scholarly journals The fate of 14C in glucose 6-phosphate synthesized from [1-14C]Ribose 5-phosphate by enzymes of rat liver

1978 ◽  
Vol 176 (1) ◽  
pp. 241-256 ◽  
Author(s):  
J F Williams ◽  
M G Clark ◽  
P F Blackmore
Keyword(s):  
Rat Liver ◽  
Pentose Phosphate Pathway ◽  
Specific Radioactivity ◽  
Early Time ◽  
Pentose Phosphate ◽  
Enzyme Extract ◽  
Reaction Sequence ◽  
Time Intervals ◽  
A Value ◽  
Initial Substrate

1. Glucose 5-phosphate was synthesized from ribose 5-phosphate by an enzyme extract prepared from an acetone-dried powder of rat liver. Three rates of ribose 5-phosphate utilization were observed during incubation for 17 h. An analysis of intermediates and products formed throughout the incubation revealed that as much as 20% of the substrate carbon could not be accounted for. 2. With [1-14C]ribose 5-phosphate as substrate, the specific radioactivity of [14C]glucose 6-phosphate formed was determined at 1, 2, 5 and 30 min and 3, 8 and 17 h. It increased rapidly to 1.9-fold the initial specific radioactivity of [1-14C]ribose 5-phosphate at 3 h and then decreased to a value approximately equal to that of the substrate at 6 h, and finally at 17 h reached a value 0.8-fold that of the initial substrate [1-14C]ribose 5-phosphate. 3. The specific radioactivity of [14C]ribose 5-phosphate decreased to approx. 50% of its inital value during the first 3 h of the incubation and thereafter remained unchanged. 4. The distribution of 14C in the six carbon atoms of [14C]glucose 6-phosphate formed from [1-14C]ribose 5-phosphate at 1, 2, 5 and 30 min and 3, 8 and 17 h was determined. The early time intervals (1–30 min) were characterized by large amounts of 14C in C-2 and in C-6 and with C-1 and C-3 being unlabelled. In contrast, the later time intervals (3–17 h) were characterized by the appearance of 14C in C-1 and C-3 and decreasing amounts of 14C in C-2 and C-6. 5. It is concluded that neither the currently accepted reaction sequence for the non-oxidative pentose phosphate pathway nor the ‘defined’ pentose phosphate-cycle mechanism can be reconciled with the labelling patterns observed in glucose 6-phosphate formed during the inital 3 h of the incubation.

scholarly journals New reaction sequences for the non-oxidative pentose phosphate pathway

1978 ◽  
Vol 176 (1) ◽  
pp. 257-282 ◽  
Author(s):  
J F Williams ◽  
P F Blackmore ◽  
M G Clark
Keyword(s):  
Volatile Compounds ◽  
Rat Liver ◽  
Pentose Phosphate Pathway ◽  
Carbon Balance ◽  
Pentose Phosphate ◽  
Oxidative Pentose Phosphate Pathway ◽  
Rat Tissues ◽  
Phosphate Metabolism ◽  
Reaction Sequence ◽  
Triose Phosphate

1. Reactions leading to the formation of 14C-labelled volatile compounds and compounds volatile under acid conditions were investigated in a system actively synthesizing hexose 6-phosphates from [U-14C]ribose 5-phosphate by reactions catalysed by enzymes prepared from acetone-dried powder of rat liver; no reactions involving 14C-labelled volatile compounds were detected. Similarly the fixation of 14C-labelled volatile compounds into hexose 6-phosphate could not be detected. 2. A complete carbon balance was made for the reactants, intermediates and products of the reactions involved in the conversion of ribose 5-phosphate into hexose 6-phosphate by enzymes of rat liver. Five additional intermediates of pentose 5-phosphate metabolism in liver were detected, namely D-manno-heptulose 7-phosphate, D-altro-heptulose 1,7-bisphosphate, D-glycero-D-ido-octulose 1,8-bisphosphate, D-glycero-D-altro-octulose 1,8-bisphosphate and D-arabinose 5-phosphate. 3. D-Arabinose 5-phosphate was found to be utilized by a rat liver enzyme preparation to produce both hexose 6-phosphate and triose phosphate. 4. D-Arabinose 5-phosphate was reversibly converted into other pentose 5-phosphates. Paper chromatographic and enzymic evidence indicated that the conversion involved an enzyme tentatively named arabinose phosphate 2-epimerase, which catalyses the following reaction: D-arabinose 5-P in equilibrium D-ribose-5-P. 5. A variety of rat tissues also utilized D-arabinose 5-phosphate to produce both hexose 6-phosphate and triose phosphate and at a rate comparable with that obtained with D-ribose 5-phosphate. 6. A new reaction sequence for the non-oxidative pentose phosphate pathway in liver is proposed.

