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Adv Physiol Educ 41: 286 –290, 2017;
doi:10.1152/advan.00180.2016.
STAYING CURRENT
Understanding the interrelationship between the synthesis of urea and
gluconeogenesis by formulating an overall balanced equation
Piero L. Ipata and Rossana Pesi
Department of Biology, Unit of Biochemistry, University of Pisa, Pisa, Italy
Submitted 10 November 2016; accepted in final form 20 March 2017
overall reaction of ureogenesis; overall reaction of gluconeogenesis;
interaction between ureogenesis and gluconeogenesis; importance of
balanced equations
Introduction: the Equations of Urea Synthesis and of
Gluconeogenesis
THE SYNTHESIS OF UREA,
often referred to as “ureagenesis” or
“urea cycle” was discovered by Krebs and Henseleit in 1932
(12). It occurs predominantly in the liver (3, 6, 11, 14, 15, 28).
Its physiological importance in handling the toxic ammonium
ions discharged during catabolism of amino acids is satisfactorily discussed in many texts. The overall equation of ureagenesis (Table 1) is:
Address for reprint requests and other correspondence: P. L. Ipata, Dept. of
Biology, Unit of Biochemistry, University of Pisa, Via San Zeno 51, 56127
Pisa, Italy (e-mail: [email protected]).
286
1⫺
4⫺
⫹ Asp1⫺
NH1⫹
4 mit ⫹ HCO3 mit ⫹ 3 ATP
cyt ⫹ H2O ¡ ureacyt
2⫺
2⫺
3⫺
⫹ 2 ADP ⫹ 2 Pi ⫹ AMP ⫹ PPi4⫺ ⫹ fumarate2⫺
cyt
⫹ 5 H1⫹(1)
where Asp is aspartate, mit is mitochondrial, and cyt is cytosolic.
Gluconeogenesis is the pathway by which glucose is generated, starting from amino acids or lactate converted to pyruvate. The overall equation (Table 2) is:
4⫺
⫹ 2 GTP4⫺ ⫹ 2 NADH
2 pyruvate1⫺
cyt ⫹ 4 ATP
⫹ 6 H2O ¡ glucosecyt ⫹ 2 NAD1⫹ ⫹ 4 ADP3⫺ ⫹ 2 GDP3⫺
1⫹
⫹ 6 P2⫺
(2)
i ⫹2H
At first glance, both equations may appear too complicated to
be memorized and discussed. However, taking into account the
familiar formulas of urea (H2N-CO-NH2) and fumarate
(⫺OOC-CH ⫽ CH-COO⫺), the two final products of ureagenesis (Eq. 1) can be “read” as follows: 1) one of the two nitrogen
atoms of urea derives from NH1⫹
4 and the other from aspartate,
and the carbon atom from HCO1⫺
3 ; 2) the (4C)carbon skeleton
of aspartate generates the (4C)fumarate molecule. Equation 2
shows that one glucose, a (6C)molecule, is synthesized from
two (3C)molecules of pyruvate. Other noncarbohydrate precursors of glucose enter the gluconeogenetic pathway at oxaloacetate and dihydroxyacetone-phosphate (Table 2). Lactate,
which accumulates during periods of intense exercise in muscle cytosol, is transformed into pyruvate by lactate dehydrogenase:
1⫺
⫹
⫹
lactate1⫺
cyt ⫹ NAD ¡ pyruvatecyt ⫹ NADH ⫹ H
and is converted to glucose by gluconeogenesis. Animals do
not convert fatty acids into glucose, however they can convert
glycerol, produced by the hydrolysis of triacylglycerol, into
dihydroxyacetone-phosphate, via the actions of glycerol kinase
and glycerol phosphate dehydrogenase:
glycerolcyt ⫹ ATP4⫺ ¡ ADP3⫺ ⫹ glycerol-3-phosphate2⫺
cyt
⫹ H⫹ 共glycerol kinase兲
glycerol-3-phosphate2⫺
cyt
⫹ NAD⫹ ¡ dihydroxyacetone-phosphate2⫺
cyt ⫹ NADH
⫹ H⫹ 共glycerol phosphate dehydrogenase兲
Equations 1 and 2 also show that both require the hydrolysis
of “high-energy” nucleoside triphosphates to render the two
processes exergonic. Students are referred to Tables 1 and 2,
reporting the enzymatic and transport steps of the two meta-
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Downloaded from http://advan.physiology.org/ by 10.220.33.3 on June 16, 2017
Ipata PL, Pesi R. Understanding the interrelationship between the
synthesis of urea and gluconeogenesis by formulating an overall
