The Warburg effect: a balance of flux analysis

Cancer metabolism is characterized by increased macromolecular syntheses through coordinated increases in energy and substrate metabolism. The observation that cancer cells produce lactate in an environment of oxygen sufficiency (aerobic glycolysis) is a central theme of cancer metabolism known as the Warburg effect. Aerobic glycolysis in cancer metabolism is accompanied by increased pentose cycle and anaplerotic activities producing energy and substrates for macromolecular synthesis. How these processes are coordinated is poorly understood. Recent advances have focused on molecular regulation of cancer metabolism by oncogenes and tumor suppressor genes which regulate numerous enzymatic steps of central glucose metabolism. In the past decade, new insights in cancer metabolism have emerged through the application of stable isotopes particularly from ¹³C carbon tracing. Such studies have provided new evidence for system-wide changes in cancer metabolism in response to chemotherapy. Interestingly, experiments using metabolic inhibitors on individual biochemical pathways all demonstrate similar system-wide effects on cancer metabolism as in targeted therapies. Since biochemical reactions in the Warburg effect place competing demands on available precursors, high energy phosphates and reducing equivalents, the cancer metabolic system must fulfill the condition of balance of flux (homeostasis). In this review, the functions of the pentose cycle and of the tricarboxylic acid (TCA) cycle in cancer metabolism are analyzed from the balance of flux point of view. Anticancer treatments that target molecular signaling pathways or inhibit metabolism alter the invasive or proliferative behavior of the cancer cells by their effects on the balance of flux (homeostasis) of the cancer metabolic phenotype.

Gleevec (STI571) influences metabolic enzyme activities and glucose carbon flow toward nucleic acid and fatty acid synthesis in myeloid tumor cells

Chronic myeloid leukemia cells contain a constitutively active Bcr-Abl tyrosine kinase, the target protein of Gleevec (STI571) phenylaminopyrimidine class protein kinase inhibitor. Here we provide evidence for metabolic phenotypic changes in cultured K562 human myeloid blast cells after treatment with increasing doses of STI571 using [1,2-13C2]glucose as the single tracer and biological mass spectrometry. In response to 0.68 and 6.8 microm STI571, proliferation of Bcr-Abl-positive K562 cells showed a 57% and 74% decrease, respectively, whereas glucose label incorporation into RNA decreased by 13.4% and 30.1%, respectively, through direct glucose oxidation, as indicated by the decrease in the m1/Sigma(m)n ratio in RNA. Based on the in vitro proliferation data, the IC50 of STI571 in K562 cultures is 0.56 microm. The decrease in 13C label incorporation into RNA ribose was accompanied by a significant fall in hexokinase and glucose-6-phosphate 1-dehydrogenase activities. The activity of transketolase, the enzyme responsible for nonoxidative ribose synthesis in the pentose cycle, was less affected, and there was a relative increase in glucose carbon incorporation into RNA through nonoxidative synthesis as indicated by the increase in the m2/Sigma(m)n ratio in RNA. The restricted use of glucose carbons for de novo nucleic acid and fatty acid synthesis by altering metabolic enzyme activities and pathway carbon flux of the pentose cycle constitutes the underlying mechanism by which STI571 inhibits leukemia cell glucose substrate utilization and growth. The administration of specific hexokinase/glucose-6-phosphate 1-dehydrogenase inhibitor anti-metabolite substrates or competitive enzyme inhibitor compounds, alone or in combination, should be explored for the treatment of STI571-resistant advanced leukemias as well as that of Bcr-Abl-negative human malignancies.

The effect of thiamine supplementation on tumour proliferation. A metabolic control analysis study

Thiamine deficiency frequently occurs in patients with advanced cancer and therefore thiamine supplementation is used as nutritional support. Thiamine (vitamin B1) is metabolized to thiamine pyrophosphate, the cofactor of transketolase, which is involved in ribose synthesis, necessary for cell replication. Thus, it is important to determine whether the benefits of thiamine supplementation outweigh the risks of tumor proliferation. Using oxythiamine (an irreversible inhibitor of transketolase) and metabolic control analysis (MCA) methods, we measured an in vivo tumour growth control coefficient of 0.9 for the thiamine-transketolase complex in mice with Ehrlich's ascites tumour. Thus, transketolase enzyme and thiamine clearly determine cell proliferation in the Ehrlich's ascites tumour model. This high control coefficient allows us to predict that in advanced tumours, which are commonly thiamine deficient, supplementation of thiamine could significantly increase tumour growth through transketolase activation. The effect of thiamine supplementation on tumour proliferation was demonstrated by in vivo experiments in mice with the ascites tumour. Thiamine supplementation in doses between 12.5 and 250 times the recommended dietary allowance (RDA) for mice were administered starting on day four of tumour inoculation. We observed a high stimulatory effect on tumour growth of 164% compared to controls at a thiamine dose of 25 times the RDA. This growth stimulatory effect was predicted on the basis of correction of the pre-existing level of thiamine deficiency (42%), as assayed by the cofactor/enzyme ratio. Interestingly, at very high overdoses of thiamine, approximately 2500 times the RDA, thiamine supplementation had the opposite effect and caused 10% inhibition of tumour growth. This effect was heightened, resulting in a 36% decrease, when thiamine supplementation was administered from the 7th day prior to tumour inoculation. Our results show that thiamine supplementation sufficient to correct existing thiamine deficiency stimulates tumour proliferation as predicted by MCA. The tumour inhibitory effect at high doses of thiamine is unexplained and merits further study.

Role of thiamin (vitamin B-1) and transketolase in tumor cell proliferation

Metabolic control analysis predicts that stimulators of transketolase enzyme synthesis such as thiamin (vitamin B-1) support a high rate of nucleic acid ribose synthesis necessary for tumor cell survival, chemotherapy resistance, and proliferation. Metabolic control analysis also predicts that transketolase inhibitor drugs will have the opposite effect on tumor cells. This may have important implications in the nutrition and future treatment of patients with cancer.

Genistein inhibits nonoxidative ribose synthesis in MIA pancreatic adenocarcinoma cells: a new mechanism of controlling tumor growth

Genistein is a plant isoflavonoid bearing potent tumor growth-regulating characteristics. This effect of genistein has been attributed partially to its tyrosine kinase-regulating properties, resulting in cell-cycle arrest and limited angiogenesis. Genistein has been used in chemotherapy-resistant cases of advanced leukemia with promising results. Here we demonstrate that genistein primarily affects nucleic acid synthesis and glucose oxidation in tumor cells using the [1,2-(13)C2]glucose isotope as the single tracer and gas chromatography/mass spectrometry to follow various intracellular glucose metabolites. The ribose fraction of RNA demonstrated a rapid 4.6%, 16.4%, and 46.3% decrease in isotope uptake through the nonoxidative branch of the pentose cycle and a sharp 4.8%. 24.6%, and 48% decrease in 13CO2 release from glucose after 2, 20, and 200 micromol/L genistein treatment, respectively. Fatty acid synthesis and the 13C enrichment of acetyl units were not significantly affected by genistein treatment. De novo glycogen synthesis from media glucose was not detected in cultured MIA cells. It can be concluded from these studies that genistein controls tumor growth primarily through the regulation of glucose metabolism, specifically targeting glucose carbon incorporation into nucleic acid ribose through the nonoxidative steps of the pentose cycle, which represents a new paradigm for the antiproliferative action of a plant phytochemical.

