GPT (ALT, Alanin-Aminotransferase): the pathway in the body

GPT (ALT, Alanin-Aminotransferase) is part of the pathway “Liver enzymes”. This page shows the whole pathway; the station of GPT (ALT, Alanin-Aminotransferase) is highlighted.

Where this laboratory value sits: GPT (ALT) — alanine aminotransferase. GPT sits in the cytoplasm of liver cells, and in much smaller amounts in muscle and kidney. It transfers the amino group of alanine to α-ketoglutarate. It enters the blood when liver cells renew themselves or their membrane becomes more permeable. Source 1, 3

In brief

Liver enzymes are enzymes whose activity is measured in the blood: GPT, GOT, LDH, γGT, alkaline phosphatase and cholinesterase. They work inside the cell, at the cell membrane or in plasma, and reach the blood through ongoing cell turnover.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Alanine → Pyruvate GPT · Vitamin B6 (PLP) GPT turns alanine into pyruvate. In the liver it serves as a starting material for new glucose or is broken down in the mitochondrion for energy. Source 3, 5↕ both, depending on amount Pyruvate is a junction: in the liver it goes into glucose formation, in the mitochondrion into the citric acid cycle, and via LDH it can become lactate. Which route runs depends on what the cell needs. established physiology Source 5
   ⚖ When the balance tips

   too much — If pyruvate builds up because the mitochondrion cannot keep up, LDH converts more of it into lactate.

   too little — If little pyruvate is available, less starting material is ready for new glucose and for the citric acid cycle.

   established physiology · Source 5

2. Aspartate → Oxaloacetate GOT · Vitamin B6 (PLP) Oxaloacetate is an intermediate of the citric acid cycle. In the liver it is also the starting point of glucose formation. Source 4, 5↑ supplies In the citric acid cycle, oxaloacetate absorbs acetyl-CoA and keeps the cycle turning; in the liver it supplies the framework for new glucose. established physiology Source 4, 5
   ⚖ When the balance tips

   too much — If much oxaloacetate is available, the citric acid cycle can absorb more acetyl-CoA and the liver can form more glucose.

   too little — If little oxaloacetate is available, the citric acid cycle absorbs less acetyl-CoA.

   established physiology · Source 4, 5

3. Pyruvate → Lactate LDH · NADH Lactate forms when LDH converts pyruvate using NADH. It leaves the cell and is reused in the liver, heart and muscles. Source 5↑ supplies Lactate is not waste but a fuel: the heart and muscles burn it, and the liver builds new glucose from it. In this way the blood distributes energy between tissues. established physiology Source 5
   ⚖ When the balance tips

   too much — If more lactate forms than the liver and muscles can absorb, it rises in the blood; this is seen after hard exercise.

   too little — If little lactate forms, pyruvate breakdown continues mainly in the mitochondrion.

   established physiology · Source 5

4. Glutathione → Cysteinylglycine γGT After the split, cysteinylglycine remains. Dipeptidases separate it into cysteine and glycine, which the cells absorb again. Source 6↑ supplies After cleavage, cysteinylglycine supplies cysteine and glycine, two of the three building blocks of glutathione. The cell builds new glutathione from them. established physiology Source 6
   ⚖ When the balance tips

   too much — If much cysteinylglycine forms, more cysteine is available to the cells.

   too little — If little forms, little cysteine returns to the cells by this route.

   established physiology · Source 6

5. Pyrophosphate → Phosphate ALP · Zinc, Magnesium The cleavage releases inorganic phosphate. In bone it combines with calcium to form the crystals that make bone hard. Source 7↑ supplies Together with calcium, free phosphate supplies the mineral of bone. established physiology Source 7
   ⚖ When the balance tips

   too much — If much phosphate is released, more calcium phosphate can form at that site.

   too little — If little phosphate is released, mineral is deposited in bone more slowly.