balanced equation. Adv Physiol Educ 41: 286 –290, 2017; doi:
10.1152/advan.00180.2016.—It is well known that a strong metabolic
interrelationship exists between ureagenesis and gluconeogenesis. In
this paper, we present a detailed, overall equation, describing a
possible metabolic link between ureagenesis and gluconeogenesis. We
adopted a guided approach in which we strongly suggest that students,
when faced with the problem of obtaining the overall equation of a
metabolic pathway, carefully account for all atoms and charges of the
single reactions, as well as the cellular localizations of the substrates,
and the related transport systems. If this suggestion is always taken
into account, a balanced, overall equation of a metabolic pathway will
be obtained, which strongly facilitates the discussion of its physiological role. Unfortunately, textbooks often report unbalanced overall
equations of metabolic pathways, including ureagenesis and gluconeogenesis. Most likely the reason is that metabolism and enzymology
have been neglected for about three decades, owing to the remarkable
advances of molecular biology and molecular genetics. In this paper,
we strongly suggest that students, when faced with the problem of
obtaining the overall reaction of a metabolic pathway, carefully
control if the single reactions are properly balanced for atoms and
charges. Following this suggestion, we were able to obtain an overall
equation describing the metabolic interrelationship between ureagenesis and gluconeogenesis, in which urea and glucose are the final
products. The aim is to better rationalize this topic and to convince
students and teachers that metabolism is an important and rewarding
chapter of human physiology.
INTERACTION BETWEEN UREA SYNTHESIS AND GLUCONEOGENESIS
287
Table 1. The reactions, enzymes, transporters, and overall equation of the urea synthesis
Reactions
NH⫹
4 mit
Enzymes
2⫺
4⫺
3⫺
⫹ HCO1⫺
3 mit ⫹ 2 ATP ¡carbamoyl phosphatemit ⫹ 2 ADP
1⫹
2⫺
⫹ carbamoyl phosphatemit
¡citrullinemit ⫹ Pi2⫺ ⫹ H⫹
ornithinemit
1
2
3
4
5
6
7
⫹ Pi
2⫺
⫹
⫹2H
citrullinemit ¡ citrullinecyt
1⫺
1⫺
¡argininosuccinatecyt
⫹ AMP2⫺ ⫹ PPi4⫺ ⫹ 2 H⫹
citrullinecyt ⫹ ATP4⫺ ⫹ Aspcyt
1⫺
2⫺
argininosuccinatecyt ¡argininecyt ⫹ fumaratecyt
1⫹
1⫹
⫹ H2O¡ureacyt ⫹ ornithinecyt
argininecyt
1⫹
1⫹
¡ornithinemit
ornithinecyt
1⫺
1⫺
4⫺
Overall equation: NH⫹
⫹ Aspcyt
⫹ H2O¡ureacyt ⫹ 2 ADP3⫺ ⫹ AMP2⫺ ⫹
4 ⫹ HCO3 ⫹ 3 ATP
2⫺
⫹ 5 H⫹
PPi4⫺ ⫹ 2 Pi2⫺ ⫹ fumaratecyt
Carbamoyl phosphate synthetase 1
Ornithine transcarbamylase
Citrulline-ornithine transporter
Argininosuccinate synthetase
Argininosuccinase
Arginase
Citrulline-ornithine transporter
The synthesis of urea is the result of the cross talk between mitochondria and the cytosolic compartment: two mitochondrial reactions (reactions 1 and 2), three
cytosolic reactions (reactions 4, 5, and 6), and the transport of two intermediates from mithocondria to cytosol and vice versa (steps 3 and 6).
gluconeogenesis, in which both cytosolic urea and glucose are
the final products. The equation also shows the energy cost of
the metabolic interrelationship between the two pathways.
The Synthesis of Phosphoenolpyruvate from the Cytosolic
Fumarate Generated by Ureagenesis
As shown in Eq. 1 and in Table 1 (see also Fig. 1),
ureagenesis produces fumarate, a dicarboxylic acid, in addition
to urea. In the cytosol, fumarate is acted upon by the successive
action of three cytosolic enzymes: fumarase (22), malate dehydrogenase (20, 21), and cytosolic phosphoenolpyruvate
(PEP) carboxy kinase (PEPCK-C) (19).