Loss of [13C]glycerol carbon via the pentose cycle. Implications for gluconeogenesis measurement by mass isotoper distribution analysis

Whereas many reports substantiated the suitability of using [2-(13)C]glycerol and Mass Isotoper Distribution Analysis for gluconeogenesis, the use of [(13)C]glycerol had been shown to give lower estimates of gluconeogenesis (GNG). The reason for the underestimation has been attributed to asymmetric isotope incorporation during gluconeogenesis as well as zonation of gluconeogenic enzymes and a [(13)C]glycerol gradient across the liver. Since the cycling of glycerol carbons through the pentose cycle pathways can introduce asymmetry in glucose labeling pattern and tracer dilution, we present here a study of the role of the pentose cycle in gluconeogenesis in Fao cells. The metabolic regulation of glucose release and gluconeogenesis by insulin was also studied. Serum-starved cells were incubated for 24 h in Dulbecco's modified Eagle's media containing 1.5 mm [U-(13)C]glycerol. Mass isotopomers of whole glucose from medium or glycogen and those of the C-1-C-4 fragment were highly asymmetrical, typical of that resulting from the cycling of glucose carbon through the pentose cycle. Substantial exchange of tracer between hexose and pentose intermediates was observed. Our results offer an alternative mechanism for the asymmetrical labeling of glucose carbon from triose phosphate. The scrambling of (13)C in hexose phosphate via the pentose phosphate cycle prior to glucose release into the medium is indistinguishable from dilution of labeled glucose by glycogen using MIDA and probably accounts for the underestimation of GNG using (13)C tracer methods.

Loss of regulation of lipogenesis in the Zucker diabetic (ZDF) rat

We present here a study on the role of leptin in the regulation of lipogenesis by examining the effect of dietary macronutrient composition on lipogenesis in the leptin receptor-defective Zucker diabetic fatty rat (ZDF) and its lean litter mate (ZL). Animals were pair fed two isocaloric diets differing in their fat-to-carbohydrate ratio providing 10 and 30% energy as fat. Lipogenesis was measured in the rats using deuterated water and isotopomer analysis. From the deuterium incorporation into plasma palmitate, stearate, and oleate, we determined de novo synthesis of palmitate and synthesis of stearate by chain elongation and of oleate by desaturation. Because the macronutrient composition and the caloric density were controlled, changes in de novo lipogenesis under these dietary conditions represent adaptation to changes in the fat-to-carbohydrate ratio of the diet. De novo lipogenesis was normally suppressed in response to the high-fat diet in the ZL rat to maintain a relatively constant amount of lipids transported. The ZDF rat had a higher rate of lipogenesis, which was not suppressed by the high-fat diet. The results suggest an important hormonal role of leptin in the feedback regulation of lipogenesis.

Transforming growth factor beta2 promotes glucose carbon incorporation into nucleic acid ribose through the nonoxidative pentose cycle in lung epithelial carcinoma cells

The invasive transformation of A-459 lung epithelial carcinoma cells has been linked to the autocrine regulation of malignant phenotypic changes by transforming growth factor beta (TGF-beta). Here we demonstrate, using stable 13C glucose isotopes, that the transformed phenotype is characterized by decreased CO2 production via direct glucose oxidation but increased nucleic acid ribose synthesis through the nonoxidative reactions of the pentose cycle. Increased nucleic acid synthesis through the nonoxidative pentose cycle imparts the metabolic adaptation of nontransformed cells to the invasive phenotype that potentially explains the fundamental metabolic disturbance in tumor cells: highly increased nucleic acid synthesis despite hypoxia and decreased glucose oxidation.

Oxythiamine and dehydroepiandrosterone induce a G1 phase cycle arrest in Ehrlich's tumor cells through inhibition of the pentose cycle

Transketolase (TK) reactions play a crucial role in tumor cell nucleic acid ribose synthesis utilizing glucose carbons, yet, current cancer treatments do not target this central pathway. Experimentally, a dramatic decrease in tumor cell proliferation after the administration of the TK inhibitor oxythiamine (OT) was observed in several in vitro and in vivo tumor models. Here, we demonstrate that pentose cycle (PC) inhibitors, OT and dehydroepiandrosterone (DHEA), efficiently regulate the cell cycle and tumor proliferation processes. Increasing doses of OT or DHEA were administered by daily intraperitoneal injections to Ehrlich's ascites tumor hosting mice for 4 days. The tumor cell number and their cycle phase distribution profile were determined by DNA flow histograms. Tumors showed a dose dependent increase in their G0-G1 cell populations after both OT and DHEA treatment and a simultaneous decrease in cells advancing to the S and G2-M cell cycle phases. This effect of PC inhibitors was significant, OT was more effective than DHEA, both drugs acted synergistically in combination and no signs of direct cell or host toxicity were observed. Direct inhibition of PC reactions causes a G1 cell cycle arrest similar to that of 2-deoxyglucose treatment. However, no interference with cell energy production and cell toxicity is observed. PC inhibitors, specifically ones targeting TK, introduce a new target site for the development of future cancer therapies to inhibit glucose utilizing pathways selectively for nucleic acid production.

Fatty acid cycling in human hepatoma cells and the effects of troglitazone

Fatty acid cycling by chain shortening/elongation in the peroxisomes is an important source of fatty acids for membrane lipid synthesis. Its role in the homeostasis of nonessential fatty acids is poorly understood. We report here a study on the cycling of saturated fatty acids and the effects of troglitazone in HepG2 cells in culture using [U-13C]stearate or [U-13C]oleate and mass isotopomer analysis. HepG2 cells were grown in the presence of 0.7 mmol/liter [U-13C]stearate or [U-13C]oleate, and in the presence and absence of 50 microM troglitazone for 72 h. Fatty acids extracted from cell pellets after saponification were analyzed by gas chromatography/mass spectrometry. Peroxisomal beta-oxidation of uniformly 13C-labeled stearate (C18:0) and oleate (C18:1) resulted in chain shortening and produced uniformly labeled palmitate (C16:0) and palmitoleate (C16:1). In untreated cells, 16% of C16:0 was derived from C18:0 and 26% of C16:1 from C18:1 by chain shortening. Such contributions were significantly increased by troglitazone to 23.6 and 36.6%, respectively (p < 0.001). Desaturation of stearate contributed 67% of the oleate, while reduction of oleate contributed little to stearate (2%). The desaturation of C18:0 to C18:1 was not affected by troglitazone. Our results demonstrated a high degree of recycling of C18:0 and C18:1 to C16:0 and C16:1 through chain shortening and desaturation. Chain shortening was accompanied by chain elongation in the synthesis of other long chain fatty acids. Troglitazone specifically increased recycling by peroxisomal beta-oxidation of C18 to C16 fatty acids, and the interconversion of long chain fatty acids was associated with reduced de novo lipogenesis.