scholarly journals Pentose Phosphate Pathway Reactions in Photosynthesizing Cells

Cells ◽  
2021 ◽  
Vol 10 (6) ◽  
pp. 1547
Author(s):  
Thomas D. Sharkey
Keyword(s):  
Pentose Phosphate Pathway ◽  
Strong Association ◽  
Pentose Phosphate ◽  
Critical Component ◽  
Reaction Sequence ◽  
Response To Stress ◽  
Labeling Pattern

The pentose phosphate pathway (PPP) is divided into an oxidative branch that makes pentose phosphates and a non-oxidative branch that consumes pentose phosphates, though the non-oxidative branch is considered reversible. A modified version of the non-oxidative branch is a critical component of the Calvin–Benson cycle that converts CO2 into sugar. The reaction sequence in the Calvin–Benson cycle is from triose phosphates to pentose phosphates, the opposite of the typical direction of the non-oxidative PPP. The photosynthetic direction is favored by replacing the transaldolase step of the normal non-oxidative PPP with a second aldolase reaction plus sedoheptulose-1,7-bisphosphatase. This can be considered an anabolic version of the non-oxidative PPP and is found in a few situations other than photosynthesis. In addition to the strong association of the non-oxidative PPP with photosynthesis metabolism, there is recent evidence that the oxidative PPP reactions are also important in photosynthesizing cells. These reactions can form a shunt around the non-oxidative PPP section of the Calvin–Benson cycle, consuming three ATP per glucose 6-phosphate consumed. A constitutive operation of this shunt occurs in the cytosol and gives rise to an unusual labeling pattern of photosynthetic metabolites while an inducible shunt in the stroma may occur in response to stress.

scholarly journals Measurement of protein turnover in rat liver. Analysis of the complex curve for decay of label in a mixture of proteins

1976 ◽  
Vol 156 (3) ◽  
pp. 657-663 ◽  
Author(s):  
P J Garlick ◽  
J C Waterlow ◽  
R W Swick
Keyword(s):  
Rat Liver ◽  
Turnover Rate ◽  
Decay Curve ◽  
Specific Radioactivity ◽  
Liver Protein ◽  
Turnover Rates ◽  
A Value ◽  
Maximum Value ◽  
Single Exponential

The curve for decay of 14C in rat liver protein labelled by injection of NaH14CO3 was analysed to obtain the average turnover rate of mixed liver protein. Three different methods of analysis were used. (1) Unlike decay curves from homogeneous proteins, the curve did not fit a single exponential, but a good fit was obtained with three exponentials. By assuming that the mixture contained three major components with different turnover rates, the calculated value for the average turnover rate (k) was close to 40% per day. (2) k was also calculated from the area under the decay curve, a method which makes no assumptions about the number of proteins in the mixture. This method also gave a value close to 40% per day. (3) It was shown empirically, both by simulation of decay of label in model mixtures of protein and with the decay curve measured in vivo, that k can be calculated from the time taken for the specific radioactivity to fall to 10% of its maximum value. This is an advantage, since the other two methods require the decay curve to be measured over a much longer period of time.

scholarly journals Regulation of the pentose phosphate cycle Cofactor that controls the inhibition of glucose-6-phosphate dehydrogenase by NADPH in rat liver