2⫺
fumarate2⫺
cyt ⫹ H2O ¡ malatecyt 共 cytosolic fumarase兲
⫹
2⫺
malate2⫺
cyt ⫹ NADcyt ¡ OAAcyt ⫹ NADHcyt
⫹
⫹ H 共cytosolic malate dehydrogenase兲
4⫺
3⫺
OAA2⫺
¡ PEP3⫺
⫹ CO2 共PEPCK-C兲
cyt ⫹ GTP
cyt ⫹ GDP
where OAA is oxaloacetate.
The overall equation of these three reactions is:
Table 2. The reactions, enzymes, transporters, and overall equation of gluconeogenesis
Reactions
1
2
2
2
1⫺
1⫺
¡2pyruvatemit
pyruvatecyt
1⫺
2⫺
⫹ 2 ADP3⫺ ⫹ 2 Pi2⫺ ⫹
⫹ 2 CO2 ⫹ 2 ATP4⫺ ⫹ 2 H2O¡2 oxaloacetatemit
pyruvatemit
1⫹
4H
2⫺
2⫺
⫹
⫹ 2 NADmit
⫹ 2 NADHmit ⫹ 2 H1⫹¡2 malatemit
2 oxaloacetatemit
2⫺
2⫺
malatemit¡2malatecyt
2⫺
1⫹
2⫺
¡2 oxaloacetatecyt
⫹ 2 NADHcyt ⫹ 2 H1⫹
⫹ 2 NADcyt
2 malatecyt
2⫺
3⫺
⫹ 2 GTP4⫺¡2 PEPcyt
⫹ 2 CO2 ⫹ 2 GDP3⫺
2 oxaloacetatecyt
3⫺
3⫺
⫹ 2 H2O¡2 2-phosphoglyceratecyt
2 PEPcyt
3⫺
3⫺
¡23-phosphoglyceratecyt
22-phosphoglyceratecyt
3⫺
4⫺
4⫺
⫹ 2 ADP3⫺
2 3-phosphoglyceratecyt ⫹ 2 ATP ¡2 1,3-bisphosphoglyceratecyt
4⫺
2⫺
⫹ 2 NADH ⫹ 2 H1⫹¡2 glyceraldehyde-3-phosphatecyt
⫹
2 1,3-bisphosphoglyceratecyt
2 NAD1⫹ ⫹ 2 Pi2⫺
2⫺
2⫺
11
¡dihydroxyacetone-phosphatecyt
glyceraldehyde-3-phosphatecyt
2⫺
2⫺
4⫺
12
⫹ dihydroxyacetone-phosphatecyt
¡fructose-1,6-bisphosphatecyt
glyceraldehyde-3-phosphatecyt
4⫺
2⫺
2⫺
13
fructose-1,6-bisphosphatecyt ⫹ H2O¡fructose-6-phosphatecyt ⫹ Pi
2⫺
2⫺
14
¡glucose-6-phosphatecyt
fructose-6-phosphatecyt
2⫺
15
⫹ H2O¡glucosecyt ⫹ Pi2⫺
glucose-6-phosphatecyt
1⫺
Overall equation: 2 pyruvatecyt
⫹ 4 ATP4⫺ ⫹ 2 GTP4⫺ ⫹ 2 NADH ⫹ 6 H2O¡glucosecyt ⫹ 2 NAD1⫹ ⫹
4 ADP3⫺ ⫹ 2 GDP3⫺ ⫹ 6 Pi2⫺ ⫹ 2 H1⫹
3
4
5
6
7
8
9
10
Enzymes
Mitochondrial pyruvate transporter
Mitochondrial pyruvate carboxylase
Mitochondrial malate dehydrogenase
Mitochondrial malate transporter
Malate dehydrogenase
PEP carboxykinase
Enolase
Phosphoglycerate mutase
Phosphoglycerate kinase
3-Phoglyceraldehyde dehydrogenase
Triose phosphate mutase
Aldolase
Fructose-1,6-bisphosphatase
Phosphoglucoisomerase
Glucose-6-phosphatase
Two (3C)molecules are needed to synthesize one (6C)glucose molecule. Therefore, reactions 1–10 are multiplied by 2. The majority of the single reactions
are catalyzed by cytosolic enzyme proteins. However, steps 1– 4 require the intervention of 2 mitochondrial transporters and 2 mitochondrial enzyme proteins.