Calibration of isotope ratio mass spectrometry working standard for 2H/1H ratio analysis

A method is described for the calibration of an isotope ratio mass spectrometry working standard for 2H/1H analysis using highly purified (> 99.99% enriched) deuterium oxide (2H2O). Serial dilution of such a compound can be used to construct a standard curve for calibration purposes using the relationship [formula: see text] where APE is the atom percent excess. It was found that the determined rWR had a precision of +/- 1%. Most of the variation came from sample handling, which includes the serial dilution of the weighed deuterium oxide, the reduction of water to hydrogen gas and the introduction of the hydrogen gas into the mass spectrometer. The use of this calibration procedure allows the direct determination of the isotope ratio rSA, and APE from a standard curve over a wide range of enrichment. Unlike the reference standards V-SMOW and SLAP, the highly purified deuterium oxide is non-exhaustible and readily available. The highly purified deuterium oxide should be used in place of secondary standards to calibrate working references for 2H/1H analysis in biological studies.

Mass isotopomer study of the nonoxidative pathways of the pentose cycle with [1,2-13C2]glucose

We present a single-tracer method for the study of the pentose phosphate pathway (PPP) using [1,2-13C2]glucose and mass isotopomer analysis. The metabolism of [1,2-13C2]glucose by the glucose-6-phosphate dehydrogenase, transketolase (TK), and transaldolase (TA) reactions results in unique pentose and lactate isotopomers with either one or two 13C substitutions. The distribution of these isotopomers was used to estimate parameters of the PPP using the model of Katz and Rognstad (J. Katz and R. Rognstad. Biochemistry 6: 2227-2247, 1967). Mass and position isotopomers of ribose, and lactate and palmitate (products from triose phosphate) from human hepatoma cells (Hep G2) incubated with 30% enriched [1,2-13C2]glucose were determined using gas chromatography-mass spectrometry. After 24-72 h incubation, 1.9% of lactate molecules in the medium contained one 13C substitution (m1) and 10% contained two 13C substitutions (m2). A similar m1-to-m2 ratio was found in palmitate as expected. Pentose cycle (PC) activity determined from incubation with [1,2-13C2]glucose was 5.73 +/- 0.52% of the glucose flux, which was identical to the value of PC (5.55 +/- 0.73%) determined by separate incubations with [1-13C] and [6-13C]glucose, 13C was found to be distributed in four ribose isotopomers ([1-13C]-, [5-13C]-, [1,2-13C2]-, and [4,5-13C2]ribose). The observed ribose isotopomer distribution was best matched with that provided from simulation by substituting 0.032 for TK and 0.85 for TA activity relative to glucose uptake into the model of Katz and Rognstad. The use of [1,2-13C2]glucose not only permits the determination of PC but also allows estimation of relative rates through the TK and TA reactions.

Fitness and the effect of exercise training on the dietary intake of healthy adolescents

OBJECTIVE

In healthy, nonobese, adolescent males and females to: (1) Determine the relationship between fitness and energy intake; (2) assess the effect of five-weeks endurance training on energy intake and food choice and (3) compare food record assessments of energy intake with doubly-labeled water (DLW) measurement of total energy expenditure (TEE).

DESIGN

(1) Cross sectional analysis of fitness and food intake and (2) Prospective, randomized, controlled interventional study of endurance-type exercise training in 44 females and 44 males (age range, 15-17 y).

MEASUREMENTS

Pre and end interventional three day food records were successfully collected from 32 females (15 controls, 17 trained) and 39 males (19 controls, 20 trained). Fitness was assessed from cycle ergometry as peak oxygen uptake normalized both to thigh muscle mass and body weight. Thigh muscle mass was measured by magnetic resonance imaging. TEE using the DLW technique was measured in 12 females (6 controls, 6 trained) and 20 males (10 controls, 10 trained) during weeks 4-5 of the exercise training program (simultaneously with the second assessment of food records). Food record data were analyzed using the Minnesota Nutrition Data System.

RESULTS

Fitness was correlated with self reported total caloric intake in males but not females. In females, there was a significant increase in fat intake (19.8+/-9%, P < 0.05) and a significant decrease in carbohydrate intake (-9.8+/-4%) in the trained subjects. No changes were observed in the control subjects. Energy expenditure (2072+/-52 kcal/d) was significantly greater than the estimated energy intake (1520+/-112 kcal/d, P < 0.007) during the intervention in the trained, but not control, subjects. However, there was no weight change in either control or trained subjects. In males, no changes were observed in food choice in either control or trained subjects. Similar to the females, energy expenditure (2425+/-22 kcal/d) was significantly greater than the estimated energy intake (2168+/-117 kcal/d, P < 0.05) during the intervention in the trained, but not control, subjects. No weight changes were observed in either group.

CONCLUSIONS

Fitness is associated with increased self-reported energy intake in males but not females, while exercise training led to alterations in food selection (greater fat and reduced carbohydrate) only in females. These observations could reflect specific gender differences, or, alternatively, the generally lower levels of fitness in the females. The apparent negative energy balance without evidence for weight loss in both the trained males and females suggests a systematic under reporting of food intake during exercise programs in adolescents, and indicates the possibility that errors in self reported food intake might be greater during transitions from one level of energy expenditure to another.

Fatty acid transport and utilization for the developing brain

To determine the transport and utilization of dietary saturated, monounsaturated, and n-6 and n-3 polyunsaturated fatty acids for the developing brain and other organs, artificially reared rat pups were fed a rat milk substitute containing the perdeuterated (each 97 atom% deuterium) fatty acids, i.e., palmitic, stearic, oleic, linoleic, and linolenic, from day 7 after birth to day 14 as previously described. Fatty acids in lipid extracts of the liver, lung, kidney, and brain were analyzed by gas chromatography-mass spectrometry to determine their content of each of the deuterated fatty acids. The uptake and metabolism of perdeuterated fatty acid lead to the appearance of three distinct groups of isotopomers: the intact perdeuterated, the newly synthesized (with recycled deuterium), and the natural unlabeled fatty acid. The quantification of these isotopomers permits the estimation of uptake and de novo synthesis of these fatty acids. Intact perdeuterated palmitic, stearic, and oleic acids from the diet were found in liver, lung, and kidney, but not in brain. By contrast, perdeuterated linoleic acid was found in all these organs. Isotopomers of fatty acid from de novo synthesis were observed in palmitic, oleic, and stearic acids in all tissues. The highest enrichment of isotopomers with recycled deuterium was found in the brain. The data indicate that, during the brain growth spurt and the prelude to myelination, the major saturated and monounsaturated fatty acids in brain lipids are exclusively produced locally by de novo biosynthesis. Consequently, the n-6 and n-3 polyunsaturated fatty acids must be transported and delivered to the brain by highly specific mechanisms.

