Creatinine: the pathway in the body · Biomarker & Urine Analysis

This page shows the biochemical pathway behind the laboratory value Creatinine: which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Creatinine is the breakdown product of creatine and creatine phosphate, the rapid energy store of muscle. It forms without an enzyme in a fairly steady amount and leaves the body via the kidney in urine. Creatine phosphate buffers ATP under sudden load.

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. Arginine and glycine → Guanidinoacetate AGAT The enzyme AGAT transfers a guanidino group from arginine to glycine. This step occurs mainly in the kidney and the pancreas. Through this step the body regulates how much creatine forms. Source 1, 2↑ supplies Guanidinoacetate is the direct precursor of creatine. The AGAT reaction sets the pace of the whole pathway; through it the body regulates how much creatine forms in total. established physiology Source 1
   ⚖ When the balance tips

   too much — If guanidinoacetate builds up because GAMT cannot keep pace, it accumulates in blood and tissues instead of being processed further into creatine.

   too little — If little guanidinoacetate forms, the liver makes less creatine; the body then relies more on creatine from food.

   established physiology · Source 1

2. Guanidinoacetate → Creatine GAMT · SAM The enzyme GAMT attaches a methyl group; it comes from SAM, the cell's methyl group donor. This forms creatine, which the liver releases into the blood. This step uses a lot of SAM. Source 1↕ both, depending on amount Making creatine draws on SAM: it is one of the largest consumers of methyl groups in the body. The finished creatine supplies the muscles with the raw material for their energy store, creatine phosphate. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of creatine arrives, for example from food, the body throttles AGAT in the kidney; less guanidinoacetate forms, and arginine, glycine and SAM are drawn on less.

   too little — If little creatine forms, less of it reaches the muscle, and the store of creatine phosphate becomes smaller.

   established physiology · Source 1

3. Creatine → Creatine in muscle CRT (SLC6A8) · Sodium, Chloride The transporter CRT carries creatine together with sodium into the muscle cell. Meat and fish provide additional creatine from food. Most of the body's store is in skeletal muscle. There it serves as raw material for the energy store. Source 1↑ supplies The transporter CRT concentrates creatine in muscle against a steep gradient; this builds the store from which creatine kinase makes creatine phosphate. The muscle cell itself makes hardly any creatine. established physiology Source 1
   ⚖ When the balance tips

   too much — If the muscle is saturated with creatine, the transporter takes up hardly any more; the excess stays in the blood and leaves the body via the kidney.

   too little — If little creatine reaches the muscle, the store of creatine phosphate falls, and less phosphate is available for the rapid resupply of ATP.

   established physiology · Source 1

4. Creatine in muscle → Creatinine Spontaneous breakdown Creatine and creatine phosphate break down slowly and without an enzyme into creatinine. The share that breaks down each day is fairly constant; the amount depends mainly on muscle mass. Creatinine has no further role after that. Source 1↓ depletes Creatinine is an end product with no further role in metabolism; through this breakdown the body loses creatine irreversibly. What breaks down each day is replaced by new synthesis or food. established physiology Source 1
   ⚖ When the balance tips

   too much — More creatinine forms when there is more muscle mass or more creatine. It builds up in the blood when the kidney filters or secretes less of it into the urine; formation itself stays steady.

   too little — If there is little muscle mass, less creatine breaks down, and correspondingly less creatinine forms.

   established physiology · Source 1, 3

5. Creatinine → Creatinine in urine Filtration, secretion Creatinine leaves the muscle for the blood. In the kidney it passes through the glomerular filter, and a smaller share is also secreted from the blood into the urine. It therefore serves as a reference for other urine values. It is hardly reabsorbed. Source 1, 3↓ depletes The kidney removes creatinine mainly through the filter; the transporters OCT2 and MATE secrete a smaller share into the urine as well. Because it is hardly reabsorbed, its excretion follows the filtration rate. established physiology Source 1, 3
   ⚖ When the balance tips

   too much — A lot of creatinine in urine occurs when a lot is formed, for example with large muscle mass, or when the urine contains little water; this is why other urine values are related to creatinine.

   too little — Little creatinine in urine occurs with low formation or dilute urine. If drugs block OCT2 and MATE, the kidney secretes less, and more stays in the blood.

