Noninvasive tracing of Krebs cycle metabolism in liver

Nuclear magnetic resonance analysis of cell metabolism

Nuclear magnetic resonance (NMR) continues to be a useful tool for the study of cellular metabolism. A variety of NMR techniques have been developed or newly applied to the analysis of cell systems. Many of these techniques are particularly useful for the analysis of immobilized cell bioreactors. The use of several NMR techniques has been an integral part of recent comprehensive metabolic studies. Novel computer-based models and methods have been developed which may make NMR study of metabolism more accessible and powerful.

Tracing hepatic gluconeogenesis relative to citric acid cycle activity in vitro and in vivo. Comparisons in the use of [3-13C]lactate, [2-13C]acetate, and alpha-keto[3-13C]isocaproate

The validity of the use of a carbon tracer for investigating liver intermediary metabolism in vivo requires that the labeling pattern of liver metabolites not be influenced by metabolism of the tracer in other tissues. To identify such specific tracer, livers from 48-h starved rats were perfused with recirculating buffer containing [3-13C]lactate, [2-13C]acetate, or alpha-keto[3-13C]isocaproate. Conscious 48-h starved rats were infused with the same tracers for 5 h. The labeling patterns of liver glutamate and extracellular glucose were assayed by gas chromatography-mass spectrometry. In vivo data were corrected for 13CO2 reincorporation into C-1 of glutamate and C-3 and C-4 of glucose, using data from control rats infused with NaH13CO3. With [3-13C]lactate the labeling pattern of liver glutamate was the same in perfused organs and in vivo. In contrast, with [2-13C]acetate and alpha-keto[3-13C]isocaproate the labeling pattern of liver glutamate in vivo was clearly influenced by the expected labeling pattern of citric acid cycle intermediates formed in non-gluconeogenic organs, presumably glutamine made in muscle. Indeed, the labeling pattern of plasma glutamine and liver glutamate were similar in experiments with [3-13C]lactate but different in experiments with [2-13C]acetate and alpha-keto[3-13C]isocaproate. Similar conclusions were drawn from the labeling patterns of glucose. Therefore, labeled lactate appears as the best tracer for studies of liver intermediary metabolism in vivo. Our data also show that a substantial fraction of alpha-ketoisocaproate metabolism occurs in peripheral tissues.

Determination of (13C) urea enrichment by gas chromatography/mass spectrometry and gas chromatography/isotope ratio mass spectrometry

We present gas chromatographic/mass spectrometric and gas chromatographic/isotope ratio mass spectrometric assays of the 13C enrichment of plasma urea converted to its dimethylaminomethylene derivative. The limits of sensitivity of the two techniques are 0.2% and 0.02%, respectively. The techniques were tested in rats and humans infused with (13C)urea or (3-13C)lactate. (13C)Urea enrichment during the infusion of (3-13 C)lactate in humans was not detectable by gas chromatography/mass spectrometry but was easily measured by gas chromatography/isotope ratio mass spectrometry. These assays should be useful for clinical investigations, in which the incorporation of a (13C)gluconeogenic substrate into glucose must be corrected for the incorporation of 13CO2 derived from the oxidation of the substrate. This correction involves measuring the low-level 13C enrichment of urea.

Hepatic metabolism during constant infusion of fructose; comparative studies with 31P-magnetic resonance spectroscopy in man and rats

A protocol of constant infusion of fructose has been carried out both in human volunteers and in the perfused rat liver, aiming at a steady-state blood fructose concentration of 6-8 mM. Localized 31P-NMR spectroscopy and biochemical analyses were used to evaluate the metabolic changes. Comparison of the model experiment and the clinical study allowed an evaluation of this protocol as a clinically relevant assessment of the metabolic function of the liver. The time course of change, as well as the quasi steady-state levels reached during fructose infusion, for phosphomonoesters (PME), ATP and inorganic phosphate (Pi) provided the following results: During fructose infusion, ATP and Pi reached a steady-state level of 74.0 +/- 5.9 and 54.6 +/- 3.3% of control respectively, in the human volunteers. The corresponding data in the rat liver was 71.3 +/- 4.3 and 54.4 +/- 4.3%. Hepatic clearance of fructose was 0.53 and 0.52 ml.g liver-1.min-1 for volunteers and rats, respectively. The time course of intracellular metabolite recovery after fructose could be approximated by a first order kinetic. The rate constants for PME and ATP change were similar during fructose infusion and recovery, while after the discontinuation of fructose infusion, Pi increased with a rate constant significantly greater than during its fructose-induced depletion in human liver (P < 0.005). Thus, this relatively simple clinically applicable protocol seems to be verifiable in the well controlled perfused rat liver model, and it is argued that it may be useful in the clinical evaluation of the metabolic functional capacity of the human liver.