Thiamine supplementation to cancer patients: a double edged sword

The objectives of this review are to (a) explain the mechanism by which thiamine (vitamin B1) promotes nucleic acid ribose synthesis and tumor cell proliferation via the nonoxidative transketolase (TK) pathway; (b) estimate the thiamine intake of cancer patients and (c) provide background information and to develop guidelines for alternative treatments with antithiamine transketolase inhibitors in the clinical setting. Clinical and experimental data demonstrate increased thiamine utilization of human tumors and its interference with experimental chemotherapy. Analysis of RNA ribose indicates that glucose carbons contribute to over 90% of ribose synthesis in cultured cervix und pancreatic carcinoma cells and that ribose is synthesized primarily through the thiamine dependent TK pathway (> 70%). Antithiamine compounds significantly inhibit nucleic acid synthesis and tumor cell proliferation in vitro and in vivo in several tumor models. The medical literature reveals little information regarding the role of the thiamine dependent TK reaction in tumor cell ribose production which is a central process in de novo nucleic acid synthesis and the salvage pathways for purines. Consequently, current thiamine administration protocols oversupply thiamine by 200% to 20,000% of the recommended dietary allowance, because it is considered harmless and needed by cancer patients. The thiamine dependent TK pathway is the central avenue which supplies ribose phosphate for nucleic acids in tumors and excessive thiamine supplementation maybe responsible for failed therapeutic attempts to terminate cancer cell proliferation. Limited administration of thiamine and concomitant treatment with transketolase inhibitors is a more rational approach to treat cancer.

Oxythiamine and dehydroepiandrosterone inhibit the nonoxidative synthesis of ribose and tumor cell proliferation

This study investigates the significance of the glucose-6-phosphate dehydrogenase (G6PD) catalyzed oxidative and the transketolase (TK) catalyzed nonoxidative pentose cycle (PC) reactions in the tumor proliferation process by characterizing tumor growth patterns and synthesis of the RNA ribose moiety in the presence of respective inhibitors of G6PD and TK. Mass spectra analysis of 13C-labeled carbons revealed that these PC reactions contribute to over 85% of de novo ribose synthesis in RNA from [1,2-(13)C]glucose in cultured Mia pancreatic adenocarcinoma cells, with the fraction synthesized through the TK pathway predominating (85%). Five days of treatment with the TK inhibitor oxythiamine (OT) and the G6PD inhibitor dehydroepiandrosterone-sulfate (0.5 microM each) exerted a 39 and a 23% maximum inhibitory effect on cell proliferation in culture, which was increased to 60% when the two drugs were administered in combination. In vivo testing of 400 mg/kg OT or dehydroepiandrosterone-sulfate in C57BL/6 mice hosting Ehrlich's ascitic tumor cells revealed a 90.4 and a 46% decrease in the final tumor mass after 3 days of treatment. RNA ribose fractional synthesis through the TK reaction using metabolites directly from glycolysis declined by 9.1 and 23.9% after OT or the combined treatment, respectively. Nonoxidative PC reactions play a central regulating role in the carbon-recruiting process toward de novo nucleic acid ribose synthesis and cell proliferation in vitro and in vivo. Therefore, enzymes or substrates regulating the nonoxidative synthesis of ribose could also be the sites to preferentially target tumor cell proliferation by new anticancer drugs.

Feasibility of analysing [13C]urea breath tests for Helicobacter pylori by gas chromatography-mass spectrometry in the selected ion monitoring mode

BACKGROUND

The [13C]urea breath test for Helicobacter pylori is nonradioactive, as well as noninvasive, but few clinical laboratories have the expensive isotope ratio mass spectrometer used for analysis.

METHODS

To demonstrate the feasibility of analysing [13C]urea breath tests with a gas chromatograph-mass spectrometer routinely used for drug testing, 13CO2 standards for breath tests and breath samples from patients in a multiple-blind study were analysed. The breath samples were also analysed by isotope ratio mass spectrometry, and the diagnoses were compared with biopsy results.

RESULTS

The precision of the enrichment measurements by gas chromatography-mass spectrometry was 1.1 parts per thousand, and the calculated differences in enrichment between standard gases equaled the certified values. The sensitivity (94%), specificity (94%), and percentage agreement (94%) for diagnosis of Helicobacter pylori (n = 34) were as high or higher than for analysis of replicate breath samples by isotope ratio mass spectrometry and comparable to the values reported for diagnosis of the bacterium by other currently accepted tests.

CONCLUSIONS

The study demonstrates that a gas chromatograph-mass spectrometer can be used to analyse [13C]urea breath tests, thus potentially lowering the cost of the test and increasing the number of laboratories that can perform the test.

Effect of exercise training on energy expenditure, muscle volume, and maximal oxygen uptake in female adolescents

OBJECTIVES

American female adolescents are at high risk of a physically inactive lifestyle that likely leads to health problems later in life. We hypothesized that a brief program of endurance exercise training in female adolescents would result in increased energy expenditure and quantifiable structural and functional adaptations.

STUDY DESIGN

Forty-four high school girls (aged 15 to 17 years, none were elite athletes) enrolled in a 5-day per week anatomy class for 5 weeks and were randomly assigned to control (n = 22) and training groups. All subjects participated in a 2-hour daily teaching program. During the remaining time (2 hours), the training group members underwent endurance-type training and control group subjects participated in a computer workshop. The intervention was assessed by (1) comparison of total energy expenditure between groups with the doubly labeled water technique, (2) determination of changes in thigh muscle volume by magnetic resonance imaging, and (3) determination of changes in maximal oxygen uptake by use of respiratory gas exchange responses.

RESULTS

Total energy expenditure was significantly greater (15.3%) in the training group compared with the control subjects (p < 0.003). Five weeks of training led to a 4.3% +/- 1% increase in thigh muscle volume (p < 0.0002) and a 12.1% +/- 3.7% increase in maximal oxygen uptake (p < 0.004); there were no changes in the control group. The training effect was most pronounced in the least fit subjects.

CONCLUSIONS

Exercise training programs for female adolescents can be successfully integrated into a high school summer curriculum. Quantifiable, substantial structural and functional responses occur with relatively short periods of training. Approximately 60% of the training response was related to factors independent of muscle size per se. These data may serve to better design physical activity programs for female adolescents.

