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pass through a cycle; "This machine automatically cycles"
an interval during which a recurring sequence of events occurs; "the never-ending cycle of the seasons" (同)rhythm, round
recur in repeating sequences
a single complete execution of a periodically repeated phenomenon; "a year constitutes a cycle of the seasons" (同)oscillation
a periodically repeated sequence of events; "a cycle of reprisal and retaliation"
a series of poems or songs on the same theme; "Schuberts song cycles"
cause to go through a recurring sequence; "cycle the laundry in this washing program"
the sport of traveling on a bicycle or motorcycle

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『周期』,循環期;一巡り / 周波,サイクル / (英雄・伝説などを扱った)一連の詩(物語) / 『自転車』;三輪車;オートバイ / 自転車に乗る / 循環する,反復する
自転車に乗ること,サイクリング
cobaltの化学記号

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/07/17 17:32:41」(JST)

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The Cori cycle (also known as Lactic acid cycle), named after its discoverers, Carl Cori and Gerty Cori, refers to the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is converted back to lactate.^[1]

Cycle[edit]

Gerty Cori and Carl Cori jointly won the 1947 Nobel Prize in Physiology or Medicine, not however for their discovery of the Cori cycle but for their discovery of the course of the catalytic conversion of glycogen.

Muscular activity requires energy, which is provided by the breakdown of glycogen in the skeletal muscles. The breakdown of glycogen, a process known as glycogenolysis, releases glucose in the form of glucose-1-phosphate (G-1-P). The G-1-P is converted to G-6-P by the enzyme phosphoglucomutase. G-6-P is readily fed into glycolysis,(or can go into the pentose phosphate pathway if G-6-P concentration is high) a process that provides ATP to the muscle cells as an energy source. During muscular activity, the store of ATP needs to be constantly replenished. When the supply of oxygen is sufficient, this energy comes from feeding pyruvate, one product of glycolysis, into the Krebs cycle.

When oxygen supply is insufficient, typically during intense muscular activity, energy must be released through anaerobic metabolism. Lactic acid fermentation converts pyruvate to lactate by lactate dehydrogenase. Most important, fermentation regenerates NAD⁺, maintaining the NAD⁺ concentration so that additional glycolysis reactions can occur. The fermentation step oxidizes the NADH produced by glycolysis back to NAD⁺, transferring two electrons from NADH to reduce pyruvate into lactate. Refer to the main articles on glycolysis and fermentation for the details.

Instead of accumulating inside the muscle cells, lactate produced by anaerobic fermentation is taken up by the liver. This initiates the other half of the Cori cycle. In the liver, gluconeogenesis occurs. From an intuitive perspective, gluconeogenesis reverses both glycolysis and fermentation by converting lactate first into pyruvate, and finally back to glucose. The glucose is then supplied to the muscles through the bloodstream; it is ready to be fed into further glycolysis reactions. If muscle activity has stopped, the glucose is used to replenish the supplies of glycogen through glycogenesis.^[2]

Overall, the glycolysis part of the cycle produces 2 ATP molecules at a cost of 6 ATP molecules consumed in the gluconeogenesis part. Each iteration of the cycle must be maintained by a net consumption of 4 ATP molecules. As a result, the cycle cannot be sustained indefinitely. The intensive consumption of ATP molecules indicates that the Cori cycle shifts the metabolic burden from the muscles to the liver.

Significance[edit]

The cycle's importance is based on the prevention of lactic acidosis in the muscle under anaerobic conditions. However, normally before this happens the lactic acid is moved out of the muscles and into the liver.^[2]

The cycle is also important in producing ATP, an energy source, during muscle activity. The Cori cycle functions more efficiently when muscle activity has ceased. This allows the oxygen debt to be repaid such that the Krebs cycle and electron transport chain can produce energy at peak efficiency.^[2]

The drug metformin can precipitate lactic acidosis in patients with renal failure because metformin inhibits the cori cycle. Normally, the excess lactate would be cleared by the kidneys, but in patients with renal failure, the kidneys cannot handle the excess lactic acid.

References[edit]

^ Nelson, David L., & Cox, Michael M.(2005) Lehninger Principles of Biochemistry Fourth Edition. New York: W.H. Freeman and Company, p. 543.
^ ^a ^b ^c "Cori Cycle". Retrieved May 3, 2008, from Elmhurst, pp. 1–3.

Sources[edit]

Smith, A.D., Datta, S.P., Smith, G. Howard, Campbell, P.N., Bentley, R., (Eds.) et al.(1997) Oxford Dictionary of Biochemistry and Molecular Biology. New York: Oxford University Press.

UpToDate Contents

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English Journal

Hypoxia and exercise provoke both lactate release and lactate oxidation by the human brain.

Overgaard M, Rasmussen P, Bohm AM, Seifert T, Brassard P, Zaar M, Homann P, Evans KA, Nielsen HB, Secher NH.SourceDepartment of Anesthesia, The Copenhagen Muscle Research Center, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
FASEB journal : official publication of the Federation of American Societies for Experimental Biology.FASEB J.2012 Jul;26(7):3012-20. Epub 2012 Mar 21.
Lactate is shuttled between organs, as demonstrated in the Cori cycle. Although the brain releases lactate at rest, during physical exercise there is a cerebral uptake of lactate. Here, we evaluated the cerebral lactate uptake and release in hypoxia, during exercise and when the two interventions we
PMID 22441982