   established physiology · Source 7

6. Choline esters → Choline CHE Cleavage leaves choline and the acid part. These fragments hardly act at the acetylcholine binding sites any more. Source 8↓ depletes Cleavage ends the action of the choline ester. How quickly this happens depends on cholinesterase activity in the blood. established physiology Source 8
   ⚖ When the balance tips

   too much — If much is split, the action of the choline ester ends quickly.

   too little — If little is split, little choline forms and the choline ester stays active for longer.

   established physiology · Source 8

Further stations

• GPT (ALT) — alanine aminotransferase GPT sits in the cytoplasm of liver cells, and in much smaller amounts in muscle and kidney. It transfers the amino group of alanine to α-ketoglutarate. It enters the blood when liver cells renew themselves or their membrane becomes more permeable. Source 1, 3↑ supplies In the liver, GPT turns alanine into pyruvate and so supplies a building block for new glucose; it passes the nitrogen on as glutamate. This is how the liver uses alanine released by muscles during fasting. established physiology Source 3, 1
  ⚖ When the balance tips

  too much — If many liver cells release their contents, GPT activity in the blood rises. Because the enzyme sits mainly in the liver, the rise mainly reflects events there; it disappears from the blood only over the course of days.

  too little — If little GPT is in the blood, only a few liver cells are releasing the enzyme. Inside the cell, GPT works only with bound PLP, the active form of vitamin B6.

  established physiology · Source 1, 9

  Field of research — GPT activity in the blood is the subject of clinical guidelines on liver conditions. Source 2

• GOT (AST) — aspartate aminotransferase GOT occurs in the liver, heart muscle and skeletal muscle, in the cytoplasm and in the mitochondria. It transfers the amino group of aspartate to α-ketoglutarate. In the blood it therefore does not come from the liver alone. Source 1, 4↕ both, depending on amount GOT works in both directions: it forms oxaloacetate for the citric acid cycle and glucose formation, or aspartate for the malate-aspartate shuttle. Which direction runs depends on the supply on either side. established physiology Source 4
  ⚖ When the balance tips

  too much — If liver, heart or skeletal muscle cells release more of their contents, GOT in the blood rises; strenuous muscle work also contributes. GOT disappears from the blood faster than GPT.

  too little — If little GOT is in the blood, few cells are releasing the enzyme. Inside the cell, GOT works only with bound PLP, the active form of vitamin B6.

  established physiology · Source 1, 9

• Alanine — arrives from muscle During fasting and exercise, muscles release nitrogen into the blood in the form of alanine. The liver absorbs it. This circuit is called the glucose-alanine cycle. Source 3↑ supplies Alanine supplies the liver with carbon for new glucose and with nitrogen, which the liver converts into urea. In this way nitrogen from muscle reaches the liver in a non-toxic form. established physiology Source 3
  ⚖ When the balance tips

  too much — If much alanine arrives, for instance during prolonged fasting or muscle work, the liver converts more of it via GPT and makes more glucose from it.

  too little — If little alanine arrives, this route contributes little glucose; the liver then mainly uses other starting materials such as lactate.

  established physiology · Source 3, 5

• Aspartate — amino acid Aspartate is an amino acid found in almost every cell. GOT moves its amino group onto α-ketoglutarate, forming oxaloacetate and glutamate. Source 4↑ supplies Aspartate supplies the amino group that GOT passes on and is part of the malate-aspartate shuttle. Through it, the cell moves hydrogen from the cytoplasm into the mitochondria. established physiology Source 4
  ⚖ When the balance tips

  too much — If plenty of aspartate is available, GOT forms more oxaloacetate and glutamate, as long as enough α-ketoglutarate is ready.

  too little — If little aspartate is available, the malate-aspartate shuttle runs more slowly and less hydrogen from the cytoplasm reaches the respiratory chain.

