Oestrogen breakdown: the pathway in the body · Biomarker & Urine Analysis

This page shows the biochemical pathway behind the laboratory value Oestrogen metabolites, β-glucuronidase: 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

Oestrogen metabolites are the conversion products of oestradiol and estrone. Liver enzymes first attach OH groups, methylate them and add sugar or sulphate groups; in the gut, bacteria can undo that linkage again. The gut flora thus helps decide how much returns.

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. Oestradiol → Estrone 17β-HSD type 2 · NAD⁺ 17β-hydroxysteroid dehydrogenases interconvert oestradiol and estrone: type 2 forms estrone, type 1 forms oestradiol. Further breakdown usually starts from estrone. In tissue it can become oestradiol again. Source 2↑ supplies Estrone binds the receptor more weakly and serves as a reserve: type 1 can turn it back into oestradiol in the tissue, type 2 steers the route towards breakdown. established physiology Source 2
   ⚖ When the balance tips

   too much — If type 2 forms a lot of estrone, the action at the receptor falls, and more material goes into breakdown by the liver enzymes.

   too little — If little estrone forms or it is quickly converted back to oestradiol, more active oestradiol remains in the tissue.

   established physiology · Source 2

2. Estrone → 2-hydroxyestrone CYP1A1, CYP1A2 · NADPH In the liver, enzymes of the cytochrome P450 family attach an OH group to the ring. CYP1A1 and CYP1A2 prefer position 2. The result is a catechol, a ring with two OH groups. It barely binds to the receptor. Source 3↓ depletes 2-Hydroxyestrone barely binds to the oestrogen receptor; the route via position 2 thus largely removes the oestrogen's action. As a catechol it can be oxidised further if COMT does not methylate it. observed in studies Source 4, 3
   ⚖ When the balance tips

   too much — If much runs via position 2, oestrogen action falls, and COMT has to methylate more catechol; it uses up SAM to do so.

   too little — If little runs via position 2, more estrone remains for the routes via positions 4 and 16.

   established physiology · Source 3, 4

3. Estrone → 4-hydroxyestrone CYP1B1 · NADPH CYP1B1 places the OH group at position 4. This is also a catechol; it is more easily oxidised further to reactive intermediates, which methylation in the next step intercepts. If it builds up, more of them form. Source 3, 4↕ both, depending on amount 4-Hydroxyestrone still acts at the oestrogen receptor. As long as COMT keeps up, it is methylated; if it builds up, it oxidises to quinones, which in cell experiments bind to DNA and form oxygen radicals. observed in studies Source 4
   ⚖ When the balance tips

   too much — If 4-hydroxyestrone builds up because COMT cannot keep up, more quinones form; in cell experiments they bind to DNA and form reactive oxygen species.

   too little — If CYP1B1 forms little of it, few quinones form as well, and more estrone remains for the other routes.

   observed in studies · Source 4

4. Estrone → 16α-hydroxyestrone CYP3A4 · NADPH CYP3A4 attaches the OH group at position 16, at the end of the second ring. Unlike the two catechols, this product still binds to the oestrogen receptor. Estriol is among the products formed from it. Source 4, 3↑ supplies 16α-Hydroxyestrone continues to act as an oestrogen: it binds to the receptor, in cell experiments even firmly and lastingly. Estriol is among the products formed from it. observed in studies Source 4, 3
   ⚖ When the balance tips

   too much — If much runs via position 16, more oestrogenic action is retained, because the product continues to bind to the receptor.

   too little — If little runs this way, more estrone is converted via the catechol routes at positions 2 and 4.

   observed in studies · Source 4, 3

5. 2-hydroxyestrone → 2-methoxyestrone COMT · SAM, magnesium Catechol-O-methyltransferase (COMT) transfers a methyl group from SAM to one of the two OH groups. This makes the molecule unreactive and easier to excrete; COMT needs magnesium. This uses up SAM. Source 4↓ depletes Methylation removes the catechol's reactivity: it can no longer be oxidised to quinones. The related form 2-methoxyestradiol barely binds to the receptor and has effects of its own in cell experiments. observed in studies Source 4
   ⚖ When the balance tips

   too much — If a lot is methylated, a lot of SAM is used up and turned into SAH; COMT needs the same methyl groups for breaking down catecholamines too.