14C-labeled propionate metabolism in vivo and estimates of hepatic gluconeogenesis relative to Krebs cycle flux

Purposes of this study were 1) to estimate in humans, using 14C-labeled propionate, the rate of hepatic gluconeogenesis relative to the rate of Krebs cycle flux; 2) to compare those rates with estimates previously made using [3-14C]lactate and [2-14C]acetate; 3) to determine if the amount of ATP required for that rate of gluconeogenesis could be generated in liver, calculated from that rate of Krebs cycle flux and splanchnic balance measurements, previously made, and 4) to test whether hepatic succinyl-CoA is channeled during its metabolism through the Krebs cycle. [2-14C]propionate, [3-14C]-propionate, and [2,3-14C]succinate were given along with phenyl acetate to normal subjects, fasted 60 h. Distributions of 14C were determined in the carbons of blood glucose and of glutamate from excreted phenylacetylglutamine. Corrections to the distributions for 14CO2 fixation were made from the specific activities of urinary urea and the specific activities in glucose, glutamate, and urea previously found on administering [14C]-bicarbonate. Uncertainties in the corrections and in the contributions of pyruvate and Cori cyclings limit the quantitations. The rate of gluconeogenesis appears to be two or more times the rate of Krebs cycle flux and pyruvate's decarboxylation to acetyl-CoA, metabolized in the cycle, less than one-twenty-fifth the rate of its decarboxylation. Such estimates were previously made using [3-14C]lactate. The findings support the use of phenyl acetate to sample hepatic alpha-ketoglutarate. Ratios of specific activities of glucose to glutamate and glucose to urinary urea and expired CO2 indicate succinate's extensive metabolism when presented in trace amounts to liver. Utilizations of the labeled compounds by liver relative to other tissues were in the order succinate = lactate > propionate > acetate. ATP required for gluconeogenesis and urea formation was approximately 40% of the amount of ATP generated in liver. There was no channeling of succinyl-CoA in the Krebs cycle in the hepatic mitochondria.

Measuring glucose and fructose-6-phosphate cycling in liver in vivo

Approaches measuring futile cycling of glucose and fructose-6-phosphate (fructose-6-P) in liver in vivo depend on assumptions about the fates of hydrogens bound to specific carbons of glucose. Thus, 3H of [2-3H]glucose has been assumed to be completely removed after its conversion to glucose-6-P, [3-3H]glucose after its conversion to fructose-1,6-bisP, and [6-3H]glucose not at all. Previous measurements have shown that these assumptions are incompletely fulfilled. Corrections to estimates of cycling can be made when detritiations of [2-3H]glucose and [3-3H]glucose are not complete, and detritiation of [6-3H]glucose occurs. How the corrections can be made is presented using data previously reported on giving labeled glucoses to humans after an overnight fast and on infusing a glucose load. Estimates of glucose cycling nearly double, and that of fructose-6-P cycling almost triples. Estimates of hepatic glucose production as measured with [6-3H]glucose decrease. Correction of estimates of cycling under other conditions may very well be similarly affected. Thus, rates of glucose and fructose-6-P cycling appear to be substantially more than previously estimated. Quantitation under a given condition requires measurements to be made of the extent to which assumptions as to the fate of labeled hydrogen of the glucoses are fulfilled. The uncertain extent of exchange of label catalyzed by transaldolase and detritiation in the pentose cycle, the failure of fructose-6-P cycling to be expressed through detritiation of 3H from [3-3H]glucose, and possible isotope effects still limit the confidence that can be placed in such estimates.

Estimating gluconeogenic rates in NIDDM

To measure the rate of gluconeogenesis in humans directly, one must administer and determine the specific activity or the enrichment in an intermediate in the gluconeogenic process and in the glucose formed, thus obtaining the fraction of the glucose formed by gluconeogenesis. By a separate determination of the rate of hepatic glucose production, the rate of gluconeogenesis can then be calculated. The closer the intermediate is to glucose-6-P, the more complete will be the measurement of the rate. Thus, if the intermediate is below the level of the triose phosphates, gluconeogenesis from glycerol will not be included in the estimate. Estimates of rates of gluconeogenesis from estimates of PEP enrichment or specific activity require a measure of the extent of exchange of label at the level of oxaloacetate. By using 14C or 13C labeled CO2 as the intermediate and estimating the relative rates of the reactions of the tricarboxylic acid cycle relative to gluconeogenesis from the distribution of 14C from [3-14]lactate in glutamine from the glutamine conjugate of phenylacetate, the enrichment or specific activity of PEP has been estimated. Correction must be made for the incorporation into the glutamine of 14CO2 formed from the [3-14C]lactate. Data support the validity of this approach toward estimating gluconeogenesis in NIDDM, but the approach is complex, time consuming and with uncertainties. Estimates that have been made using [2-14C] acetate are invalid because of the extensive metabolism of [2-14C]acetate in other than liver. Other approaches have promise, but technical problems may exist in their use and other problems, such as hepatic zonation and exchange reactions, may compromise their application.