3⫺
⫹ 4 H2O ⫹ 2 ATP4⫺ ⫹ 2 NADH ⫹
The overall equation of gluconeogenesis starting from 2 cytosolic PEP molecules (reactions 7–15) is 2 PEPcyt
2 H1⫹¡glucose ⫹ 2 ADP3⫺ ⫹ 4 Pi2⫺ ⫹ 2 NAD1⫹.
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bolic pathways, to get a deeper insight into this topic. Moreover, they should be aware that discussing and memorizing the
overall equation of a pathway may facilitate the understanding
of its physiogical role. However, it is mandatory that the
overall equation, as well as the single reactions, account for all
atoms and charges, to obey the Lavoisier principle and, more
importantly, to rationalize the discussion. H1⫹ and H2O may be
omitted only when dealing with thermodynamic parameters,
not when the stoichiometry of a metabolic pathway is being
investigated (1). These fundamental rules are not always
obeyed. A cursory check showed that only in one textbook is
the ureagenesis overall equation properly balanced (24). Unbalanced overall equations are found in other textbooks and in
specialized articles (2, 9, 17, 23, 26, 29, 32), causing ambiguities, misunderstanding, and frustration in the most consciensious students. See Refs. 13 and 16 for a discussion on this
issue. The same holds true for gluconeogenesis. Its balanced
overall equation reported above (Eq. 2) is that reported in Ref.
5. Overall unbalanced equations are reported in other textbooks. In this paper, we propose a single balanced overall
equation, based on our present knowledge on ureagenesis and
288
INTERACTION BETWEEN UREA SYNTHESIS AND GLUCONEOGENESIS
1⫹
fumarate2⫺
⫹ GTP4⫺ ¡ NADH ⫹ H1⫹
cyt ⫹ H2O ⫹ NAD
⫹ PEP3⫺ ⫹ GDP3⫺ ⫹ CO2
By adding this equation to Eq. 1, we can state that one turn
of ureagenesis may be coupled to the synthesis of one cytosolic
PEP:
1⫺
4⫺
⫹ 2 H2O ⫹ GTP4⫺ ⫹ Asp1⫺
NH1⫹
4 mit ⫹ HCO3 ⫹ 3 ATP
cyt
3⫺
3⫺
⫹ NAD1⫹
⫹ AMP2⫺ ⫹ PP4⫺
cyt ¡ ureacyt ⫹ PEPcyt ⫹ 2 ADP
⫹ 2 P2⫺ ⫹ GDP3⫺ ⫹ CO2 ⫹ NADHcyt ⫹ 6 H1⫹ (3)
The other cytosolic PEP molecule, needed to synthesize one
glucose molecule by gluconeogenesis (see Table 2 legend) is
synthesized starting from mitochondrial pyruvate (Pyr),
through one of the two possible pathways shown in Fig. 1. In
the first one (the PEPCK-C-mediated gluconeogenesis), pyruvate is acted upon by mitochondrial pyruvate kinase (7, 27),
followed by mitochondrial malate dehydrogenase (20), malate
transfer into the cytosolic compartment (25), cytosolic malate
dehydrogenase (20), and PEPCK-C (19):
2⫺
4⫺
3⫺
Pyr1⫺
⫹ HCO1⫺
⫹ P2⫺
mit ⫹ ATP
3 ¡ OAAmit ⫹ ADP
i
⫹ H1⫹共pyruvate carboxylase兲
1⫹
OAA2⫺
¡ malate2⫺
mit ⫹ NADHmit ⫹ H
mit
⫹ NAD1⫹
mitochondrial
malate dehydrogenase兲
共
mit
2⫺
malate2⫺
mit ¡ malatecyt 共 malate 3-oxoglutarate carrier兲
1⫹
2⫺
malate2⫺
cyt ⫹ NADcyt ¡ OAAcyt ⫹ NADHcyt
1⫹
⫹ H 共cytosolic malate dehydogenase兲
4⫺
3⫺
3⫺
OAA2⫺
cyt ⫹ GTPcyt ¡ PEPcyt ⫹ GDPcyt ⫹ CO2共 PEPCK ⫺ C兲
By summarizing these five steps, we obtain the following
overall reaction for the synthesis of the second PEP molecule,
starting with pyruvate:
4⫺
4⫺
3⫺
Pyr1⫺
⫹ HCO1⫺
¡ PEP3⫺
mit ⫹ ATP
3 ⫹ GTP
cyt ⫹ ADP
2⫺
3⫺
1⫹
⫹ Pi ⫹ GDP ⫹ H ⫹ CO2 (4)
The second pathway [often named the PEPCK-M (mitochondrial phosphoenolpyruvate carboxy kinase)-mediated PEP
gluconeogenesis] (see Fig. 1) is composed of the following
three steps:
2⫺
4⫺
3⫺
Pyr1⫺
⫹ HCO1⫺
⫹ P2⫺
mit ⫹ ATP
3 ¡ OAAmit ⫹ ADP
i
1⫹
⫹ H (pyruvate carboxylase)
4⫺
3⫺
3⫺
OAA2⫺
⫹ CO2共PEPCK-M兲
mit ⫹ GTPmit ¡ PEPmit ⫹ GDP
3⫺
PEP3⫺
mit ¡ PEPcyt 共 PEP transporter兲
Interestingly, the overall equation of these three reactions is
Eq. 4 (see Fig. 1).