   too little — If COMT works slowly, for example when little SAM or magnesium is available, more catechols remain unmethylated and can oxidise further.

   observed in studies · Source 9, 4

6. 4-hydroxyestrone → 4-methoxyestrone COMT · SAM, magnesium The same COMT also methylates the 4-form. Both routes thus end in a molecule that is not oxidised any further. The 4-form then no longer yields reactive intermediates. Source 4↓ depletes Here too the methyl group ends oxidation: the reactive 4-form becomes a molecule that no longer forms quinones and is ready for conjugation. observed in studies Source 4
   ⚖ When the balance tips

   too much — If a lot of 4-methoxyestrone forms, a correspondingly large amount of 4-hydroxyestrone is intercepted before it oxidises to quinones.

   too little — If little forms although CYP1B1 makes a lot of 4-hydroxyestrone, more of it stays unmethylated and can oxidise to quinones.

   observed in studies · Source 4

7. 2-methoxyestrone → Oestrogen glucuronide UGT, SULT · UDP-glucuronic acid UGT enzymes attach a sugar residue, sulfotransferases a sulphate group. Only then are the breakdown products water-soluble enough to leave via urine and bile. Once conjugated, oestrogens no longer act at the receptor. Source 5↓ depletes Conjugation makes oestrogens inactive and water-soluble: glucuronides and sulphates do not bind to the receptor and leave via kidney and bile. Sulphates also serve as a reserve in the blood. established physiology Source 5
   ⚖ When the balance tips

   too much — If a lot is conjugated, more oestrogens leave the body via urine and bile; to do so UGT and SULT use up UDP-glucuronic acid and PAPS.

   too little — If little is conjugated, oestrogens and catechols stay fat-soluble for longer and thus remain longer in the tissue.

   established physiology · Source 5

8. Glucuronide in the gut → Free oestrogen β-glucuronidase Many gut bacteria produce the enzyme β-glucuronidase. It splits the sugar residue off again. The oestrogen is then free and can pass back through the gut wall into the blood. In this way oestrogen returns to the circulation. Source 6, 7↑ supplies β-glucuronidase makes oestrogen active again: once released, it is absorbed through the gut wall and returns via the portal vein. The gut flora thus helps determine how much oestrogen stays in the body. observed in studies Source 6, 7
   ⚖ When the balance tips

   too much — If the gut bacteria make a lot of β-glucuronidase, more oestrogen is released and absorbed back into the blood; less leaves the body.

   too little — If they make little, more oestrogen stays bound and leaves with the stool; less returns to the blood.

   observed in studies · Source 6, 7

9. Glucuronide in the gut → Out with the stool Whatever stays bound and is not reabsorbed leaves the body with the stool. The more stays bound, the more oestrogen is lost here. Source 7↓ depletes This exit removes oestrogen from the body for good. The more stays bound in the gut, the more is lost here instead of returning to the circulation. established physiology Source 7
   ⚖ When the balance tips

   too much — If a lot leaves with the stool, little returns, and the amount of oestrogen in the circulation falls.

   too little — If little leaves because much is split and reabsorbed, more oestrogen remains in the circulation.

   established physiology · Source 7

Further stations

• Oestradiol — most potent oestrogen Oestradiol is made from androgens: the enzyme aromatase (CYP19A1) remodels the first ring of the steroid, turning testosterone into oestradiol and androstenedione into estrone. It binds firmly to the oestrogen receptors. Source 1↑ supplies Oestradiol binds firmly to the oestrogen receptors and makes cells in the uterus, breast, bone and vessel wall, for example, read certain genes. It is the starting point of all breakdown routes on this map. established physiology Source 4, 1
  ⚖ When the balance tips

  too much — If plenty of oestradiol is present, more receptors are occupied; at the same time 17β-HSD type 2 and the liver enzymes convert more of it.

