Metabolism of glucose, glutamine, long-chain fatty acids and ketone bodies by lungs of the rat.

The maximum activities of some key enzymes of metabolism were studied in lungs of fed and 48-h-starved rats. The maximum activity of hexokinase in the lung is similar to that of other tissues of the body, but lower than that of phosphorylase and 6-phosphofructokinase. High activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were found in lung tissue, suggesting the importance of the pentose phosphate pathway in the lung. The activities of hexokinase and 6-phosphofructokinase were decreased whereas that of phosphorylase increased in response to starvation. Of the enzymes of the tricarboxylic acid cycle whose activities were measured, that of oxoglutarate dehydrogenase was the lowest, yet its activity (approximately 4.2 nmol/min per mg protein at 37 degrees C) was considerably greater than the flux through the cycle (0.46 nmol/min per mg protein at 37 degrees C; calculated from oxygen consumption by incubated lung slices). The activities of both oxoglutarate dehydrogenase and citrate synthase were decreased by starvation. The activities of 3-oxoacid CoA-transferase and acetoacetyl-CoA thiolase were low in lung tissue compared to those of other tissues (eg kidney, brain) and that of 3-hydroxybutyrate dehydrogenase was very low. The activity of carnitine palmitoyl transferase is higher in the lung, suggesting that fatty acids (and possibly acetoacetate) could provide acetyl-CoA as substrate for the tricarboxylic acid cycle. Very low rates of utilization of 3-hydroxybutyrate were observed during incubation of lung slices, but that of oleate was 1.2 nmol/h per mg of protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enzymatic activities related to intermediary metabolism of glucose in circulating mononuclear cells from obese humans: relationship to enzyme activity in adipose tissue

In order to assess whether enzyme activities of glucose metabolism measured in mononuclear blood cells reflect those in a typical insulin target tissue, we studied hexokinase, 6-phosphofructokinase, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase activities in lymphomonocytes and in hypogastric adipose tissue from 15 nondiabetic obese women. Statistically significant relationships were found in the activities of hexokinase (r = 0.53, p less than 0.05), 6-phosphofructokinase (r = 0.85, p less than 0.01), and 6-phosphogluconate dehydrogenase (r = 0.72, p less than 0.01) between the two tissues. These results suggest that mononuclear blood cells may be suitable as a model for studying cytosolic key enzymes involved in the glucose metabolism of humans.     

Regulation of glucose-6-phosphate dehydrogenase activity during caloric restriction in human adipose tissue

Glucose-6-phosphate dehydrogenase activity was significantly lower in adipose tissue of human subjects after 7 days of severe caloric restriction on low-carbohydrate diets and had returned to normal values 4 days after the subjects resumed normal diets. Three other enzyme activities (malate-NADP dehydrogenase, oxaloacetate decarboxylating; ATP-citrate lyase and 6-phosphofructokinase) were not significantly affected by these dietary changes. These results are consistent with separate control of glucose-6-phosphate dehydrogenase activity versus other 'lipogenic' enzyme activities in human adipose tissue.     

Metabolism of glucose, glutamine, long-chain fatty acids and ketone bodies by murine macrophages.

Maximum activities of some key enzymes of metabolism were studied in elicited (inflammatory) macrophages of the mouse and lymph-node lymphocytes of the rat. The activity of hexokinase in the macrophage is very high, as high as that in any other major tissue of the body, and higher than that of phosphorylase or 6-phosphofructokinase, suggesting that glucose is a more important fuel than glycogen and that the pentose phosphate pathway is also important in these cells. The latter suggestion is supported by the high activities of both glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. However, the rate of glucose utilization by 'resting' macrophages incubated in vitro is less than the 10% of the activity of 6-phosphofructokinase: this suggests that the rate of glycolysis is increased dramatically during phagocytosis or increased secretory activity. The macrophages possess higher activities of citrate synthase and oxoglutarate dehydrogenase than do lymphocytes, suggesting that the tricarboxylic acid cycle may be important in energy generation in these cells. The activity of 3-oxoacid CoA-transferase is higher in the macrophage, but that of 3-hydroxybutyrate dehydrogenase is very much lower than those in the lymphocytes. The activity of carnitine palmitoyltransferase is higher in macrophages, suggesting that fatty acids as well as acetoacetate could provide acetyl-CoA as substrate for the tricarboxylic acid cycle. No detectable rate of acetoacetate or 3-hydroxybutyrate utilization was observed during incubation of resting macrophages, but that of oleate was 1.0 nmol/h per mg of protein or about 2.2% of the activity of palmitoyltransferase. The activity of glutaminase is about 4-fold higher in macrophages than in lymphocytes, which suggests that the rate of glutamine utilization could be very high. The rate of utilization of glutamine by resting incubated macrophages was similar to that reported for rat lymphocytes, but was considerably lower than the activity of glutaminase.     