We are, therefore, faced with two cytosolic PEP molecules
generated by Eq. 3 (the synthesis of one cytosolic PEP via one
turn of the ureagenesis coupled with fumarate conversion to
PEP), and by Eq. 4 (the synthesis of one cytosolic PEP
molecule mediated either by PEPCK-C or by PEPCK-M). It
has long been known that a strong interrelationship exists
between ureagenesis and gluconeogenesis (18). By summarizing Eqs. 3 and 4 to gluconeogenesis equation, starting from 2
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Fig. 1. Metabolic relationship between the ureagenesis and the gluconeogenesis pathways. The figure shows that one turn of the urea cycle generates one
cytosolic fumarate and one cytosolic urea. Urea is excreted by urine, whereas fumarate and one mitochondrial pyruvate generate two cytosolic PEPs, which are
then converted to glucose by gluconeogenesis. Cytosolic fumarate is converted to cytosolic PEP via reactions 12 (cytosolic fumarase), 13 (cytosolic malate
dehydrogenase), and 14 (PEPCK-C). The second PEP molecule is synthesized via the successive action of either reaction 7 (mitochondrial pyruvate carboxylase),
followed by reaction 10 (PEPCK-M), and by the transfer of mitochondrial PEP in the cytosol (step 11) (the PEPCK-M-mediated gluconeogenesis), or via reaction
7, followed by reaction 8 (mitochondrial malate dehydrogenase), the transport of mitochondrial malate into cytosol (step 13), and by PEPCK-C (reaction 14)
(the PEPCK-C mediated gluconeogenesis). Other enzymes and transporters are as follows: 1) carbamoyl-phosphate synthetase; 2) ornithine transcarbamylase;
3) citrulline-ornithine transporter; 4) argininosuccinate synthetase; 5) argininosuccinase; 6) arginase. The overall reaction of the gluconeogenetic pathway starting
from two PEPs is reported in Table 1, legend.
INTERACTION BETWEEN UREA SYNTHESIS AND GLUCONEOGENESIS
PEP (see Table 2 legend), we obtain, to our knowledge, for the
first time, a single balanced equation describing the molecular
basis of the interaction between ureagenesis and gluconeogenesis networks, with the production of both urea and glucose.
