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The Cahill cycle, also known as the alanine cycle or glucose-alanine cycle, [1] is the series of reactions in which amino groups and carbons from muscle are transported to the liver. [2] It is quite similar to the Cori cycle in the cycling of nutrients between skeletal muscle and the liver. [ 1 ]
The liver is responsible for clearing the blood of unconjugated bilirubin, by 'conjugating' it (modified to make it water-soluble) through an enzyme named UDP-glucuronyl-transferase. When the total bilirubin level exceeds 17 μmol/L, it indicates liver disease.
As an example, consider alanine. Alanine is a glucogenic amino acid that the liver's gluconeogenesis process can use to produce glucose. Muscle cells break down their protein when their blood glucose levels fall, which happens during fasting or periods of intense exercise. The breakdown process releases alanine, which is then transferred to the ...
The glucose cycle can occur in liver cells due to a liver specific enzyme glucose-6-phosphatase, which catalyse the dephosphorylation of glucose 6-phosphate back to glucose. Glucose-6-phosphate is the product of glycogenolysis or gluconeogenesis , where the goal is to increase free glucose in the blood due body being in catabolic state.
In similar manner, in muscles the use of pyruvate for transamination gives alanine, which is carried by the bloodstream to the liver (the overall reaction being termed glucose-alanine cycle). Here other transaminases regenerate pyruvate, which provides a valuable precursor for gluconeogenesis.
[1] [2] Other terms include transaminasemia, [3] and elevated liver enzymes (though they are not the only enzymes in the liver). Normal ranges for both ALT and AST vary by gender, age, and geography and are roughly 8-40 U/L (0.14-0.67 μkal/L). [ 4 ]
Alanine transaminase (ALT), also known as alanine aminotransferase (ALT or ALAT), formerly serum glutamate-pyruvate transaminase (GPT) or serum glutamic-pyruvic transaminase (SGPT), is a transaminase enzyme (EC 2.6.1.2) that was first characterized in the mid-1950s by Arthur Karmen and colleagues. [1]
Glucose-6-phosphate can be used in other metabolic pathways or dephosphorylated to free glucose. Whereas free glucose can easily diffuse in and out of the cell, the phosphorylated form (glucose-6-phosphate) is locked in the cell, a mechanism by which intracellular glucose levels are controlled by cells.