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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 ]
Enzyme activity is commonly used for e.g. liver function tests like AST, ALT, LD and γ-GT in Sweden. [ 5 ] Percentages and time-dependent units (mol/s) are used for calculated derived parameters, e.g. for beta cell function in homeostasis model assessment or thyroid's secretory capacity .
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]
Liver function tests (LFTs or LFs), also referred to as a hepatic panel or liver panel, are groups of blood tests that provide information about the state of a patient's liver. [1] These tests include prothrombin time (PT/INR), activated partial thromboplastin time (aPTT), albumin , bilirubin (direct and indirect), and others.
Muscle sources of the enzymes, such as intense exercise, are unrelated to liver function and can markedly increase AST and ALT. [5] Cirrhosis of the liver or fulminant liver failure secondary to hepatitis commonly reach values for both ALT and AST in the >1000 U/L range; however, many people with liver disease have normal transaminases.
Lactate is transported back to the liver where it is converted into pyruvate by the Cori cycle using the enzyme lactate dehydrogenase. Pyruvate, the first designated substrate of the gluconeogenic pathway, can then be used to generate glucose. [9]
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.
The proportion of AST to ALT in hepatocytes is about 2.5:1, but because AST is removed from serum by the liver sinusoidal cells twice as quickly (serum half-life t 1/2 = 18 hr) compared to ALT (t 1/2 = 36 hr), so the resulting serum levels of AST and ALT are about equal in healthy individuals, resulting in a normal AST/ALT ratio around 1.