The molecular origin of standard metabolic rate and thermogenesis in mammals is examined. It is pointed out that there are important differences and distinctions between the cellular reactions that 1) couple to oxygen consumption, 2) uncouple metabolism, 3) hydrolyze ATP, 4) control metabolic rate, 5) regulate metabolic rate, 6) produce heat, and 7) dissipate free energy. The quantitative contribution of different cellular reactions to these processes is assessed in mammals. We estimate that approximately 90% of mammalian oxygen consumption in the standard state is mitochondrial, of which approximately 20% is uncoupled by the mitochondrial proton leak and 80% is coupled to ATP synthesis. The consequences of the significant contribution of proton leak to standard metabolic rate for tissue P-to-O ratio, heat production, and free energy dissipation by oxidative phosphorylation and the estimated contribution of ATP-consuming processes to tissue oxygen consumption rate are discussed. Of the 80% of oxygen consumption coupled to ATP synthesis, approximately 25-30% is used by protein synthesis, 19-28% by the Na(+)-K(+)-ATPase, 4-8% by the Ca2(+)-ATPase, 2-8% by the actinomyosin ATPase, 7-10% by gluconeogenesis, and 3% by ureagenesis, with mRNA synthesis and substrate cycling also making significant contributions. The main cellular reactions that uncouple standard energy metabolism are the Na+, K+, H+, and Ca2+ channels and leaks of cell membranes and protein breakdown. Cellular metabolic rate is controlled by a number of processes including metabolic demand and substrate supply. The differences in standard metabolic rate between animals of different body mass and phylogeny appear to be due to proportionate changes in the whole of energy metabolism. Heat is produced by some reactions and taken up by others but is mainly produced by the reactions of mitochondrial respiration, oxidative phosphorylation, and proton leak on the inner mitochondrial membrane. Free energy is dissipated by all cellular reactions, but the major contributions are by the ATP-utilizing reactions and the uncoupling reactions. The functions and evolutionary significance of standard metabolic rate are discussed.

Cellular energy utilization and molecular origin of standard metabolic rate in mammals

In early studies on energy metabolism of tumor cells, it was proposed that the enhanced glycolysis was induced by a decreased oxidative phosphorylation. Since then it has been indiscriminately applied to all types of tumor cells that the ATP supply is mainly or only provided by glycolysis, without an appropriate experimental evaluation. In this review, the different genetic and biochemical mechanisms by which tumor cells achieve an enhanced glycolytic flux are analyzed. Furthermore, the proposed mechanisms that arguably lead to a decreased oxidative phosphorylation in tumor cells are discussed. As the O(2) concentration in hypoxic regions of tumors seems not to be limiting for the functioning of oxidative phosphorylation, this pathway is re-evaluated regarding oxidizable substrate utilization and its contribution to ATP supply versus glycolysis. In the tumor cell lines where the oxidative metabolism prevails over the glycolytic metabolism for ATP supply, the flux control distribution of both pathways is described. The effect of glycolytic and mitochondrial drugs on tumor energy metabolism and cellular proliferation is described and discussed. Similarly, the energy metabolic changes associated with inherent and acquired resistance to radiotherapy and chemotherapy of tumor cells, and those determined by positron emission tomography, are revised. It is proposed that energy metabolism may be an alternative therapeutic target for both hypoxic (glycolytic) and oxidative tumors.

https://febs.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1742-4658.2007.05686.x

Energy metabolism in tumor cells

Cancer metabolism: facts, fantasy, and fiction.

Abstract The degradation of denatured globin in reticulocyte lysates is markedly stimulated by ATP. This system has now been resolved into two components, designated fractions I and II, in the order of their elution from DEAE-cellulose. Fraction II has a neutral protease activity but is stimulated only slightly by ATP, whereas fraction I has no proteolytic activity but restores ATP-dependent proteolysis when combined with fraction II. The active principle of fraction I is remarkably heat-stable, but it is non-dialysable, precipitable with ammonium sulfate and it is destroyed by treatment with proteolytic enzymes. In gel filtration on Sephadex-G-75, it behaves as a single component with a molecular weight of approximately 9,000.

A heat-stable polypeptide component of an ATP-Dependent Proteolytic System from reticullocytes

Intracellular proteolytic systems may function as secondary antioxidant defenses: An hypothesis

The report deals with a detailed balance of ATP production and consumption of the rabbit reticulocyte.