1⫺
1⫺
NH1⫹
⫹ 6 ATP4⫺ ⫹ 6 H2O
4 mit ⫹ 2 HCO3 mit ⫹ Pyr
1⫹
⫹ 2 NADHcyt
⫹ 2 GTP4⫺ ⫹ Asp1⫺
cyt ⫹ 2 NAD
1⫹
⫹ 2 H ¡ ureacyt ⫹ glucosecyt ⫹ 5 ADP3⫺ ⫹ AMP2⫺
1⫹
⫹ PP4⫺ ⫹ 7 P2⫺
⫹ 2 CO2 ⫹ 2 GDP3⫺
i ⫹ NAD
⫹ NAD⫹ ⫹ 7 H1⫹ (5)
Discussion
In the last three decades, the remarkable advances of molecular biology and molecular genetics have somewhat
eclipsed areas of traditional biology, such as enzymology and
metabolism. In recent years metabolism has reemerged as a
central topic in biology (4, 30, 31). Nevertheless, recurring
errors create problems, when students try to balance the reactions of a metabolic pathway (13, 16). In this article, we
present for the first time an overall equation describing the
relationship between urea synthesis and gluconeogenesis. We
suggest that students always ensure if the single reactions of a
metabolic pathway are balanced for atoms and charges, so that
the overall equation will also result in being balanced. We are
aware that some textbooks may simply be geared to an audience that may require a more general and simple knowlwdge or
understanding. Therefore, this article would benefit students in
a graduate metabolism course or a professional health course
such as those taught in medical schools. According to a
proposed lesson plan, an educator could use this as an assignment for their students to get a better understanding of the
connection between ureagenesis and gluconeogenesis, including two modules. In the first module, students can use the
single reactions of Tables 1 and 2 to develop an overall
equation for each pathway. The section, Introduction: the
Equations of Urea Synthesis and of Gluconeogenesis, contains
most of the information related to this issue. In the second
module, students will formulate an overall balanced equation
encompassing both pathways. They can discuss the possible
pathophysiological perturbation, the catabolism of the amino
acids leading to fumarate and oxalacetate, as well as the three
carbons of odd chain fatty acids to succinyl-CoA during
catabolism.
ACKNOWLEDGMENTS
This work was supported by local funds of the University of Pisa. Authors
are grateful to Prof. Umberto Mura for precious advice.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
AUTHOR CONTRIBUTIONS
P.L.I. and R.P. drafted manuscript; P.L.I. and R.P. edited and revised
manuscript; P.L.I. and R.P. approved final version of manuscript; R.P. prepared figures.
REFERENCES
1. Alberty RA, Cornish-Bowden A, Goldberg RN, Hammes GG, Tipton
K, Westerhoff HV. Recommendations for terminology and databases for
biochemical thermodynamics. Biophys Chem 155: 89 –103, 2011. doi:
10.1016/j.bpc.2011.03.007.
2. Anand U, Anand CV. The energy cost of the urea synthesis. Biochem
Educ 21: 198 –199, 1993. doi:10.1016/0307-4412(93)90095-H.
3. Ash DE. Structure and function of arginases. J Nutr 134, Suppl: 2760S–
2764S, 2004.
4. Bar-Even A, Flamholz A, Noor E, Milo R. Rethinking glycolysis: on the
biochemical logic of metabolic pathways. Nat Chem Biol 8: 509 –517,
2012. doi:10.1038/nchembio.971.
5. Berg JM, Tymoczko, JL, Stryer L. Biochemistry (7th Ed.). Basingstoke,
UK: Palgrave MacMillan, 2012, p. 425.
6. Brusilow SW, Horwich AL. Urea cycle enzymes (8th Ed.). In: The
Metabolic and Molecular Bases of Inherited Disease (Scriver CR, Beaudet
AL, Sly WS, Valle D, editors). New York: McGraw-Hill, 2001, vol. 2, p.
1897–1963.
7. Buc HA, Demaugre F, Moncion A, Leroux JP. Metabolic consequences of
pyruvate kinase inhibition by oxalate in intact rat hepatocytes. Biochimie 63:
595– 602, 1981. doi:10.1016/S0300-9084(81)80057-0.
8. Chakravarty K, Cassuto H, Reshef L, Hanson RW. Factors that control
the tissue-specific transcription of the gene for phosphoenolpyruvate
carboxykinase-C. Crit Rev Biochem Mol Biol 40: 129 –154, 2005. doi:
10.1080/10409230590935479.
9. Devlin TM. Textbook of Biochemistry with Clinical Correlation (3rd Ed.).
Hoboken, NJ: Wiley-Liss, 1992, p. 483.
10. Devlin TM. Textbook of Biochemistry with Clinical Correlation (6th Ed.).
Hoboken, NJ: Wiley-Liss, 2006, p. 593.
11. Haines RJ, Pendleton LC, Eichler DC. Argininosuccinate synthase:
at the center of arginine metabolism. Int J Biochem Mol Biol 2: 8 –23,
2011.
12. Holmes FL. Hans Krebs: Architect of the Intermediary Metabolism,
1933–1937. New York: Oxford University Press, 1993, vol. 2.