Mass isotopomer study of glutamine oxidation and synthesis in primary culture of astrocytes

The metabolism of [1, 2-13C2] acetate via the tricarboxylic acid (TCA) cycle leads to the formation of a number of key mass isotopomers of glutamate. The distribution of these isotopomers which is a function of pyruvate carboxylase, pyruvate dehydrogenase and pyruvate recycling was used to determine the relative anaplerotic flux and glutamine oxidation of astrocytes in culture under different substrate conditions. Combinatory analysis of mass isotopomers formed from the condensation of labeled oxaloacetate with labeled acetyl-CoA was used to determine precursor enrichment and fractional glutamine synthesis. When glucose or glutamine was supplied in the medium, the effective anaplerotic flux (Y') was about 1.5 times that of the TCA cycle flux. Under substrate-limiting conditions, Y' and glutamine synthesis was significantly reduced. A unique feature of the use of [1, 2-13C2] acetate in this study is the formation of singly labeled isotopomer of glutamine in the C4 or C5 position when glutamine is irreversibly loss in net oxidation. We observed very little [4-13C] or [5-13C] glutamine either because of the lack of pyruvate recycling or the lack of pyruvate dehydrogenase activity. The lack of 13C recycling to the C4 and C5 position of glutamine suggests that less than 10% of the glutamine is oxidized into astrocytes for energy production. Therefore, glutamine is not a major energy substrate for astrocytes in culture.

In vivo study of the biosynthesis of long-chain fatty acids using deuterated water

To determine the contributions of preexisting fatty acid, de novo synthesis, and chain elongation in long-chain fatty acid (LCFA) synthesis, the synthesis of LCFAs, palmitate (16:0), stearate (18:0), arachidate (20:0), behenate (22:0), and lignocerate (24:0), in the epidermis, liver, and spinal cord was determined using deuterated water and mass isotopomer distribution analysis in hairless mice and Sprague-Dawley rats. Animals were given 4% deuterated water for 5 days or 8 wk in their drinking water. Blood was withdrawn at the end of these times for the determination of deuterium enrichment, and the animals were killed to isolate the various tissues for lipid extraction for the determination of the mass isotopomer distributions. The mass isotopomer distributions in LCFA were incompatible with synthesis from a single pool of primer. The synthesis of palmitate, stearate, arachidate, behenate, and lignocerate followed the expected biochemical pathways for the synthesis of LCFAs. On average, three deuterium atoms were incorporated for every addition of an acetyl unit. The isotopomer distribution resulting from chain elongation and de novo synthesis can be described by the linear combination of two binomial distributions. The proportions of preexisting, chain elongation, and de novo-synthesized fatty acids as a percentage of the total fatty acids were determined using multiple linear regression analysis. Fractional synthesis was found to vary, depending on the tissue type and the fatty acid, from 47 to 87%. A substantial fraction (24-40%) of the newly synthesized molecules was derived from chain elongation of unlabeled (recycled) palmitate.

Isotopomer study of lipogenesis in human hepatoma cells in culture: contribution of carbon and hydrogen atoms from glucose

Recent developments in the application of stable isotopes and mass spectrometry have permitted the estimation of precursor enrichment and fractional synthesis of the product through mass isotopomer analysis. Thus, the application of isotopomer analysis in studies with 2H- and 13C-labeled glucose may potentially overcome the limitations of traditional methods which can only estimate the fractional use of carbon and hydrogen from glucose for lipogenesis. To illustrate this approach, isotope incorporation and mass isotopomer distribution were determined in fatty acids and cholesterol from a hepatoma cell line (Hep G2) grown in media containing specific (C1 or C6) 2H- or 13C-labeled glucose. Using the binomial model, the respective precursor enrichment, and fractional synthesis of palmitate, stearate and cholesterol were determined using mass isotopomer distribution analysis. In 1 week, 80% of palmitate, 65.5% of stearate, and 50% of cholesterol molecules in the cell extract were derived from de novo synthesis. Under serum-free condition, glucose contributed about 80% of the carbon of the newly synthesized lipids. Using the relative isotope yield of [1-13C] and [6-13C]glucose and a standard formula, the contribution of the pentose pathway to glucose catabolism was calculated to be 4.7%. Fractional syntheses of palmitate, stearate, and cholesterol determined using [1-2H]glucose agreed well with values determined using 13C-labeled glucose. After correcting for the contribution of deuterium label from the glycolytic pathway, the deuterium from [1-2H]glucose contributed 4.7% of the total reducing equivalents for lipogenesis. Unlike radioisotope studies, the stable isotope approach provides information from the perspective of the product and insight into the economy of acetyl units and reducing equivalents which were otherwise not available.

Correlation of spinal mobility with the severity of chronic lower back pain

The purpose of this study is to show the correlation between the range of spinal motion and the severity of chronic lower back pain. The subjects of this study were 40 female patients with chronic lower back pain over a 6 months' duration. The range of spinal and hip joint motion was measured with a electrogoniometer, and the severity of back pain was evaluated with the Rolland's score and Pollard's pain disability index. Results were as follows. There was a correlation between the severity of pain and the range of lumbar lateral flexion, rotation, and extension (p < 0.05). Age, height, weight and body mass index had no correlation with the range of spinal motion. These results suggest that the range of lumbar spinal motion can be used as an objective measure for the evaluation of classifying chronic lower back pain patients and for planning and following their treatment.

Utilization of aspartate as a nitrogen source in Escherichia coli. Analysis of nitrogen flow and characterization of the products of aspartate catabolism

15N NMR, reverse-phase high performance liquid chromatography and gas chromatography/mass spectrometry have been used to follow nitrogen metabolism in Escherichia coli labeled on medium containing L-[15N]aspartate. The flow of 15N through various nitrogen-containing metabolites was followed over the course of the labeling period. For wild-type E. coli labeled on L-[15N]aspartate as sole source of nitrogen, significant 15N labeling was detected only in the intracellular L-glutamate, L-alanine, L-aspartate, and putrescine pools. Intracellular concentrations of L-aspartate and L-glutamate differed significantly in extracts of an arginine auxotroph (argG-), which is deficient in a potential aspartate-assimilating reaction. When the L-[15N]aspartate containing labeling medium was supplemented with unlabeled arginine, extracts of wild-type E. coli were shown to contain significant amounts of unlabeled ammonia and putrescine. There was substantial dilution of 15N in the glutamate pool. The observation that glutamate, aspartate, and alanine are 15N-labeled, but ammonia is not, suggests that these amino acids are not the immediate source of ammonia. Our results suggest that arginine may be an intermediate for the degradation of some of the aspartate and that arginine may be an intermediate for ammonia production during nitrogen-limited growth. Our results also strongly suggest the presence of a previously uncharacterized pathway of arginine degradation.

