Biochemistry, comparative Carbohydrate metabolism

Soengas, J.L., Otero, J. Fuentes, J., Andrds, M.D. and Aldegunde, M. (1991). Preliminary studies on carbohydrate metabolism changes in domesticated rainbow trout (Oncorhynchus mykiss) transferred to diluted sea water (12 p.p.t). Comparative Biochemistry and Physiology 98B, 53-57. 

Why are these isotopes important in biochemistry and medicine The isotopes we have mentioned occur at very low natural abundance , e.g. in the world around us only about 1 carbon atom in 10 (a million million) is C. However, with the advent of nuclear physics and specifically the Manhattan Project, the atomic bomb project in World War 11, radioactive isotopes started to be produced artificially, and this meant that chemical compounds could be radioactively labelled , either uniformly (e.g. in every carbon position) or selectively (i.e. with radioactive enrichment in particular positions). In the case of carbohydrate metabolism, it was possible to study the relative importance of glycolysis and PPP by comparing the release of radioactivity from glucose, specifically labelled either in carbon 1 or in carbon 6. If you look at Topic 28, you will see that in the initial reactions of the PPP the CO2 that is produced comes entirely from the Cl position. Over time, as the later molecular rearrangements come into play, C6 atoms could also eventually be released but not initially. On the other hand, if you revisit Topics 13 and 14, you will see that, because the sugar phosphate is split down the middle into two triose phosphate halves that are then handled identically, CO2 released in the oxidation of pyruvate to acetyl CoA will be derived equally from Cl and C6. This allows biochemists to assess the relative activities of PPP and glycolysis in different tissues or in the same tissue over time. This is how it was possible to estimate (Topic 28) that 30% of glucose breakdown in liver is via PPP. 

Saz, H.J. and Lescure, O.L. (1966) Interrelationships between the carbohydrate and lipid metabolism of Ascaris lumbricoides egg and adult stages. Comparative Biochemistry and Physiology 18, 845-857. 

Ethanoic acid is activated for biosynthesis by combination with the thiol, coenzyme A (CoASH, Figure 18-7) to give the thioester, ethanoyl (acetyl) coenzyme A (CH3COSC0A). You may recall that the metabolic degradation of fats also involves this coenzyme (Section 18-8F) and it is tempting to assume that fatty acid biosynthesis is simply the reverse of fatty acid metabolism to CH3COSCoA. However, this is not quite the case. In fact, it is a general observation in biochemistry that primary metabolites are synthesized by different routes from those by which they are metabolized (for example, compare the pathways of carbon in photosynthesis and metabolism of carbohydrates, Sections 20-9,10). 

Lewander, K., Dave, G., Johansson, M.-L., Larsson, A. and Lidman, U. (1974). Metabolic and haematological studies on the yellow and silver phases of the European eel, Anguilla anguilla. -1. Carbohydrate, lipid, protein and inorganic ion metabolism. Comparative Biochemistry and Physiology 47B, 571-581. 

J. Preiss D. A. Walsh, The Comparative Biochemistry of Glycogen and Starch Metabolism and Their Regulation In Biology of Carbohydrate, V. Ginsburg, Ed. John Wiley Sons, Inc. New York, 1981 Vol. 1, pp 199-314. 

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