  established physiology · Source 4

• LDH — lactate dehydrogenase LDH is found in the cytoplasm of almost all cells, especially red blood cells, muscle, heart and liver. It converts pyruvate into lactate and back. In the blood its activity therefore comes from many tissues. Source 5↕ both, depending on amount LDH works in both directions: when oxygen is scarce it turns pyruvate into lactate and regains NAD⁺ so that glycolysis can continue; in the liver and heart, lactate becomes pyruvate again. established physiology Source 5
  ⚖ When the balance tips

  too much — If many cells release their contents, LDH activity in the blood rises; because almost every tissue contains LDH, it does not show where it comes from. Phagocytes in the liver remove the enzyme from the blood again.

  too little — If little LDH is in the blood, few cells are releasing the enzyme. Lactate formation inside the cell does not depend on the blood value but on the enzyme each cell makes itself.

  established physiology · Source 5, 10

• γGT — gamma-glutamyltransferase γGT sits on the outer side of the cell membrane, especially in the bile ducts, liver, kidney and pancreas. It splits glutathione outside the cell. Alcohol and some medicines can stimulate its production in the liver. Source 6, 1↑ supplies γGT breaks down glutathione outside the cell and so returns cysteine, which the cell needs for new glutathione. It transfers the glutamyl residue to other amino acids or to water. established physiology Source 6
  ⚖ When the balance tips

  too much — If the liver makes more γGT or bile backs up, more enzyme passes from the membranes into the blood. The rise then reflects stimulated enzyme production or disturbed bile flow.

  too little — If little γGT is in the blood, the enzyme is neither being made in larger amounts nor released from the membranes in larger amounts.

  established physiology · Source 6, 1

  Field of research — γGT activity in the blood is studied in conditions of the liver and bile ducts. Source 2

• Glutathione — outside the cell Glutathione is made of glutamate, cysteine and glycine. Cells release it outwards, for example into bile and urine. There γGT breaks it down. Source 6↑ supplies Outside the cell, glutathione is a store of cysteine. Via γGT and dipeptidases, its building blocks return to the cells. established physiology Source 6
  ⚖ When the balance tips

  too much — If there is much glutathione outside the cells, γGT has more substrate and more cysteine is recovered.

  too little — If little glutathione is present outside, little cysteine returns by this route; the cell then relies more on cysteine from the blood.

  established physiology · Source 6

• ALP — alkaline phosphatase Alkaline phosphatase sits on cell membranes in the liver and bile ducts, bone, gut and placenta. It works best in a slightly alkaline setting and needs zinc and magnesium. In the blood it comes mainly from liver and bone. Source 7, 1↑ supplies ALP splits off phosphate groups, for example from pyrophosphate and from PLP. In bone it thus removes an inhibitor of mineralisation and supplies phosphate; it makes PLP suitable for uptake into cells. established physiology Source 7
  ⚖ When the balance tips

  too much — If bone makes more ALP, for instance during growth, or bile backs up, more enzyme reaches the blood. Which source contributes can be separated by its isoforms.

  too little — If ALP activity is low, less pyrophosphate is broken down; mineral is then deposited more slowly in bones and teeth.

  established physiology · Source 7, 1

• Pyrophosphate — and other phosphate esters Pyrophosphate consists of two linked phosphate groups. It inhibits the formation of crystals from calcium and phosphate. Source 7↓ depletes Pyrophosphate slows the deposition of calcium phosphate. Mineral therefore forms mainly where ALP breaks down the inhibitor. established physiology Source 7
  ⚖ When the balance tips

  too much — If pyrophosphate builds up because little ALP splits it, mineralisation of bones and teeth is slowed.

  too little — If little pyrophosphate is present, the brake is missing and calcium phosphate can be deposited more easily.

  established physiology · Source 7

• CHE — cholinesterase from the liver Cholinesterase in the blood, more precisely butyrylcholinesterase, is made by the liver and released into plasma. It is not the same as acetylcholinesterase at nerve endings. Its amount follows the liver's protein production. Source 8↓ depletes In the blood, cholinesterase splits choline esters such as the anaesthetic drug succinylcholine. In this way it intercepts such substances before they act at nerve endings and muscles. established physiology Source 8
  ⚖ When the balance tips

  too much — If the liver makes much protein, there is also more cholinesterase in the blood; it then splits choline esters correspondingly faster.