In vivo measurement of fatty acids and cholesterol synthesis using D2O and mass isotopomer analysis

The synthesis of palmitate, stearate, and cholesterol in liver and nervous tissues (brain, cord, and nerve) of Sprague-Dawley rats was determined using deuterated water (D2O) and mass isotopomer analysis. Rats were given 4% deuterium in their drinking water after each receiving an intraperitoneal priming dose. Animals were killed at 1, 2, 4, and 8 wk for deuterium enrichment in body water and determination of mass isotopomer distribution in lipids from various tissues. In 1 wk, the enrichment in the body water reached a plateau of 2.6%, which is 65% of that in the drinking water. We observed the maximum incorporation number (N) in all lipids to be higher than those previously observed, being 22, 24, and 30 for liver palmitate, stearate, and cholesterol, respectively, and N may vary among tissues. Using a single exponential model, we found the half-time (t1/2) and the plateau levels of the newly synthesized lipids of the nervous tissues (t1/2 values ranging from 5 to 28 days) to be different from those of the liver (t1/2 values < or = 4 days) in this relatively long-term study. Mass isotopomer distribution analysis and D2O can be used not only to quantitate the replacement rate of many lipids in various compartments but may also be used to elucidate the tissue-specific synthetic pathways from N.

Measurement of fractional lipid synthesis using deuterated water (2H2O) and mass isotopomer analysis

Fractional biosynthesis of palmitate, stearate, and cholesterol was determined with deuterated water (2H2O) using mass isotopomer analysis in Hep G2 and MCA sarcoma cells in culture. The method employed differs from previous ones in that the number of deuterium atoms from 2H2O incorporated into newly synthesized molecules was determined and not assumed. After correction for background natural abundances, the isotopomer distribution due to deuterium incorporation in fatty acids and cholesterol was shown to follow a simple binomial distribution depending on the deuterium enrichment in water (p) and the maximum number of deuterium atoms incorporated per molecule (N). Under a wide range of 2H2O enrichments, N could be determined to be 17 for palmitate, 20 for stearate, and 20 for cholesterol by regression analysis or from a series of consecutive mass isotopomer ratios. The fraction derived from de novo synthesis was given by the ratio of the observed to the theoretical deuterium enrichment, which is the product (N x p). The new synthesized fraction of palmitate and stearate by Hep G2 cells for the length of the experiment was found to be 77 and 65%, respectively. These values were confirmed by experiments with [U-13C]glucose as the precursor. In MCA sarcoma cells grown in lipid-poor medium, the average values for fractional synthesis of palmitate, stearate, and cholesterol were 70, 35, and 70%, respectively. This approach should be generally applicable to the simultaneous determined of fractional synthesis of a number of compounds with either deuterium or 13C tracers. Its application is only limited by the accuracy of mass spectrometric analysis.

Isotopomer studies of gluconeogenesis and the Krebs cycle with 13C-labeled lactate

Fasted rats were intragastrically infused with either [2,3-13C]lactate or [1,2,3-13C]lactate. The infusate also contained 14C-labeled lactate and [3-3H]glucose. Glucose, alanine, glutamate, and glutamine were isolated from liver and blood. There was near complete equilibration of lactate and alanine, and the relative specific activities and relative enrichments were the same in blood and liver. Glucose was cleaved enzymatically to lactate. The compounds were examined by gas chromatography-mass spectroscopy. From the mass isotopomer spectra of the lactate, glutamate, and glutamine and their cleavage fragments the positional isotopomer composition of these compounds was obtained. The enrichment and isotopomer pattern in the lactate from cleaved glucose represents that in phosphoenolpyruvate (PEP). When [1,2,3-13C]lactate was infused the mass isotopomer spectrum of glutamates consisted only of compounds containing either one, two, or three 13C carbons per molecule (m1, m2, and m3). There was little 13C in C-4 and C-5 of glutamate. The rate of pyruvate decarboxylation is low, and 3-4% of the acetyl-CoA flux in the Krebs cycle is contributed by lactate carbon. The major isotopomers in lactate, alanine, and PEP were m3 and m2 with 13C in C-2 and C-3. The predominant isotopomer in PEP from [2,3-13C]lactate was m2 with 13C in C-2 and C-3. There was much more of m1 isotopomer with 13C in C-3 and C-2 than the m1 isotopomer with 13C in C-1. There was very little m3, the isotopomer with 13C in all three carbons. Most of the 13C in C-3 and C-4 of glucose and C-1 of glutamate was introduced via 13CO2 fixation. From the isotopomer distribution and the rate of glucose turnover we deduced, applying the analysis described in the "Appendix," the absolute rates of gluconeogenic pathways, recycling of PEP and the Cori cycle, and flux in the Krebs cycle. The flux from oxaloacetate (OAA)-->PEP was seven times that of OAA-->citrate, and about half of PEP was recycled to pyruvate via pyruvate kinase. The mass isotopomer patterns in glutamate and glutamine were similar but differed from those of lactate and glucose. It appears that the glutamates are derived from alpha-ketoglutarate from a different Krebs cycle pool than PEP. The flux from OAA to PEP in this pool was two to three times that of OAA to citrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitation of positional isomers of deuterium-labeled glucose by gas chromatography/mass spectrometry

A method for determining the site and extent of deuterium (D) labeling of glucose by GC/MS and mass fragmentography was developed. Under chemical and electron impact ionization, ion clusters m/z 328, 242, 217, 212, and 187 of glucose aldonitrile pentaacetate and m/z 331 and 169 of pentaacetate derivative were produced. From the mass spectra of 13C- and D-labeled reference compounds, glucose carbon and hydrogen (C-H) positions included in these fragments were deduced to be m/z 328 = C1-C6, 2,3,4,5,6,6-H6; m/z 331 = C1-C6, 1,2,3,4,5,6,6-H7; m/z 169 = C1-C6, 1,3,4,5,6,6-H6; m/z 187 = C3-C6, 3,4,5,6,6-H5; m/z 212 = C1-C5, 2,3,4,5-H4; m/z 217 = C4-C6, 4,5,6,6-H4; and m/z 242 = C1-C4, 2,3,4-H3. After correction for isotope discrimination and deuterium-hydrogen exchange, the D enrichment of these fragments can be quantitated using selective ion monitoring, and the D enrichment of all C-H positions can be obtained by the difference in enrichment of the corresponding ion pairs. The validity of this approach was tested by examining D enrichment of known mixtures of 1-d1-, 2-d1-, 3-d1-, and 5,6,6-d3-glucose with unlabeled glucose and D enrichment of perdeuterated glucose using these fragments. This method was used to determine deuterium incorporation in C1 through C6 of blood glucose in fasted (24 h) rats infused with deuterated water. The distribution of deuterium was similar to that found by Postle and Bloxham (1980, Biochem. J. 192, 65-73). Approximately one deuterium atom was incorporated into C5 and only 75% deuterium atom was incorporated into C2. The enrichment of C2 and C6 of glucose relative to that of water indicated that 74 +/- 9% of plasma glucose was newly formed 4 h after the onset of deuterium infusion, and gluconeogenesis accounted for about 76 +/- 7% of the glucose 6-phosphate flux.