Degradation of endogenous fructose during catabolism of sucrose and mannitol in halophilic archaebacteria

Metabolism of fructose arising endogenously from sucrose or mannitol was studied in halophilic archaebacteria Haloarcula vallismortis and Haloferax mediterranei. Activities of the enzymes of Embden-Meyerhof-Parnas (EMP) pathway, Entner-Doudoroff (ED) pathway and Pentose Phosphate (PP) pathway were examined in extracts of cells grown on sucrose or mannitol and compared to those grown on fructose and glucose. Sucrase and NAD-specific mannitol dehydrogenase were induced only when sucrose or mannitol respectively were the growth substrates. Endogenously arising fructose was metabolised in a manner similar to that for exogenously supplied fructose i.e. a modified EMP pathway initiated by ketohexokinase. While the enzymes for modified EMP pathway viz. ketohexokinase, 1-phosphofructokinase and fructose 1,6-bisphosphate aldolase were present under all growth conditions, their levels were elevated in presence of fructose. Besides, though fructose 1,6-bisphosphatase, phosphohexoseisomerase and glucose 6-phosphate dehydrogenase were present, the absence of 6-phosphogluconate dehydratase precluded routing of fructose through ED pathway, or through PP pathway directly as 6-phosphogluconate dehydrogenase was lacking. Fructose 1,6-bisphosphatase plays the unusual role of a catabolic enzyme in supporting the non-oxidative part of PP pathway. However the presence of constitutive levels of glucose dehydrogenase and 2-keto 3-deoxy 6-phosphogluconate aldolase when glucose or sucrose were growth substrates suggested that glucose breakdown took place via the modified ED pathway.Abbreviations EMP Embden Meyerhof Parnas - ED Entner Doudoroff - PP pentose phosphate - KHK ketohexokinase - 1-PFK 1-phosphofructokinase - PEP-PTS phosphoenolpyruvate phosphotransferase - 6-PFK 6-phosphofructokinase - FBPase fructose 1,6-bisphosphatase - PHI phosphohexoseisomerase - G6P-DH glucose 6-phosphate dehydrogenase - 6PG-DH 6-phosphogluconate dehydrogenase - GAPDH glyceraldehyde 3-phosphate dehydrogenase - FIP fructose 1-phosphate - GSH reduced glutathione - 2-ME -mercaptoethanol - FBP fructose 1,6-bisphosphate - KDPG 2-keto 3-deoxy 6-phosphogluconate - F6P fructose 6-phosphatez     

Fructose catabolism in Azospirillum brasilense and Azospirillum lipoferum.

The pathways for catabolism of fructose were investigated in the type strains of Azospirillum lipoferum and Azospirillum brasilense grown aerobically with (NH4)2SO4 as the nitrogen source. When grown on fructose, the former species possessed a complete Entner-Doudoroff pathway, whereas the latter species lacked activity for glucose-6-phosphate dehydrogenase. Both species possessed a complete catabolic Embden-Meyerhof-Parnas pathway. Neither species possessed the key enzyme of the hexose monophosphate pathway, 6-phosphogluconate dehydrogenase. Both species could phosphorylate fructose to fructose-1-phosphate by means of a phosphoenolpyruvate-phosphotransferase system, and high activities of 1-phosphofructokinase occurred. Both species possessed glucokinase activity, but only A. lipoferum had hexokinase activity; moreover, the cells of A. brasilense were nearly impermeable to glucose, accounting for the inability of this species to grow on glucose. Both species possessed pyruvate dehydrogenase, a complete tricarboxylic acid cycle, a glyoxylate shunt, and malic enzyme. Analysis of the acidic end products for both species indicated the formation of only small amounts of various organic acids, and most of the titratable acidity was due to utilization of the ammonium ions of the medium. Gluconic acid was not formed during growth of either species on fructose but was detected during growth of A. lipoferum on glucose; this species also possessed an NADP-linked glucose dehydrogenase and gluconokinase.     