1986 ◽  
Vol 239 (3) ◽  
pp. 553-558 ◽  
Author(s):  
M Nogueira ◽  
G Garcia ◽  
C Mejuto ◽  
M Freire
Keyword(s):  
Ion Exchange ◽  
Rat Liver ◽  
Pentose Phosphate Pathway ◽  
Reductase Activity ◽  
Gel Filtration ◽  
Ion Exchange Chromatography ◽  
Exchange Chromatography ◽  
Pentose Phosphate ◽  
Glutathione Reductase Activity ◽  
Glucose 6 Phosphate Dehydrogenase

A cofactor of Mr 10(4), characterized as a polypeptide, was found to co-operate with GSSG to prevent the inhibition of glucose-6-phosphate dehydrogenase by NADPH, in order to ensure the operation of the oxidative phase of the pentose phosphate pathway, in rat liver [Eggleston & Krebs (1974) Biochem. J. 138, 425-435; Rodriguez-Segade, Carrion & Freire (1979) Biochem. Biophys. Res. Commun. 89, 148-154]. This cofactor has now been partially purified by ion-exchange chromatography and molecular gel filtration, and characterized as a protein of Mr 10(5). The lighter cofactor reported previously was apparently the result of proteolytic activity generated during the tissue homogenization. The heavier cofactor was unstable, and its amount increased in livers of rats fed on carbohydrate-rich diet. Since the purified cofactor contained no glutathione reductase activity, the involvement of this enzyme in the deinhibitory mechanism of glucose-6-phosphate dehydrogenase by NADPH should be ruled out.

scholarly journals Biosynthesis of phosphatidylethanolamines and phosphatidylcholines from ethanolamine and choline in rat liver

1975 ◽  
Vol 146 (2) ◽  
pp. 309-315 ◽  
Author(s):  
R Sundler ◽  
B Akesson
Keyword(s):  
Rat Liver ◽  
Molecular Species ◽  
Kinetic Models ◽  
Quantitative Data ◽  
Specific Radioactivity ◽  
Similar Rate ◽  
Time Intervals ◽  
Previous Suggestion ◽  
Kinetics Of

1. The kinetics of phosphatidylcholine and phosphatidylethanolamine synthesis in rat liver were followed 5-60 min after the intraportal injection of [14-C]choline and [3-H]-ethanolamine. 2. At all time-intervals the specific radioactivity of CDP-choline was only about half that of phosphorylcholine. This indicated that CDP-choline was formed at a similar rate from phosphorylcholine and phosphatidylcholines, the latter probably through the reverse reaction of cholinephosphotransferase (EC 2.7.8.2.). In view of recent data obtained from experiments in vitro this implies a significant role for the cholinephosphotransferase reaction in the turnover of molecular species of phosphatidylcholine. 3. The specific radioactivity of CDP-ethanolamine was about twice that of phosphorylethanolamine at all time-intervals studied. This supports a previous suggestion that the liver phosphorylethanolamine pool is subject to compartmentation and shows that there is no rapid equilibration between different pools. In contrast with a recent study, no evidence was found for any significant methylation of phosphoryl-or CDP-ethanolamine to the corresponding choline derivative. 4. Quantitative data on the biosynthesis of molecular species of phosphoLIPIDS via CDP derivatives were calculated according to simple kinetic models. They were in the same range as those calculated from earlier data on precusors incorporated via diacylglycerols. 5. The proportion of radioactive phosphatidylethanolamines appearing in the plasma was approximately ten times lower than that for phosphatidylcholines. No selectivity was observed in the transfer into plasma of different molecular species of phosphatidylethanolamine.

scholarly journals The pentose phosphate pathway in rabbit liver. Studies on the metabolic sequence and quantitative role of the pentose phosphate cycle by using a system in situ

1971 ◽  
Vol 123 (5) ◽  
pp. 923-943 ◽  
Author(s):  
J. F. Williams ◽  
K. G. Rienits ◽  
P. J. Schofield ◽  
M. G. Clark
Keyword(s):  
Pentose Phosphate Pathway ◽  
Time Course ◽  
Specific Radioactivity ◽  
Glucose Production ◽  
Pentose Phosphate ◽  
Rate Of Change ◽  
Exchange Reactions ◽  
High Concentration ◽  
Pentose Phosphate Cycle ◽  
Definition Of

