Suberinat (Suberinsäure, Korksäure): the pathway in the body

Suberinat (Suberinsäure, Korksäure) is part of the pathway “Carnitine”. This page shows the whole pathway; the station of Suberinat (Suberinsäure, Korksäure) is highlighted.

Where this laboratory value sits: Dicarboxylic acids — e.g. suberic acid, in urine. A small part of the fatty acids is oxidised from the other end in the endoplasmic reticulum (ω-oxidation). Peroxisomes shorten the resulting dicarboxylic acids to adipic, suberic and sebacic acid, which pass into the urine. Source 5, 6

In brief

Carnitine is a small nitrogen-containing molecule that the body forms from lysine and methionine and also obtains from food. It carries long-chain fatty acids into the mitochondria, where they are broken down. With little carnitine, fewer fatty acids get there.

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. Lysine in protein → Trimethyllysine Methyltransferases · SAM Methyltransferases transfer three methyl groups from SAM to the lysine in the protein. When the protein is later broken down, trimethyllysine is released. Its methyl groups later form the nitrogen head of carnitine. Source 1↑ supplies Trimethyllysine is the actual starting compound of carnitine formation. The three methyl groups from SAM later become the trimethylammonium group, the nitrogen head of carnitine. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of trimethyllysine is released, only part of it takes the route to carnitine; the rest leaves via the kidneys.

   too little — If little trimethyllysine is released, for example because little methylated protein is broken down, TMLH has hardly any substrate, and the body makes less carnitine.

   established physiology · Source 1

2. Trimethyllysine → Hydroxytrimethyllysine TMLH · Iron, Ascorbate The enzyme TMLH attaches an OH group. It contains iron, uses up α-ketoglutarate and is kept in its reactive form by ascorbate (vitamin C). With little ascorbate, TMLH works more slowly. Source 1, 2↑ supplies Hydroxylation by TMLH is the first step that serves only carnitine formation. Ascorbate keeps the iron in the enzyme reactive, and α-ketoglutarate is used up to form succinate. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If a lot of hydroxy-trimethyllysine is present, the aldolase processes it further; its pace and the available pyridoxal phosphate set the limit.

   too little — If little ascorbate is present, more of the iron in TMLH stays in the unreactive form, and less trimethyllysine is converted; in animal models carnitine formation then falls.

   observed in studies · Source 2, 1

3. Hydroxytrimethyllysine → Trimethylaminobutanal HTML aldolase · Vitamin B6 (PLP) An aldolase splits the molecule in two: an aldehyde and the amino acid glycine. The enzyme needs pyridoxal phosphate, the active form of vitamin B6. With little of it, the aldolase works more slowly. Source 1↑ supplies The aldolase splits off glycine and leaves an aldehyde, the backbone of the later carnitine. Which enzyme performs this step in humans has not yet been firmly established. observed in studies Source 1
   ⚖ When the balance tips

   too much — If a lot of aldehyde forms, the dehydrogenase rapidly converts it with NAD⁺ into γ-butyrobetaine, so it hardly accumulates.

   too little — If little pyridoxal phosphate is present, the aldolase works more slowly, and less aldehyde is available for the following steps.

   established physiology · Source 1

4. Trimethylaminobutanal → γ-Butyrobetaine TMABA dehydrogenase · NAD⁺ A dehydrogenase converts the aldehyde into an acid; NAD⁺ accepts the electrons. γ-Butyrobetaine travels via the blood mainly to the liver and kidney, which perform the last step. Source 1↑ supplies γ-Butyrobetaine is the direct precursor of carnitine. Many tissues make it, but only the liver, kidney and, in humans, the brain have BBOX for the last step; it reaches them via the blood. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of γ-butyrobetaine is present, BBOX converts it as long as iron, α-ketoglutarate and ascorbate are available; these substances then set the limit.

   too little — If little γ-butyrobetaine is present, BBOX makes hardly any carnitine, and the body relies more on carnitine from food.

   established physiology · Source 1

5. γ-Butyrobetaine → Carnitine BBOX · Iron, Ascorbate The enzyme BBOX adds the last OH group. This enzyme also contains iron, uses up α-ketoglutarate and works with ascorbate. Part of the carnitine also comes from food. It carries long-chain fatty acids into the mitochondria. Source 1, 2↑ supplies Carnitine is the carrier that brings long-chain fatty acids into the mitochondria. It also binds surplus acyl groups as acylcarnitines, which leave the cell and are excreted via the kidneys. established physiology Source 1, 3
   ⚖ When the balance tips

   too much — If more carnitine arrives than the kidneys reclaim, they release the excess in the urine; the blood level thus stays largely the same.

   too little — If little carnitine is present, fewer long-chain fatty acids reach the mitochondria; the cells switch more to glucose, and fat remains in the cytoplasm.