Mannitol Metabolism in Lentinus edodes, the Shiitake Mushroom

Mannitol metabolism was evaluated in fruiting bodies of Lentinus edodes. Cell extracts were prepared from fruiting bodies, and key enzymes involved in mannitol metabolism were assayed, including hexokinase, mannitol dehydrogenase, mannitol-1-phosphate dehydrogenase, mannitol-1-phosphatase, and fructose-6-phosphatase. Mannitol dehydrogenase, fructose-6-phosphatase, mannitol-1-phosphatase, and hexokinase activities were found in extracts of fruiting bodies. However, mannitol-1-phosphate dehydrogenase activity was not detected. Mycelial cultures were grown in an enriched liquid medium, and enzymes of the mannitol cycle were assayed in cell extracts of rapidly growing cells. Mannitol-1-phosphate dehydrogenase activity was also not found in mycelial extracts. Hence, evidence for a complete mannitol cycle both in vegetative mycelia and during mushroom development was lacking. The pathway of mannitol synthesis in L. edodes appears to utilize fructose as an intermediate.     

The effect of endurance-training on the maximum activities of hexokinase, 6-phosphofructokinase,citrate synthase,and oxoglutarate dehydrogenase in red and white muscles of the rat

Adult female rats were subjected to an eleven-week endurance-training programme, and, for the first time, the maximum activities of enzymes that can indicate the quantitative capacities of both anaerobic glycolysis and the Krebs cycle in muscle (viz. 6-phosphofructokinase and oxoglutarate dehydrogenase respectively) were measured in heart plus white and fast-oxidative skeletal muscle. No changes were observed in heart muscle. In fast-oxidative skeletal muscle, activities of hexokinase, citrate synthase, and oxoglutarate dehydrogenase were increased by 51, 26, and 33% respectively but there was no effect on 6-phosphofructokinase. These results demonstrate that in red muscle there is no effect of this training programme on the anaerobic capacity but that of the aerobic system is increased by one third. In white skeletal muscle, only the activity of citrate synthase was increased, which indicates that this activity may not provide even qualitative information about changes in capacity of the Krebs cycle.     

The pentose phosphate pathway of glucose metabolism. Hormonal and dietary control of the oxidative and non-oxidative reactions of the cycle in liver

1. Measurements were made of the non-oxidative reactions of the pentose phosphate cycle in liver (transketolase, transaldolase, ribulose 5-phosphate epimerase and ribose 5-phosphate isomerase activities) in a variety of hormonal and nutritional conditions. In addition, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities were measured for comparison with the oxidative reactions of the cycle; hexokinase, glucokinase and phosphoglucose isomerase activities were also included. Starvation for 2 days caused significant lowering of activity of all the enzymes of the pentose phosphate cycle based on activity in the whole liver. Re-feeding with a high-carbohydrate diet restored all the enzyme activities to the range of the control values with the exception of that of glucose 6-phosphate dehydrogenase, which showed the well-known ;overshoot' effect. Re-feeding with a high-fat diet also restored the activities of all the enzymes of the pentose phosphate cycle and of hexokinase; glucokinase activity alone remained unchanged. Expressed as units/g. of liver or units/mg. of protein hexokinase, glucose 6-phosphate dehydrogenase, transketolase and pentose phosphate isomerase activities were unchanged by starvation; both 6-phosphogluconate dehydrogenase and ribulose 5-phosphate epimerase activities decreased faster than the liver weight or protein content. 2. Alloxan-diabetes resulted in a decrease of approx. 30-40% in the activities of 6-phosphogluconate dehydrogenase, ribose 5-phosphate isomerase, ribulose 5-phosphate epimerase and transketolase; in contrast with this glucose 6-phosphate dehydrogenase, transaldolase and phosphoglucose isomerase activities were unchanged. Treatment of alloxan-diabetic rats with protamine-zinc-insulin for 3 days caused a very marked increase to above normal levels of activity in all the enzymes of the pentose phosphate pathway except ribulose 5-phosphate epimerase, which was restored to the control value. Hexokinase activity was also raised by this treatment. After 7 days treatment of alloxan-diabetic rats with protamine-zinc-insulin the enzyme activities returned towards the control values. 3. In adrenalectomized rats the two most important changes were the rise in hexokinase activity and the fall in transketolase activity; in addition, ribulose 5-phosphate epimerase activity was also decreased. These effects were reversed by cortisone treatment. In addition, in cortisone-treated adrenalectomized rats glucokinase activity was significantly lower than the control value. 4. In thyroidectomized rats both ribose 5-phosphate isomerase and transketolase activities were decreased; in contrast with this transaldolase activity did not change significantly. Hypophysectomy caused a 50% fall in transketolase activity that was partially reversed by treatment with thyroxine and almost fully reversed by treatment with growth hormone for 8 days. 5. The results are discussed in relation to the hormonal control of the non-oxidative reactions of the pentose phosphate cycle, the marked changes in transketolase activity being particularly outstanding.     