1. The reactions of the pentose phosphate cycle were investigated by the intraportal infusion of specifically labelled [14C]glucose or [14C]ribose into the liver of the anaesthetized rabbit. The sugars were confined in the liver by haemostasis and metabolism was allowed to proceed for periods up to 5min. Metabolism was assessed by measuring the rate of change of the specific radioactivity of CO2, the carbon atoms of glucose 6-phosphate, fructose 6-phosphate and tissue glucose. 2. The quotient oxidation of [1-14C]glucose/oxidation of [6-14C]glucose as measured by the incorporation into respiratory CO2 was greater than 1.0 during most of the time-course and increased to a maximum of 3.1 but was found to decrease markedly upon application of a glucose load. 3. The estimate of the pentose phosphate cycle from C-1/C-2 ratios generally increased during the time-course, whereas the estimate of the pentose phosphate cycle from C-3/C-2 ratios varied depending on whether the ratios were measured in glucose or hexose 6-phosphates. 4. The distribution of 14C in hexose 6-phosphate after the metabolism of [1-14C]ribose showed that 65–95% of the label was in C-1 and was concluded to have been the result of a rapidly acting transketolase exchange reaction. 5. Transaldolase exchange reactions catalysed extensive transfer of 14C from [2-14C]glucose into C-5 of the hexose 6-phosphates during the entire time-course. The high concentration of label in C-4, C-5 and C-6 of the hexose 6-phosphates was not seen in tissue glucose in spite of an unchanging rate of glucose production during the time-course. 6. It is concluded that the reaction sequences catalysed by the pentose phosphate pathway enzymes do not constitute a formal metabolic cycle in intact liver, neither do they allow the definition of a fixed stoicheiometry for the dissimilation of glucose.

scholarly journals A Cytochemical Study on the Pancreas of the Guinea Pig

1960 ◽  
Vol 7 (4) ◽  
pp. 619-630 ◽  
Author(s):  
Philip Siekevitz ◽  
George E. Palade
Keyword(s):  
Guinea Pig ◽  
Microsomal Fraction ◽  
Specific Radioactivity ◽  
Early Time ◽  
Acid Extraction ◽  
Time Intervals ◽  
Cytochemical Study ◽  
Cell Compartments ◽  
Exocrine Cell ◽  
Cell Fractions

Chymotrypsinogen synthesis in the exocrine cell of the guinea pig pancreas was studied under the following conditions: Animals fed after a fast of ∼48 hours received ∼1 hour after feeding an intravenous injection of DL-leucine-1-C14. At various time intervals (1 to 45 minutes) after the injection, the glands were removed and fractionated into a series of cell fractions of known cytological significance. Ten to twelve animals were used for each time point. From each cell fraction, the chymotrypsinogen was isolated by acid extraction and purified by (NH4)2SO4 fractionation, isoelectric precipitation, and chromatography. Because of the minuteness of the quantities involved, chymotrypsinogen amounts were calculated from enzymatic activity figures, and a carrier method was used to precipitate and count the enzyme. The chymotrypsinogen isolated from the attached ribonucleoprotein particles of the microsomal fraction had the highest specific radioactivity at the early time points (1 to 3 minutes). After long intervals (at 15 to 45 minutes), the specific radioactivity of the enzyme increased in the microsomal contents and finally in the zymogen granules. The results are compatible with the view that the chymotrypsinogen is synthesized in or on the attached RNP particles and subsequently transported to other cell compartments.

Glucose 6-Phosphate Formation by L-Type Pentose Phosphate Pathway Reactions of Rat Liver in vitro: Further Evidence

1984 ◽  
Vol 365 (2) ◽  
pp. 1425-1434 ◽  
Author(s):  
John F. WILLIAMS ◽  
Michael G. CLARK ◽  
Krishan K. ARORA ◽  
Ian C. REICHSTEIN
Keyword(s):  
Rat Liver ◽  
Pentose Phosphate Pathway ◽  
Pentose Phosphate
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