Gluconeogenesis in hepatocytes determined with [2-13C]acetate and quantitative 13C NMR spectroscopy

1. In the present study the major metabolic pathways of glucose metabolism were determined in isolated liver cells using [2-13C]acetate and 13C magnetic resonance spectroscopy. 2. The relative reaction rates of glucose synthesis to the TCA cycle were determined from the 13C distribution in glucose where the overall 13C enrichment of glucose was 6.41 +/- 1.94% (mean +/- SD; n = 6) and the mean 13C enrichment of C1, C2, C5, C6 to C3, C4 was 2.63 +/- 0.30. 3. Since the distribution of tracer in glucose is a function of the relative entry rates of pyruvate to acetyl-CoA into the oxaloacetate pool this was calculated to be 0.32 +/- 0.15 and the factor for carbon exchange (1/P) between the gluconeogenic pathway and the TCA cycle was calculated to be 1.03 +/- 0.20. 4. With this carbon exchange factor and the approximated 13C enrichment of acetyl-CoA the intramitochondrial 13C enrichment of phosphoenolpyruvate was calculated and the "true" rate of hepatic gluconeogenesis from phosphoenolpyruvate estimated. 5. Since acetate was metabolized solely in liver cells the 13C enrichment of acetyl-CoA could be approximated from that of 3-hydroxybutyrate. 6. The carbon 13 enrichment of 3-hydroxybutyrate and phosphoenolpyruvate was 5.89 +/- 0.90% and 5.96 +/- 1.67%, respectively. 7. The per cent gluconeogenesis from phosphoenolpyruvate calculated as the ratio of the 13C enrichment of glucose to that of 3-hydroxybutyrate times 1/P was 107 +/- 8%.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of 14CO2 in estimating rates of hepatic gluconeogenesis

Estimating the rate of hepatic gluconeogenesis in vivo from the incorporation of 14C from 14CO2 into glucose requires determination of the rates in liver of equilibration of oxaloacetate with fumarate, conversion of oxaloacetate to phosphoenolpyruvate (PEP), and conversion of PEP to pyruvate, all relative to the rate of tricarboxylic acid cycle flux. With the use of a model of mitochondrial metabolism and gluconeogenesis, expressions are derived relating specific activity of carboxyl of PEP from 14CO2 to those rates and specific activity of mitochondrial CO2. If those rates and specific activity of mitochondrial CO2 are known, specific activity of PEP, calculated using the expressions, should, on a mole basis, be one-half the specific activity of the glucose formed. At steady state, in the 60-h fasted individual, where glucose formation is solely by gluconeogenesis, twice estimated specific activity of PEP should then approximate that of blood glucose. Estimates of relative rates in 60-h fasted humans, previously made from distribution of 14C in glutamate from phenylacetylglutamine excreted when [3-14C]lactate and phenylacetate were given, were applied to the expressions. Specific activity of mitochondrial CO2 was equated to that of CO2 expired by 60-h fasted subjects given NaH14CO3 and alpha-[1-14C]ketoisocaproate. Predicted specific activities approximated actual specific activities of blood glucose when NaH14CO3 was administered. alpha-[1-14C]ketoisocaproate administrations gave underestimates. This is attributable to differences between specific activities of hepatic mitochondrial CO2 and expired CO2, which is evidenced by higher incorporations of 14C in glucose than in expired CO2 from alpha-[1-14C]ketoisocaproate than from NaH14CO3.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of non-invasive probes and 13C nuclear magnetic resonance spectroscopy to assess liver metabolism in humans

Two new methods to assess liver metabolism in humans will be discussed, non-invasive probes and (13)C NMR (nuclear magnetic resonance) spectroscopy. Hepatic UDP-glucose and alpha-ketoglutarate can be sampled by non-invasive probes. This method has been used to quantitate the contribution of the indirect pathway to liver glycogen formation and to estimate relative flow rates and carbon exchange in the Krebs cycle. (13)C NMR spectroscopy has been developed to measure liver glycogen concentrations in vivo. Using this method in combination with measurements of whole body glucose production during fasting, the contribution of gluconeogenesis has been calculated.