   established physiology · Source 3

6. Acyl-CoA → Acylcarnitine CPT1 The enzyme CPT1 on the outer membrane of the mitochondrion swaps coenzyme A for carnitine. This forms acylcarnitine, which can pass through the membrane. CPT1 sets the pace of fat breakdown; malonyl-CoA slows it. Source 3, 4↑ supplies CPT1 is the rate-limiting step of fat breakdown. Only as acylcarnitine does the fatty acid reach the inner membrane; malonyl-CoA slows CPT1 when sugar is plentiful. established physiology Source 4, 3
   ⚖ When the balance tips

   too much — If acylcarnitine builds up, the cell releases it into the blood; the kidneys excrete it, and carnitine is lost with it.

   too little — If CPT1 forms little acylcarnitine, for example because malonyl-CoA slows it, the fatty acids stay in the cytoplasm and are built into fats instead of broken down.

   established physiology · Source 3, 4

7. Acylcarnitine → Acylcarnitine inside CACT The translocase CACT carries acylcarnitine across the inner membrane and in exchange moves free carnitine out. This keeps carnitine circulating. Source 3↑ supplies CACT swaps one for one: for every acylcarnitine that goes in, one free carnitine comes out. The carnitine pool thus keeps circulating on both sides of the membrane. established physiology Source 3
   ⚖ When the balance tips

   too much — If more acylcarnitine arrives than CACT can carry, it builds up outside, and the cell releases it into the blood.

   too little — If CACT works slowly, less fatty acid reaches the matrix, and carnitine stays bound outside as acylcarnitine.

   established physiology · Source 3

8. Acylcarnitine inside → Acyl-CoA in the matrix CPT2 · Coenzyme A The enzyme CPT2 swaps carnitine back for coenzyme A. The free carnitine is then available again for the next fatty acid. If acyl-CoA builds up, acylcarnitines form here in reverse. Source 3, 4↑ supplies CPT2 gives the fatty acid in the matrix its coenzyme A back and so makes it accessible to β-oxidation. At the same time carnitine is released and returns outward via CACT. established physiology Source 3, 4
   ⚖ When the balance tips

   too much — If acyl-CoA builds up in the matrix because β-oxidation cannot keep up, CPT2 runs in reverse and forms acylcarnitines; they leave the mitochondrion and the cell.

   too little — If CPT2 works little, acylcarnitines remain in the matrix and pass into the blood; β-oxidation receives less substrate.

   established physiology · Source 4, 3

9. Acyl-CoA in the matrix → β-oxidation In the matrix, the fatty acid is shortened by two carbons in each round. This produces acetyl-CoA as well as NADH and FADH₂, which feed into the respiratory chain. In the liver, part of the acetyl-CoA becomes ketone bodies. Source 4↑ supplies β-oxidation yields acetyl-CoA as well as NADH and FADH₂, from which the respiratory chain makes ATP. In the liver, part of the acetyl-CoA goes into ketone bodies during fasting, which other organs use as fuel. established physiology Source 4
   ⚖ When the balance tips

   too much — If β-oxidation yields more acetyl-CoA than the citric acid cycle absorbs, the liver makes ketone bodies from it; intermediates are released as acylcarnitines.

   too little — If β-oxidation runs slowly, heart and muscle draw more of their energy from glucose, and the liver makes hardly any ketone bodies.

   established physiology · Source 4

10. Acyl-CoA → Dicarboxylic acids ω-oxidation, CYP4A · NADPH A small part of the fatty acids is oxidised from the other end in the endoplasmic reticulum (ω-oxidation). Peroxisomes shorten the resulting dicarboxylic acids to adipic, suberic and sebacic acid, which pass into the urine. Source 5, 6↓ depletes ω-oxidation is an alternative route: it breaks down fatty acids that do not enter mitochondrial β-oxidation and makes them water-soluble and excretable. established physiology Source 5, 6
    ⚖ When the balance tips

    too much — If more fatty acids arrive than the mitochondria can break down, for example during fasting or when CPT1 or β-oxidation is slowed, more goes through ω-oxidation and more suberic acid appears in the urine.

    too little — If the mitochondria break down fatty acids promptly, little is left for ω-oxidation, and only small amounts of dicarboxylic acids pass into the urine.

    established physiology · Source 5

Further stations

• Lysine in protein — Amino acid in proteins The body makes carnitine from the amino acid lysine. At the start, the lysine is still part of a protein molecule. It provides the backbone of carnitine. Source 1↑ supplies Lysine provides the carbon backbone and the nitrogen of carnitine. Only lysine that is built into proteins and methylated there enters this route; free lysine is not used directly. established physiology Source 1
  ⚖ When the balance tips

  too much — More lysine in protein does not bring unlimited carnitine: how much forms depends on how much lysine is methylated and how much methylated protein is broken down.