  too little — If little is present, for example when aromatase converts few androgens, the receptors are occupied less often, and the dependent genes are read less often.

  established physiology · Source 2, 3, 1

• Glucuronide in the gut — arrived with the bile Some of the conjugates leave the liver in the bile and reach the gut. In this form the molecule can no longer pass through the gut wall. Gut bacteria decide whether it leaves or returns. Source 7↓ depletes In the gut the glucuronide is firmly bound; it does not act and cannot pass through the gut wall. Whether it leaves the body or returns is decided by the enzymes of the gut bacteria. established physiology Source 7, 6
  ⚖ When the balance tips

  too much — If a lot of glucuronide arrives in the gut, β-glucuronidase has more starting material; whatever is not split leaves with the stool.

  too little — If little arrives, for example when more is excreted via the kidney, little oestrogen enters this circuit between liver and gut.

  established physiology · Source 7

Cofactors in this pathway

• SAM (from methionine) — Donor of the methyl group that COMT attaches to the catechol oestrogens; it becomes SAH in the process Source 4, 9In the ORY catalogue as a laboratory value: Methionin
• Magnesium — Metal in the active site of COMT; without it COMT does not methylate Source 9, 4In the ORY catalogue as a laboratory value: Magnesium
• NADPH — Electron donor of the cytochrome P450 enzymes that attach the OH groups; it is used up in the process Source 3, 8
• Iron — Central atom in the haem of aromatase and of the P450 enzymes of breakdown; it binds the oxygen there Source 8, 1In the ORY catalogue as a laboratory value: Eisen
• NAD⁺ — Hydrogen acceptor of 17β-HSD type 2, which converts oestradiol into estrone; it becomes NADH in the process Source 2In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• UDP-glucuronic acid — Provides the sugar group that the UGT enzymes attach; this makes oestrogens water-soluble Source 5
• PAPS (active sulphate) — Provides the sulphate group that the sulfotransferases attach; this forms oestrogen sulphates Source 5

Sources

1. Simpson ER, Mahendroo MS et al. Aromatase cytochrome P450, the enzyme responsible for estrogen biosynthesis. Endocr Rev 1994 · PubMed 8076586
2. Vihko P, Isomaa V et al. Structure and function of 17beta-hydroxysteroid dehydrogenase type 1 and type 2. Mol Cell Endocrinol 2001 · PubMed 11165013
3. Tsuchiya Y, Nakajima M et al. Cytochrome P450-mediated metabolism of estrogens and its regulation in human. Cancer Lett 2005 · PubMed 16112414
4. Zhu BT, Conney AH. Functional role of estrogen metabolism in target cells: review and perspectives. Carcinogenesis 1998 · PubMed 9472688
5. Raftogianis R, Creveling C et al. Estrogen metabolism by conjugation. J Natl Cancer Inst Monogr 2000 · PubMed 10963623
6. Ervin SM, Li H et al. Gut microbial β-glucuronidases reactivate estrogens as components of the estrobolome that reactivate estrogens. J Biol Chem 2019 · PubMed 31636122
7. Baker JM, Al-Nakkash L et al. Estrogen-gut microbiome axis: Physiological and clinical implications. Maturitas 2017 · PubMed 28778332
8. Iyanagi T, Xia C, Kim JJ. NADPH-cytochrome P450 oxidoreductase: prototypic member of the diflavin reductase family. Arch Biochem Biophys 2012 · PubMed 22982532
9. Nissinen E, Männistö PT. Biochemistry and pharmacology of catechol-O-methyltransferase inhibitors. Int Rev Neurobiol 2010 · PubMed 21095460

Related pathways

• Estradiol — estrone
• DHEA — estrone
• Dopamine, noradrenaline, adrenaline — Methionin, Magnesium
• Carnitine — Methionin, Eisen
• Citric acid cycle — Magnesium, Eisen

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