13. Ipata PL, Pesi R. What is the true nitrogenase reaction? A guided
approach. Biochem Mol Biol Educ 43: 142–144, 2015. doi:10.1002/
bmb.20843.
14. Jackson MJ, Beaudet AL, O’Brien WE. Mammalian urea cycle
enzymes. Annu Rev Genet 20: 431– 464, 1986. doi:10.1146/annurev.ge.20.
120186.002243.
15. Kimball ME, Jacoby LB. Purification and properties of argininosuccinate
synthetase from normal and canavanine-resistant human lymphoblasts.
Biochemistry 19: 705–709, 1980. doi:10.1021/bi00545a015.
16. Lane AN, Fan TWM, Higashi RM. Metabolic acidosis and the importance of balancing equations. Metabolomics 5: 163–165, 2009. doi:
10.1007/s11306-008-0142-2.
17. Mathews CK, Van Holde KE. Biochemistry (2nd Ed.). Rozzano, Italy:
Editrice Ambrosiana, 1999, p. 720.
18. Meijer AJ, Gimpel JA, Deleeuw G, Tischler ME, Tager JM, Williamson JR. Interrelationships between gluconeogenesis and ureogenesis in
isolated hepatocytes. J Biol Chem 253: 2308 –2320, 1978.
19. Méndez-Lucas A, Duarte JA, Sunny NE, Satapati S, He T, Fu X,
Bermúdez J, Burgess SC, Perales JC. PEPCK-M expression in mouse
liver potentiates, not replaces, PEPCK-C mediated gluconeogenesis. J
Hepatol 59: 105–113, 2013. doi:10.1016/j.jhep.2013.02.020.
20. Minárik P, Tomásková N, Kollárová M, Antalík M. Malate dehydrogenases—structure and function. Gen Physiol Biophys 21: 257–265,
2002.
21. Musrati RA, Kollárová M, Mernik N, Mikulásová D. Malate dehydrogenase: distribution, function and properties. Gen Physiol Biophys 17:
193–210, 1998.
22. Yogev O, Naamati A, Pines O. Fumarase: a paradigm of dual targeting
and dual localized functions. FEBS J 278: 4230 – 4242, 2011. doi:10.1111/
j.1742-4658.2011.08359.x.
23. Nelson DL, Cox MM. Lehninger Principles of Biochemistry (4th Ed.).
San Francisco, CA: Freeman, 2004, p. 684.
24. Newsholme EA, Leech AR. Biochemistry for the Medical Sciences (4th
Ed.). New York: Wiley, 1988, p. 490.
25. Palmieri F. Mitochondrial carrier proteins. FEBS Lett 346: 48 –54, 1994.
doi:10.1016/0014-5793(94)00329-7.
26. Pattabiraman TN. The energy cost of urea synthesis: a reappraisal.
Biochem Educ 23: 24 –25, 1995. doi:10.1016/0307-4412(94)00093-5.
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The cost is the hydrolysis of nine “high-energy” phosphodiesteric bonds (admitting that the PPi is hydrolyzed by inorganic
pyrophosphatase). However, if the two cytosolic NADH are
oxidized via the glycerophosphate shuttle, they yield three
ATP molecules (10)
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INTERACTION BETWEEN UREA SYNTHESIS AND GLUCONEOGENESIS
27. Pizzuto R, Paventi G, Atlante A, Passarella S. Pyruvate kinase in pig
liver mitochondria. Arch Biochem Biophys 495: 42– 48, 2010. doi:
10.1016/j.abb.2009.12.016.
28. Rubio V, Ramponi G, Grisolia S. Carbamoyl phosphate synthetase I of
human liver. Purification, some properties and immunological crossreactivity with the rat liver enzyme. Biochim Biophys Acta 659: 150 –160,
1981. doi:10.1016/0005-2744(81)90279-5.
29. Rawn JD. Biochemistry. Burlington, NC: Neil Patterson, 1989, p. 463.
30. Ray LB. Metabolism is not boring. Science 330: 1337, 2010. doi:10.1126/
science.330.6009.1337.
31. Vander Heiden MG. Targeting cancer metabolism: a therapeutic window
opens. Nat Rev Drug Discov 10: 671– 684, 2011. doi:10.1038/nrd3504.
32. Voet D, Voert JG, Pratt CW. Fundamentals of Bochemistry (4th Ed.).
New York: Wiley, 2013, p. 761.
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