Mass isotopomer pattern and precursor-product relationship

The synthesis of a homonucleus polymer from its labeled precursor will lead to the formation of molecules with different masses. The distribution of these mass isotopomers is strictly a function of the enrichment of the 13C-labeled precursor, and can thus be used for the determination of the precursor enrichment and product dilution in the de novo synthesis of the polymer. We present here a study of the isotopomer pattern of a polymer of acetate in the form of glucose pentaacetate synthesized from 13C-enriched acetic anhydride. The molecular ion contains four acetyl units. Its synthesis is analogous to that of octanoic acid from acetyl coenzyme A. The process of obtaining the mass isotopomer distribution in the tetraacetyl moiety from the ion cluster of m/z 331 of glucose pentaacetate is illustrated. After correcting for the contribution of 13C natural abundance, the plot of the ratio of mass isotopomers (m4/m2) against the observed enrichment of the tetraacetate moiety yielded a straight line with a slope of 1.45. The ratio was not altered by dilution with pre-existing unenriched product, as predicted. The slope of the observed linear relationship agreed with the general formula (N-(j-1))/j for the ratio of any two consecutive mass isotopomers (mj/mj-1). Theoretical and practical aspects of determining precursor enrichment from isotopomer pattern in polymers are discussed.

Application of mass isotopomer analysis for determination of pathways of glycogen synthesis

An elementary exposition of the application of mass spectroscopy to studies with substrates labeled uniformly with 13C is presented. A procedure to obtain mass isotopomer spectra, corrected for natural abundance, of products labeled with 13C in several positions is outlined. The calculation for enrichment, a term equivalent to specific activity with radioisotopes, is shown. Examples of mass isotopomer patterns of blood glucose and glycogen are presented, and calculations of the contribution of the direct path to hepatic glycogen synthesis and the dilution of glucose-derived pyruvate are shown. The analysis of mass isotopomer patterns recently offered by C. Des Rosiers, B. R. Landau, and H. Brunengraber [Am. J. Physiol. 259 (Endocrinol. Metab. 22): E757-E762, 1990] is critically examined.

Mass isotopomer analysis: theoretical and practical considerations

A theory of mass isotopomer analysis based on the well-known principle of isotope dilution mass spectrometry is reviewed. An algorithm for the determination of isotope incorporation into a metabolic substrate from a labeled precursor using mass isotopomer analysis is presented. The steps include the determination of the contribution of the derivatization reagent to the observed spectrum of the derivatized substrate and the correction of contribution from 13C natural abundance using multiple linear regression analysis. Examples of the application of this theory to determine the spectrum of the trimethylsilyl derivative of the 'pure unlabeled' or mononuclidic cholesterol, and the calculation of mass isotopomer distribution in cholesterol due to tracer incorporation using this 'pure unlabeled' spectrum, are also provided.

Glucose isotope, carbon recycling, and gluconeogenesis using [U-13C]glucose and mass isotopomer analysis

Experimental determinations of glucose carbon recycling using 14C or 13C glucose tracer often underestimate true Cori cycle activity because of dilution and exchange of isotope tracer through the tricarboxylic acid (TCA) cycle. The term glucose isotope recycling therefore is used to distinguish recycling of isotope from recycling of glucose carbon, the actual quantity of circulating glucose recycled. Recently, per-labeled glucose ([U-13C6]glucose) has been used to estimate glucose appearance rate and glucose isotope recycling. Chemical structural information determined by mass isotopomer analysis has been used to correct for dilution of isotope through the TCA cycle. In this report, we present experiments in the study of glucose turnover and recycling using [U-13C6]glucose. Methods of single injection and continuous infusion of [U-13C6]glucose are compared. A formula for the calculation of a dilution factor using TCA cycle parameters is derived. In this study of six rabbits, glucose turnover rate ranged from 3.4 to 8.8 mg/kg/min, and glucose m + 3 mass isotopomer recycling from 7 to 12%. The rate of pyruvate carboxylation (Y) was comparable to that of citrate synthetase, having an average relative flux of 0.89. Applying the correction factor for tracer dilution to the observed mass isotopomer recycling, we determined glucose carbon recycling (or Cori cycle activity) to be 22-35% of hepatic glucose output.

Determination of pathways of glycogen synthesis and the dilution of the three-carbon pool with [U-13C]glucose

Rats were infused with glucose at 30 mg/min, containing 18% enriched [U-13C]glucose and [1-14C]- and [3-3H]glucose. The mass isotopomer patterns of 13C-labeled blood glucose and liver glycogen were determined by gas chromatography/mass spectroscopy. The contribution of the direct pathway to glycogen was calculated from the three tracers, and the values by all three were nearly identical, about 50%. The 14C specific activity in carbon 6 of glycogen glucose was about 6% that of carbon 1. The [3H]glucose/[1-14C]glucose ratio in glycogen was 80-90% that in blood glucose. The enrichment of 13C and the specific activity of 14C in glycogen formed by the indirect path were 20-25% of glycogen formed directly from glucose. The dilution is of two kinds: (i) an exchange of labeled carbon with unlabeled carbon in the tricarboxylic acid cycle and (ii) dilution by unlabeled nonglucose carbon. Methods to calculate the two types of dilution are presented. In control rats the dilution factor by exchange in the tricarboxylic acid cycle is 1.4, and the dilution by unlabeled carbon is 2.5- to 3.0-fold, with the overall dilution about 4-fold. In rats preinjected with glucagon, the dilution through the tricarboxylic acid cycle was unaffected but that by nonglucose carbon was decreased.

Blood glucose turnover during high- and low-intensity exercise

We hypothesized that whole body glucose uptake (Rd) during exercise is not related in a simple, linear manner to O2 uptake (VO2). To test this, seven healthy male subjects (age range 23-34 yr) were studied in the postabsorptive but not glycogen-depleted state. Three conditions were examined: 1) rest, 2) 40 min of constant exercise in which the work rates were carefully chosen to consist of low-intensity exercise (no elevated blood lactate, a mean of 40% maximal VO2), and 3) 40 min of high-intensity exercise (markedly elevated blood lactate, 79% maximal VO2). Gas exchange was measured breath by breath, and glucose uptake and production were measured using [6,6-2H2]glucose. Low-intensity exercise (n = 7) resulted in a small but not statistically significant increase in mean Rd [3.06 +/- 0.37 (SE) mg.min-1.kg-1] compared with resting values (2.87 +/- 0.39 mg.min-1.kg-1) despite a fourfold increase in the production of CO2 and VO2. By contrast, the high-intensity exercise Rd (n = 5, 6.98 +/- 0.67 mg.min-1.kg-1) was significantly greater than the resting value (3.03 +/- 0.56 mg.min-1.kg-1). Results of glucose production were virtually the same. Similarly, mean levels of epinephrine and norepinephrine increased significantly above resting values during high- but not low-intensity exercise. Our data demonstrate that whole body glucose dynamics and regulation during 40 min of exercise do not change in a simple linear manner with respect to metabolic rate.