  too little — If little lysine is available, it is used to build proteins, and less trimethyllysine forms as starting material.

  established physiology · Source 1

• Carnitine in the cell — enters via OCTN2 The transporter OCTN2 carries carnitine together with sodium into the cells of muscle, heart and liver. In the kidney, OCTN2 reclaims carnitine from the urine. Source 3↑ supplies OCTN2 concentrates carnitine in muscle and heart far above the blood level. In the kidney the same transporter reclaims carnitine from the primary urine, so that hardly any is lost. established physiology Source 3
  ⚖ When the balance tips

  too much — If OCTN2 is at capacity, the carnitine content of the cells rises no further; the kidneys then release more carnitine in the urine.

  too little — If OCTN2 works little, carnitine in muscle and heart falls, and the kidneys lose more of it in the urine; fatty acids then reach the mitochondria more slowly.

  established physiology · Source 3

• Acyl-CoA — activated fatty acid Before a fatty acid is broken down, the cell attaches coenzyme A, which costs ATP. In this form, however, it cannot cross the inner membrane of the mitochondrion. It then goes into breakdown or into building fat. Source 3, 4↕ both, depending on amount Acyl-CoA is the activated fatty acid: in the cytoplasm it goes either via CPT1 into breakdown or into building fats. Malonyl-CoA, which forms when sugar is plentiful, slows CPT1 and steers it into storage. established physiology Source 4, 3
  ⚖ When the balance tips

  too much — If acyl-CoA builds up in the cytoplasm, more of it is built into fats and stored; at the same time it binds coenzyme A, which other steps then cannot use.

  too little — If little acyl-CoA is present, CPT1 has hardly any substrate, and the mitochondria obtain their acetyl-CoA mainly from glucose.

  established physiology · Source 4

Cofactors in this pathway

• Vitamin C — As ascorbate, keeps the iron in TMLH and BBOX in its reactive form; with little of it, formation falls Source 1, 2In the ORY catalogue as a laboratory value: Vitamin C (Ascorbinsäure)
• Iron — Metal centre of the two hydroxylases TMLH and BBOX, which form carnitine from trimethyllysine Source 1In the ORY catalogue as a laboratory value: Eisen
• Vitamin B6 — As pyridoxal phosphate, cofactor of the aldolase that cleaves hydroxy-trimethyllysine; without PLP this step stalls Source 1In the ORY catalogue as a laboratory value: Vitamin B6
• Niacin (NAD⁺) — NAD⁺ accepts the electrons when the aldehyde is oxidised to γ-butyrobetaine; without NAD⁺ the step stalls Source 1In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• α-Ketoglutarate — Consumed by TMLH and BBOX in each hydroxylation, forming succinate Source 1In the ORY catalogue as a laboratory value: Alpha-Ketoglutarat (2-Oxoglutarat)
• Methionine (SAM) — As S-adenosylmethionine, donates the three methyl groups for trimethyllysine; these form the nitrogen head Source 1In the ORY catalogue as a laboratory value: Methionin
• Lysine — Backbone of carnitine, built into proteins and methylated there; it is released only when protein is broken down Source 1In the ORY catalogue as a laboratory value: Lysin
• Pantothenic acid (coenzyme A) — Building block of coenzyme A, which CPT1 and CPT2 exchange for carnitine; this frees carnitine again Source 3, 4

Sources

1. Strijbis K, Vaz FM, Distel B. Enzymology of the carnitine biosynthesis pathway. IUBMB Life 2010 · PubMed 20306513
2. Rebouche CJ. Ascorbic acid and carnitine biosynthesis. Am J Clin Nutr 1991 · PubMed 1962562
3. Longo N, Frigeni M, Pasquali M. Carnitine transport and fatty acid oxidation. Biochim Biophys Acta 2016 · PubMed 26828774
4. Houten SM, Wanders RJ. A general introduction to the biochemistry of mitochondrial fatty acid β-oxidation. J Inherit Metab Dis 2010 · PubMed 20195903
5. Wanders RJ, Komen J, Kemp S. Fatty acid omega-oxidation as a rescue pathway for fatty acid oxidation disorders in humans. FEBS J 2011 · PubMed 21156023
6. Miura Y. The biological significance of ω-oxidation of fatty acids. Proc Jpn Acad Ser B Phys Biol Sci 2013 · PubMed 24126285

Whole pathway: Carnitine

Related pathways

• Vitamin B5 — acyl-coa
• Trans fatty acids — acyl-coa
• Dopamine, noradrenaline, adrenaline — Vitamin C (Ascorbinsäure), Eisen
• Spermidine — Eisen, Vitamin B6
• Alanine — Vitamin B6, NAD⁺ (Nicotinamidadenindinukleotid)

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