  too little — If the liver makes less protein, or an inherited variant makes the enzyme less active, succinylcholine is split more slowly and acts for longer.

  established physiology · Source 8

• Choline esters — e.g. succinylcholine Choline esters are compounds of choline and an acid. They include drugs such as succinylcholine and some local anaesthetics. Source 8↓ depletes At the muscle fibre, succinylcholine occupies the binding sites for acetylcholine. As long as it is not split, the muscle stays relaxed. established physiology Source 8
  ⚖ When the balance tips

  too much — If more choline ester arrives than cholinesterase can split, more of it stays active.

  too little — If little choline ester arrives, cholinesterase has little to do; its activity in the blood does not change as a result.

  established physiology · Source 8

Cofactors in this pathway

• Vitamin B6 (PLP) — Cofactor of GPT and GOT; bound as PLP, it carries the amino group from one partner to the other Source 9In the ORY catalogue as a laboratory value: Vitamin B6
• α-Ketoglutarate — Accepts the amino group in the GPT and GOT reactions and becomes glutamate Source 3, 4In the ORY catalogue as a laboratory value: Alpha-Ketoglutarat (2-Oxoglutarat)
• NADH and NAD⁺ — Partner of LDH: when pyruvate becomes lactate, NADH turns into NAD⁺, which keeps glycolysis running Source 5In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Zinc — Sits in the active centre of alkaline phosphatase; without zinc it splits off no phosphate groups Source 7In the ORY catalogue as a laboratory value: Zink
• Magnesium — Third metal ion in the active centre of alkaline phosphatase, alongside two zinc ions Source 7In the ORY catalogue as a laboratory value: Magnesium
• Glutathione — Substrate of γGT; from it, γGT recovers cysteine for the cells outside the cell Source 6In the ORY catalogue as a laboratory value: Glutathion (GSH)

Sources

1. Giannini EG, Testa R, Savarino V. Liver enzyme alteration: a guide for clinicians. CMAJ 2005 · PubMed 15684121
2. Kwo PY, Cohen SM, Lim JK. ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries. Am J Gastroenterol 2017 · PubMed 27995906
3. Felig P. The glucose-alanine cycle. Metabolism 1973 · PubMed 4567003
4. Borst P. The malate-aspartate shuttle (Borst cycle): How it started and developed into a major metabolic pathway. IUBMB Life 2020 · PubMed 32916028
5. Adeva-Andany M, López-Ojén M, Funcasta-Calderón R et al. Comprehensive review on lactate metabolism in human health. Mitochondrion 2014 · PubMed 24929216
6. Whitfield JB. Gamma glutamyl transferase. Crit Rev Clin Lab Sci 2001 · PubMed 11563810
7. Millán JL. Alkaline Phosphatases: Structure, substrate specificity and functional relatedness to other members of a large superfamily of enzymes. Purinergic Signal 2006 · PubMed 18404473
8. Lockridge O. Review of human butyrylcholinesterase structure, function, genetic variants, history of use in the clinic, and potential therapeutic uses. Pharmacol Ther 2015 · PubMed 25448037
9. Percudani R, Peracchi A. A genomic overview of pyridoxal-phosphate-dependent enzymes. EMBO Rep 2003 · PubMed 12949584
10. Smit MJ, Duursma AM, Bouma JM et al. Receptor-mediated endocytosis of lactate dehydrogenase M4 by liver macrophages: a mechanism for elimination of enzymes from plasma. J Biol Chem 1987 · PubMed 2820961

Whole pathway: Liver enzymes

Related pathways

• Lactate and pyruvate — oxaloacetate
• Alanine — alanine
• Biotin — oxaloacetate
• Citric acid cycle — oxaloacetate
• Vitamin B6 — alanine

As of 2026-10-05. Draft written by Claude to schema v2; sources checked in PubMed; expert approval pending
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