H+ production by antimycin-inhibited mitochondria.

1. The maximum activity of hexokinase in lymphocytes is similar to that of 6-phosphofructokinase, but considerably greater than that of phosphorylase, suggesting that glucose rather than glycogen is the major carbohydrate fuel for these cells. Starvation increased slightly the activities of some of the glycolytic enzymes. A local immunological challenge in vivo (a graft-versus-host reaction) increased the activities of hexokinase, 6-phosphofructokinase, pyruvate kinase and lactate dehydrogenase, confirming the importance of the glycolytic pathway in cell division. 2. The activities of the ketone-body-utilizing enzymes were lower than those of hexokinase or 6-phosphofructokinase, unlike in muscle and brain, and were not affected by starvation. It is suggested that the ketone bodies will not provide a quantitatively important alternative fuel to glucose in lymphocytes. 3. Of the enzymes of the tricarboxylic acid cycle whose activities were measured, that of oxoglutarate dehydrogenase was the lowest, yet its activity (about 4.0μmol/min per g dry wt. at 37°C) was considerably greater than the flux through the cycle (0.5μmol/min per g calculated from oxygen consumption by incubated lymphocytes). The activity was decreased by starvation, but that of citrate synthase was increased by the local immunological challenge in vivo. It is suggested that the rate of the cycle would increase towards the capacity indicated by oxoglutarate dehydrogenase in proliferating lymphocytes. 4. Enzymes possibly involved in the pathway of glutamine oxidation were measured in lymphocytes, which suggests that an aminotransferase reaction(s) (probably aspartate aminotransferase) is important in the conversion of glutamate into oxoglutarate rather than glutamate dehydrogenase, and that the maximum activity of glutaminase is markedly in excess of the rate of glutamine utilization by incubated lymphocytes. The activity of glutaminase is increased by both starvation and the local immunological challenge in vivo. This last finding suggests that metabolism of glutamine via glutaminase is important in proliferating lymphocytes.     

Glycolysis at the climacteric of bananas.

This work was carried out to investigate the relative roles of phosphofructokinase and pyrophosphate-fructose-6-phosphate 1-phosphotransferase during the increased glycolysis at the climacteric in ripening bananas (Musa cavendishii Lamb ex Paxton). Fruit were ripened in the dark in a continuous stream of air in the absence of ethylene. CO2 production, the contents of glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, phosphoenolpyruvate and PPi; and the maximum catalytic activities of pyrophosphate-fructose-6-phosphate 1-phosphotransferase, 6-phosphofructokinase, pyruvate kinase and phosphoenolpyruvate carboxylase were measured over a 12-day period that included the climacteric. Cytosolic fructose-1,6- bisphosphatase could not be detected in extracts of climacteric fruit. The peak of CO2 production was preceded by a threefold rise in phosphofructokinase, and accompanied by falls in fructose 6-phosphate and glucose 6-phosphate, and a rise in fructose 1,6-bisphosphate. No change in pyrophosphate-fructose-6-phosphate 1-phosphotransferase or pyrophosphate was found. It is argued that phosphofructokinase is primarily responsible for the increased entry of fructose 6-phosphate into glycolysis at the climacteric.     

Distribution of some enzymes concerning carbohydrate metabolism in sea urchin eggs

As an aid to the elucidation of the mechanism of activation of glycolysis upon fertilization, the activity and the distribution of the enzymes concerned were measured in unfertilized and fertilized eggs of Hemicentrotus pulcherrimus and Pseudocentrotus depressus. The enzymes investigated were phosphorylase, exo-1,4-α-glucosidase, hexokinase, phosphoglucomutase, glucose-6-phosphate dehydrogenase, glucosephosphate isomerase, 6-phosphofructokinase, hexosediphosphatase, fructose-bisphosphate aldolase, pyruvate kinase, and lactate dehydrogenase.Phosphorylase and pyruvate kinase were the enzymes which were activated upon fertilization. Glucose-6-phosphate dehydrogenase and a part of aldolase changed their distribution from the particulate to the soluble fraction upon fertilization. Advantages of enzyme activation over changes in enzyme distribution upon fertilization were discussed as a mechanism for the fertilization-induced activation of glycolysis.     