Studies of glycogen synthesis and the Krebs cycle by mass isotopomer analysis with [U-13C]glucose in rats

Starved rats were infused intragastrically via indwelling duodenal cannulae with glucose at a rate of 30 mg/min/kg. The infusate contained [U-13C]glucose at an enrichment of 32 or 17%. At the end of the infusion, after 160 min, glucose and lactate were isolated from arterial and portal blood and from liver, and liver glycogen was isolated and hydrolyzed to glucose. The enrichment in glucose and lactate and the isotopomer distribution in glucose of masses from 180 to 186 were determined by gas chromatography-mass spectrometry (GC-MS). From analysis of these data we determined (a) gluconeogenesis proceeds at half the basal rate in the presence of a large infused glucose load, (b) one-quarter of the hepatic pyruvate pool is derived from nonglucose carbon, (c) half of the labeled molecules in liver glycogen are of mass 186 from the infused glucose and half are of masses 181-183, (d) the contribution of the indirect path from pyruvate when corrected for synthesis from unlabeled pyruvate ranges from 55 to 65%, (e) the rate of pyruvate carboxylase averages 90% that of citrate synthase, and (f) the rate of exchange of oxaloacetate with fumarate is about three times the rate of flux in the Krebs cycle (four times in the "forward" direction), and the enrichment in carbon 1 of oxaloacetate was 2.3 times that in carbon 4. In the Appendix a method to obtain the isotopomer distribution of newly formed glucose and glycogen glucose is described. An algorithm to correct for the contribution of natural abundance of 13C and the presence of 12C in commercial [U-13C]glucose is presented. A novel mathematical analysis to obtain the parameters of the Krebs cycle from the isotopomer distribution is developed in the Appendix. Equations to calculate the relative rates of pyruvate carboxylase (y), and the equilibration of oxaloacetate with fumarate from the isotopomer distribution are derived. Mass isotopomer analysis provides a novel and powerful tool for the study of carbohydrate metabolism and the operation of the Krebs cycle.

Absolute radioassay of extended sources: an equivalent point-source coincidence-counting approach with application to the thyroid

A general methodology is provided for the absolute assay of radioisotopes decaying with coincident photons in an extended source. In the determination of the source activity, the method requires neither the detailed consideration of the geometric and self-attenuation processes taking place between the source component points nor a knowledge of the distribution of activity across the source. It derives from the concept of the "equivalent point source," that is a fictitious point source whose activity would equal that measured for the actual extended source. It has been developed for an arbitrary number of coincident photon types displaying an arbitrary degree of mutual correlation, and for arbitrary detection geometry. A unifying formalism is developed for both point and extended sources and for single and dual detecting systems. It is found that in all cases the various instrumental and spectroscopic uncertainties appear within a composite parameter (herein called F factor) that can be determined by standard calibration procedures; this factor is in turn only weakly dependent on its own component parameters. New expressions and relationships are obtained that provide a greater physical insight into coincidence-counting methods.

Simultaneous determination of free thyroxine and capacity of thyroxine-binding globulin

A simple method is described for the simultaneous determination of capacity thyroxine-binding of globulin (TBG) and free thyroxine concentration (FT4). The ratio of bound to free T4 (B/F) is first determined for two total-T4 concentrations using a Sephadex G-25 competitive-binding technique. TBG capacity and FT4 can both the calculated assuming a known value of affinity constant of TBG. The method is linear over a twenty-fold serum dilution. FT4 calculated is identical to that calculated using the method of Irvine. TBG capacity is shown to be linearly correlated to TBG concentration as determined by radioimmunoassay and is consistent with a molecular weight of 69,000 Daltons and one T4 binding site per molecule. FT4 is found to correlate with the free thyroxine index in a complicated way, depending on the degree of TBG saturation.

A new method of calculating absolute thyroid activity in intravenous I-123 uptake tests

A technique for determining the absolute activity of I-123 in early thyroidal uptake tests is presented. The method is independent of geometry, tissue attenuation, an extrathyroidal neck activity (ENA). It utilizes the efficiency factors (ExT and E gamma T) derivable for the thyroid gland using coincidence counting techniques, and a quantity, alpha, characteristics of the vascularity of the individual patient. Although ENA is not determined in absolute terms (e.g., percent of injected dose), count rates due to ENA are determined. After subtracting the ENA from the total count rates thyroidal uptake can be determined as absolute activity by means of the efficiency factors. The validity and usefulness of the technique have been demonstrated in phantom and patient studies. Thyroid uptakes of less than 0.1% of injected dose can be measured accurately in our patient studies.

Inherited abnormal thyroid hormone-binding protein causing selective increase of total serum thyroxine

A 9-yr-old boy is described in whom increased serum T4 concentration, increased T3 uptake, and increased free T4 index were associated with a euthyroid clinical state with normal total serum T3. T4-binding globulin (TBG), measured by RIA, was decreased. Reverse flow paper electrophoresis of serum proteins after reaction with radioactively labeled T4 demonstrated increased binding of T4 to a protein with electrophoretic mobility corresponding to albumin. Displacement of serum protein-bo-nd [125I]T4 activity by increasing concentrations of T4 revealed the presence of a low affinity, high binding capacity system with an association constant similar to that of T4-binding prealbumin. This low affinity binding protein cochromatographed with TBG on a DEAE-Sephadex column which normally separates TBG from T4-binding prealbumin. At free T4 concentrations equivalent to those present in the plasma of normal individuals, the T4 bound to free ratio is higher in the patient than in normals and the total serum T4 level is increased in the presence of normal free T4 concentrations. The relative affinity of this abnormal T4-binding protein for T3 is low compared to that of TBG. The patient's father had the same abnormal binding protein, which was not found in his mother or fraternal twin brother. These data suggest an autosomal dominant mode of inheritance of an aberration leading to synthesis of a new protein instead of normal TBG. The new protein is different from TBG in electrophoretic mobility, T4 and T3 binding, and antigenic properties.

Use of I-123 in early radioiodide uptake and its suppression in children and adolescents with hyperthyroidism

Absolute activity measurement of I-123 by coincidence counting was used to study the early thyroidal iodide uptake in 20 hyperthyroid children. Patients were pretreated either with methimazole or propylthiouracil before injection of Na123I. The usual method of analysis of the early uptake was modified to account for a rapidly equilibrating compartment, to give thyroidal iodide trapping rate constant (K1) and absolute iodide uptake (AIU). The suppressibility of the early uptake by triiodothyronine (T3) was evaluated in some patients. The upper limit of normal for K1 was 0.03 min-1 and for AIU was 0.04 microgram/min. In the hyperthyroid subjects, K1 and AIU were in the hyperthyroid range before and after T3 suppression. For patients with suppressible uptake, remission from hyperthyroidism was maintained for 6 mo to 2 1/2 yr. Only two patients with nonsuppressible uptake achieved remission from hyperthyroidism, perhaps because of coexistence of thyroiditis.