   established physiology · Source 1, 3

6. Creatine phosphate → ATP Creatine kinase · ADP When the muscle cell needs ATP quickly, the same reaction runs in reverse: creatine phosphate passes its phosphate group to ADP. This keeps the ATP content almost unchanged at first. Source 1↑ supplies ATP is the cell's energy currency; muscle fibres use it up when contracting. Creatine kinase renews it from ADP so quickly that the ATP content hardly falls at first. ATP is turned over as a magnesium complex. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — If plenty of ATP is present, creatine kinase runs in the opposite direction and recharges creatine to creatine phosphate.

   too little — If ATP is used faster than it is regenerated, ADP accumulates; creatine kinase then releases phosphate from creatine phosphate until this store is empty.

   established physiology · Source 1

Further stations

• Arginine and glycine — two amino acids The pathway starts with two amino acids from protein metabolism: arginine and glycine. Both are used up to make creatine. Source 1, 2↑ supplies Arginine gives up its guanidino group, glycine provides the backbone; both amino acids are used up in the process. Creatine synthesis is one of the pathways that claim a large share of the body's arginine. established physiology Source 1, 2
  ⚖ When the balance tips

  too much — If plenty of arginine and glycine is available, AGAT still does not make unlimited guanidinoacetate: how much AGAT the kidney makes is throttled by the creatine already present.

  too little — If little arginine is available, creatine synthesis competes with other pathways for the same amino acid, and less guanidinoacetate forms.

  established physiology · Source 1, 2

• Creatine phosphate — rapid energy store Creatine kinase transfers a phosphate group from ATP to creatine. Creatine phosphate is available faster than energy from sugar or fat. At rest the store is refilled. Source 1↑ supplies Creatine phosphate buffers ATP in muscle and brain: at rest creatine kinase charges the store, and under sudden load it returns its phosphate to ADP within seconds. established physiology Source 1
  ⚖ When the balance tips

  too much — If the store is full, creatine kinase cannot deposit any more phosphate in it; additional creatine leaves the muscle or gradually breaks down to creatinine.

  too little — If little creatine phosphate is available, the buffer is exhausted sooner during short, intense load; the cell then regenerates ATP earlier from sugar and fat, which is slower.

  established physiology · Source 1

Cofactors in this pathway

• Arginine — Provides AGAT with the guanidino group for guanidinoacetate and is used up in the process Source 1, 2In the ORY catalogue as a laboratory value: Arginin
• Glycine — Backbone to which AGAT attaches the guanidino group; it is used up in the process Source 1In the ORY catalogue as a laboratory value: Glycin
• Methionine (SAM) — As S-adenosylmethionine, provides GAMT with the methyl group for creatine; SAM becomes SAH in the process Source 1In the ORY catalogue as a laboratory value: Methionin
• Sodium — Drives the transporter CRT, which carries creatine into the muscle cell; without a sodium gradient, no uptake Source 1In the ORY catalogue as a laboratory value: Natrium (intrazellulär)
• Chloride — Carried by the transporter CRT together with sodium and creatine; required for uptake Source 1
• Magnesium — Forms the complexes with ATP and ADP that creatine kinase acts on – when charging and discharging the store Source 1, 4In the ORY catalogue as a laboratory value: Magnesium

What acts on this pathway

• Transporter inhibitors — Some medicines block the transporters OCT2 and MATE in the kidney. The kidney then releases less creatinine into the urine, although the filter works unchanged. This relationship has been described. Source 3

Sources

1. Wyss M, Kaddurah-Daouk R. Creatine and creatinine metabolism. Physiol Rev 2000 · PubMed 10893433
2. Brosnan ME, Brosnan JT. Renal arginine metabolism. J Nutr 2004 · PubMed 15465786
3. Nakada T, Kudo T, Ito K. Quantitative Consideration of Clinical Increases in Serum Creatinine Caused by Renal Transporter Inhibition. Drug Metab Dispos 2023 · PubMed 36859345
4. de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev 2015 · PubMed 25540137

Related pathways

• Calcium — Natrium (intrazellulär), Magnesium
• Estradiol — Methionin, Magnesium
• GABA, glutamate and glutamine — Natrium (intrazellulär), Magnesium
• Dopamine, noradrenaline, adrenaline — Methionin, Magnesium
• Methylation: methionine and homocysteine — Glycin, Magnesium

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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