Enzyme histochemistry of the sheep uterus during the oestrous cycle

Enzyme histochemical techniques were applied to frozen sheep uteri from different stages of the oestrous cycle. The localization and activities of succinate, lactate, glucose-6-phosphate, and isocitrate (NADP+) dehydrogenases and acid and alkaline phosphatases were studied in the luminal and glandular epithelia, caruncle and myometrium. Enzyme activity in the sections was scored on a scale of 0--5. In general the enzyme activity in the uterine caruncles and epithelia was higher than in the myometrium. The myometrium did not show any alkaline phosphatase activity and isocitrate dehydrogenase (NADP+) activity was negligible. The low activities of acid phosphatase and lactate dehydrogenase and the moderate levels of glucose-6-phosphate and succinate dehydrogenases in the myometrium were constant. The caruncular tissue showed high levels of phosphatases and glucose-6-phosphate dehydrogenase, moderate levels of lactate and succinate dehydrogenases, and low levels of isocitrate dehydrogenase (NADP+) throughout the oestrous cycle. Much lower phosphatase and isocitrate dehydrogenase (NADP+) levels were found in the epithelium of deep glands compared with superficial glands. The high activity of acid and alkaline phosphatases in the luminal epithelium and the superficial glands was constant from mid-cycle to ovulation, but a significant decrease was observed immediately after ovulation. The level of dehydrogenases in epithelia was generally high and did not change during the oestrous cycle.     

Maximal activities of hexokinase, 6-phosphofructokinase,oxoglutarate dehydrogenase,and carnitine palmitoyltransferase in rat and avian muscles

The maximum activities of 6-phosphofructokinase and oxoglutarate dehydrogenase in muscle provide quantitative indices of the maximum capacities of anaerobic glycolysis and the Krebs cycle (i.e. the aerobic capacity) respectively. These activities were measured in red, white, and cardiac muscle of birds and the rat. The activities in the white pectoral muscle of the domestic fowl suggest that the Krebs cycle plus electron transfer could provide only about 1% of the rate of ATP production provided by anaerobic glycolysis whereas in pigeon pectoral muscle the predicted maximal rates from the two processes are similar. In contrast to domestic-fowl pectoral muscle, the white rat muscle, epitrochlearis, contains a significant activity of oxoglutarate dehydrogenase, which indicates that the Krebs cycle could provide about 12% of the maximum rate of ATP formation. This may be explained by a higher proportion of type-I and -IIA fibres in the rat muscle compared to the avian muscle. In the aerobic muscles of the rat the maximum activities of carnitine palmitoyl transferase indicate that fatty-acid oxidation could provide a high rate of ATP formation.     

Irreversible transitions in the 6-phosphofructokinase/fructose 1,6-bisphosphatase cycle.

The dynamics of the fructose 6-phosphate fructose-1,6-bisphosphate cycle operating in an open and homogeneous system reconstituted from purified enzymes was extensively studied. In addition to 6-phosphofructokinase and fructose-1,6-bisphosphatase, pyruvate kinase, adenylate kinae and glucose-6-phosphate isomerase were involved. In that multi-enzyme system, the main source of non-linearity is the reciprocal effect of AMP on the activities of 6-phosphofructokinase and fructose-1,6-bisphosphatase. Depending upon the experimental parameter values, stable attractors, various types of multiple states and sustained oscillations were shown to occur. In the present report we show that irreversible transitions are also likely to occur for realistic operating conditions. Two parameters of the system, that is the adenylate energy charge of the influx and the fructose-1,6-bisphosphatase maximal activity, are potential candidates to provoke such irreversible transitions from one steady state to the other: (a) when varying the maximal activity of fructose-1,6-bisphosphatase, the system can jump irreversibly from a low to a high stable steady state, and (b) when the adenylate energy charge of the influx is the changing parameter, irreversible transitions occur from a high stable steady state to a stable oscillatory state (limit cycle motion). This behavior can be predicted by constructing the loci of limit points and Hopf bifurcation points.     

Regulation of glucose, fructose and sucrose catabolism in Rhodopseudomonas capsulata.

