chromatography followed by analysis with sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a successive purification of the enzyme to homogeneity with a molecular size of 98 kilodaltons. Enzyme kinetics with glucose or 2-deoxyglucose (2-DG) as the substrate were measured spectrophotometrically by coupling the appropriate reactions to either NADPH or NAD+ formation. The Km of hexokinase with glucose as the substrate in the intracerebral glioma (0.138 mM) and subcutaneous glioma (0.183 mM) tissues was 2.1-2.7-fold higher than that observed in normal brain tissue (0.067 mM) (p less than 0.001). No significant differences were observed in the Km for hexokinase with 2-DG as the substrate in the glioma and normal brain tissue. The phosphorylation ratio for normal brain was 0.320 and was increased in the intracerebral glioma to 0.694 and in the subcutaneous glioma to 0.519. The ratios of deoxyglucose and glucose volumes of distribution in normal brain and intracerebral glioma tissues were 1.70 and 1.85, respectively. The lumped constants calculated directly from the phosphorylation ratios and the volumes of distribution of deoxyglucose and glucose were 0.517 in normal brain and 1.168 in intracerebral glioma. Our results indicate the lumped constant is increased 2.26-fold in intracerebral glioma compared with normal brain.