Quantitative measurement of the L-type pentose phosphate cycle with [2-14C]glucose and [5-14C]glucose in isolated hepatocytes

Longenecker, John P.; Williams, John F.

doi:10.1042/bj1880859

Cited by 37 publications

(17 citation statements)

References 18 publications

(20 reference statements)

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“…Its concentration in control liver perfusions was ~10 nmol/g tissue ( fig.2) which is similar to 5-7 nmol/g tissue found in [20]. This intermediate plays a r61e in the proposed L-type pentose pathway [35,36], and the existence of octulose phosphates [19,20] support the operation of this pathway. However, there does appear to be some doubt regarding the metabolic fate of arabinose 5-P another key intermediate in the L-type pentose pathway [33,34].…”

Section: Resultssupporting

confidence: 67%

“…Since there are no convincing control mechanisms demonstrated for controlling flux of carbon through the non-oxidative pentose pathway [37], the operation of the pathway shown in fig.3 would serve to regulate the interconversion of hexose and pentose phosphates since phosphofructokinase and fructose bisphosphatase are under hormonal control [1][2][3][4][5][6][7]. The labelling experiments performed in hepatocytes [36] are consistent with, but do not prove, the operation of the scheme in fig.3.…”

Section: Resultsmentioning

confidence: 99%

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Regulation of hepatic altro heptulose 1,7‐bisphosphate levels and control of flux through the pentose pathway by fructose 2,6‐bisphosphate

Blackmore

Shuman

1982

FEBS Letters

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Section: Resultssupporting

confidence: 67%

Section: Resultsmentioning

confidence: 99%

Regulation of hepatic altro heptulose 1,7‐bisphosphate levels and control of flux through the pentose pathway by fructose 2,6‐bisphosphate

Blackmore

Shuman

1982

FEBS Letters

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“…gluconeogenesis) or from pentose phosphates (i.e. via the L-type pentose cycle, as proposed by Longenecker and Williams [7,8]), a fast kinetic component (tip 20 s) should be observed. The time lag following termination of tracer infusion and the absence of any fast component in 14C02 production from [6-'4C]glucose (although present in ['4C]lactate production, Fig.…”

Section: Kinetics Of 14c02 Productionmentioning

confidence: 93%

Rates of Flux through the Pentose Cycle in Perfused Rat Liver

Kuehn¹,

Scholz²

1982

European Journal of Biochemistry

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A method for the determination of substrate flux through the pentose cycle was developed employing [l‐14C]glucose in experiments with perfused rat livers. The method consists first of a kinetic analysis which differentiates between the production of 14CO2 from [l‐14C]glucose via the pentose cycle and via the citrate cycle and, second of a calculation of the specific radioactivity of the hexose monophosphate pool from measured rates of glycolysis and the specific radioactivity of lactate released into the perfusate. The method was validated by experiments comparing the results of tracer infusions with [l‐14C]glucose, [6‐14C]glucose and [3‐14C]pyruvate. In livers from fed rats perfused with 10 mM glucose, the rate of substrate flux through the pentose cycle was around 0.2 μmol ×min−1× g−1; it was about 20% of the substrate flux via glycolysis. The kinetic data were inconsistent with the existence of an L‐type pentose cycle in liver.

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“…1) using rat liver acetone powder preparation as a source of pathway enzymes [12,241. Detailed investigations of the pathway reactions were also made using rat liver cytosol [21] and rat hepatocytes [22]. The data of Table 1 show the maximum activities of each of the 'carbonyl-group-transferring' enzymes in these liver preparations, together with the calculated enzyme-catalyzed exchange activities that each of these enzymes contributes.…”

Section: Aldolase Exchangementioning

confidence: 99%

“…Spinach (Spinacia oleracea L.) was grown in water culture as described [5]. Rat liver enzymes [12,20], cytosol [21] and hepatocytes [22] were prepared by the method described in the references.…”

Section: Turnour Plant and Animal Tissues Animals And Dietmentioning

confidence: 99%

Exchange reactions catalyzed by group‐transferring enzymes oppose the quantitation and the unravelling of the identity of the pentose pathway

Flanigan

Collins

Arora

et al. 1993

European Journal of Biochemistry

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1. The distributions and rates of transfer of carbon isotopes from a selection of specifically labelled ketosugar-phosphate substrates by exchange reactions catalyzed by the pentose and photosynthetic carbon-reduction-pathway group-transferring enzymes transketolase, transaldolase and aldolase have been measured using I3C-NMR spectroscopy.2. The rates of these exchange reactions were 5, 4 and 1.5 pmol min-' mg-' for transketolase exchange, transaldolase exchange and aldolase exchange, respectively.3. A comparison of the exchange capacities contributed by the activities of these enzymes in three in v i m liver preparations with the maximum non-oxidative pentose pathway flux rates of the preparations shows that transketolase and aldolase exchanges exceeded flux by 9-19 times in liver cytosol and acetone powder enzyme preparations and by 5 times in hepatocytes. Transaldolase was less effective in the comparison of exchange versus flux rates: transaldolase exchange exceeded flux by 1.6 and 5 in catalysis by liver cytosol and acetone powder preparations, respectively, but was only 0.6 times the flux in hepatocytes.4. Values of group enzyme exchange and pathway flux rates in the above three preparations are important because of the feature role of liver and of these particular preparations in the establishment, elucidation and measurement of a proposed reaction scheme for the fat-cell-type pentose pathway in biochemistry.5. It is the claim of this paper that the excess of exchange rate activity (particularly transketolase exchange) over pathway flux will overturn attempts to unravel, using isotopically labelled sugar substrates, the identity, reaction sequence and quantitative contribution of the pentose pathway to glucose metabolism.6. The transketolase exchange reactions relative to the pentose pathway flux rates in normal, regenerating and foetal liver, Morris hepatomas, mammary carcinoma, melanoma, colonic epithelium, spinach chloroplasts and epididymal fat tissue show that transketolase exchange may exceed flux in these tissues by factors ranging over 5 -600 times.7. The confusion of pentose pathway theory by the effects of transketolase exchange action is illustrated by the 13C-NMR spectrum of the hexose 6-phosphate products of ribose 5-phosphate dissimilation, formed after 30 min of liver enzyme action, and shows I3C-labelling in carbons 1 and 3 of glucose 6-phosphate with ratios which range over 2.1 -6.4 rather than the mandatory value of 2 which is imposed by the theoretical mechanism of the pathway.

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Quantitative measurement of the L-type pentose phosphate cycle with [2-14C]glucose and [5-14C]glucose in isolated hepatocytes

Cited by 37 publications

References 18 publications

Regulation of hepatic altro heptulose 1,7‐bisphosphate levels and control of flux through the pentose pathway by fructose 2,6‐bisphosphate

Regulation of hepatic altro heptulose 1,7‐bisphosphate levels and control of flux through the pentose pathway by fructose 2,6‐bisphosphate

Rates of Flux through the Pentose Cycle in Perfused Rat Liver

Exchange reactions catalyzed by group‐transferring enzymes oppose the quantitation and the unravelling of the identity of the pentose pathway

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