Regulation of glucose, fructose and sucrose catabolism was studied in Rhodopseudomonas capsulata grown under phototrophic conditions. The sequence of preference for the utilization of the sugar substrates was fructose, glucose, sucrose. The presence of a preferred substrate did not completely suppress the utilization of the less preferred. Glucose-6-phosphate dehydrogenase, the key enzyme of glucose and sucrose catabolism, exhibited sigmoidal substrate saturation curves and was inhibited by phosphoenolpyruvate, whereas 1-phosphofructokinase, the key enzyme of fructose catabolism, exhibited hyperbolic substrate saturation curves and was not inhibited by phosphoenolpyruvate. Since phosphoenolpyruvate is a common intermediate of glucose, fructose and sucrose catabolism, the control of glucose-6-phosphate dehydrogenase may be responsible for the preferential utilization of fructose.     

Mannitol oxidation in two Micromonospora isolates and in representative species of other actinomycetes.

Mannitol kinase and mannitol-1-phosphate dehydrogenase activities were detected in two Micromonospora isolates. The presence of these enzyme activities indicates that mannitol is catabolized first to mannitol-1-phosphate and then to fructose-6-phosphate. Mannitol-oxidizing enzymes were also surveyed in representative species of four other genera of actinomycetes. Mannitol-1-phosphate dehydrogenase was detected in cell-free extracts of Streptomyces lactamdurans. In contrast, cell-free extracts of Mycobacterium smegmatis, Nocardia erythrophila, Streptomyces lavendulae, and Actinoplanes missouriensis contained mannitol dehydrogenase activity but no detectable mannitol-1-phosphate dehydrogenase activity. The mannitol dehydrogenase activities in the latter species support the operation of a pathway for catabolism of mannitol that involves the oxidation of mannitol to fructose, followed by phosphorylation to fructose-6-phosphate.     

Mannitol oxidation in two Micromonospora isolates and in representative species of other actinomycetes.

Mannitol kinase and mannitol-1-phosphate dehydrogenase activities were detected in two Micromonospora isolates. The presence of these enzyme activities indicates that mannitol is catabolized first to mannitol-1-phosphate and then to fructose-6-phosphate. Mannitol-oxidizing enzymes were also surveyed in representative species of four other genera of actinomycetes. Mannitol-1-phosphate dehydrogenase was detected in cell-free extracts of Streptomyces lactamdurans. In contrast, cell-free extracts of Mycobacterium smegmatis, Nocardia erythrophila, Streptomyces lavendulae, and Actinoplanes missouriensis contained mannitol dehydrogenase activity but no detectable mannitol-1-phosphate dehydrogenase activity. The mannitol dehydrogenase activities in the latter species support the operation of a pathway for catabolism of mannitol that involves the oxidation of mannitol to fructose, followed by phosphorylation to fructose-6-phosphate.     

Evidence for a higher glycolytic than oxidative metabolic activity in white matter of rat brain

Different values exist for glucose metabolism in white matter; it appears higher when measured as accumulation of 2-deoxyglucose than when measured as formation of glutamate from isotopically labeled glucose, possibly because the two methods reflect glycolytic and tricarboxylic acid (TCA) cycle activities, respectively. We compared glycolytic and TCA cycle activity in rat white structures (corpus callosum, fimbria, and optic nerve) to activities in parietal cortex, which has a tight glycolytic-oxidative coupling. White structures had an uptake of [(3)H]2-deoxyglucose in vivo and activities of hexokinase, glucose-6-phosphate isomerase, and lactate dehydrogenase that were 40-50% of values in parietal cortex. In contrast, formation of aspartate from [U-(14)C]glucose in awake rats (which reflects the passage of (14)C through the whole TCA cycle) and activities of pyruvate dehydrogenase, citrate synthase, alpha-ketoglutarate dehydrogenase, and fumarase in white structures were 10-23% of cortical values, optic nerve showing the lowest values. The data suggest a higher glycolytic than oxidative metabolism in white matter, possibly leading to surplus formation of pyruvate or lactate. Phosphoglucomutase activity, which interconverts glucose-6-phosphate and glucose-1-phosphate, was similar in white structures and parietal cortex ( approximately 3 nmol/mg tissue/min), in spite of the lower glucose uptake in the former, suggesting that a larger fraction of glucose is converted into glucose-1-phosphate in white than in gray matter. However, the white matter glycogen synthase level was only 20-40% of that in cortex, suggesting that not all glucose-1-phosphate is destined for glycogen formation.