1. The sum-total of ATP produced amounts to 135mmol · l−1· h−1.



2. About 70% of the ATP consumption has been accounted for by specific processes. The main contributing processes are the globin synthesis with about 28%, the Na+, K+-ATPase with 23% and the proteolysis with more than 15%. 30% of ATP consumption has not been accounted for.



3. Cycloheximide (20 μM) leads to a dissociation between synthesis and degradation of proteins, which argues against any obligatory connection between these processes.



4. More than 90% of the lysine liberated from mitochondria by proteolysis were reutilized for the globin synthesis demonstrating the high nitrogen economy of reticulocytes.



5. Each of the ATP-consuming processes studied appears to control ATP production in an independent manner without competition with each other.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1984.tb07982.x

Accounting for the ATP-consuming processes in rabbit reticulocytes

ATP production of Ehrlich ascites tumour cells was estimated on the basis of their coupled respiration and lactate formation. ATP-consuming processes were assessed from the effects of selective inhibitors of RNA synthesis, protein synthesis and proteolysis, Na+/K+-ATPase on respiration. The extent of protein synthesis and proteolysis were also determined directly. From these values and those of the inhibition of respiration by selective inhibitors, a P/O ratio of 2.2 was calculated. About 75% of the total ATP consumption could be assigned to specific processes. The major ATP-consuming processes of tumour cells in an amino-acid-enriched medium, in which they are in an approximate steady state, are protein synthesis with about 30% of total ATP consumption, and Na+/K+-ATPase with about 20%, while RNA synthesis, ATP-dependent proteolysis and Ca2+-ATPase contribute about 10% each. In an amino-acid-free glucose medium, protein synthesis is reduced to a third, with a corresponding decrease of respiration, whereas the rate of the other ATP-consuming processes is unchanged.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1986.tb10496.x

Quantification of ATP-producing and consuming processes of Ehrlich ascites tumour cells.

In this work it is demonstrated that glucose constitutes the main substrate of energy metabolism of rabbit reticulocytes under aerobic conditions in the presence of 5 mM glucose. Amino acids and fatty acids are minor sources of energy. The shares of processes utilizing glucose in reticulocytes were estimated from tracer experiments. A new mathematical technique used permits the derivation of closed terms for the specific radioactivity of single positions of C atoms of the metabolites of the citrate cycle. By means of regression analysis, the undetermined flux rates in the citrate cycle were calculated. On the basis of the data an overall balance sheet of glucose utilization and of ATP generation is given. About 45% of the glucose of reticulocytes is catabolized via the citrate cycle, about the same percentage yields lactate. Only 2% of the glucose was oxidized in the oxidative pentose pathway whereas the remainder is used for the formation of serine and glycine required for hemoglobin synthesis. These results are related to knowledge about the main processes utilizing ATP in reticulocytes, i.e. the synthesis of hemoglobin and the energy-dependent proteolysis. Our approach to the investigation of metabolic relations in the reticulocytes can be applied to other tissues in which equilibria between large metabolite pools play a role.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1982.tb06631.x

Quantification of pathways of glucose utilization and balance of energy metabolism of rabbit reticulocytes.

In this work it is demonstrated the NH3 potentially available from the oxidation of amino acids by the reticulocyte is utilized for the new-synthesis of serine via transamination reactions with hydroxypyruvate and phosphohydroxypyruvate. These compounds are derived from glucose, which furnishes in this manner the carbon skeleton of serine. It was shown that serine is mainly degraded via glycine, while glycine is in the main formed from serine. Furthermore it was found that serine synthesis is localized in the cytosol, while the reversible transformation of serine to glycine takes place in the mitochondria. A transfer of methylenetetrahydrofolate occurs in both directions across the mitochondrial membrane. A concentrative uptake of serine by the mitochondria was found, while glycine is transported slowly in both directions. On the basis of the data an overall balance is given of the quantitative relations between protein breakdown and hemoglobin synthesis as well as for the new-formation and utilization of serine and glycine. The new-formation of serine may amount to about one tenth of the glucose utilized by the reticulocyte and furnishes about one-half of the serine and glycine moieties required for the synthesis of heme and globin. The remainder is provided by the energy-dependent protein breakdown.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1980.tb04741.x

Renate Dumdey

Papers

Accounting for the ATP-consuming processes in rabbit reticulocytes

Quantification of ATP-producing and consuming processes of Ehrlich ascites tumour cells.

Quantification of pathways of glucose utilization and balance of energy metabolism of rabbit reticulocytes.

Nitrogen Economy and the Metabolism of Serine and Glycine in Reticulocytes of Rabbits