The Bile Acid Family: CA, DCA, LCA, HDCA and TUDCA Mapped
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The bile acids get treated as a list of unconnected molecules with similar names, which is why the names keep getting mixed up. They are not a list. Almost every member of the family is defined by which of three positions on the steroid skeleton carries a hydroxyl group, and whether that hydroxyl points α, points β, or is oxidised to a ketone. Change one of those and you have a different substance with a different name, a different CAS number and often a different pharmacopoeial entry.
Read the family that way and much of the confusion disappears, including a few things that look like errors on specification sheets but are not.
The four positions that generate the family
The skeleton is a C24 bile acid, 5β-cholan-24-oic acid. The 5β means the A and B rings are fused in the cis configuration, which is standard for this family and is one of the things that distinguishes it from the 5α versions. The carboxylic acid sits at C24. Everything else of interest happens at three hydroxyl positions, with one extra that matters for pig-derived material.
| Common name | Abbrev. | CAS | Hydroxyl pattern | Note |
|---|---|---|---|---|
| Cholic acid | CA | 81-25-4 | 3α, 7α, 12α | The tri-hydroxylated parent; UDCA impurity B |
| Chenodeoxycholic acid | CDCA | 474-25-9 | 3α, 7α | UDCA impurity A; the usual synthetic starting point |
| Ursodeoxycholic acid | UDCA | 128-13-2 | 3α, 7β | The C7 epimer of CDCA |
| Deoxycholic acid | DCA | 83-44-3 | 3α, 12α | UDCA impurity E |
| Lithocholic acid | LCA | 434-13-9 | 3α only | UDCA impurity C; the mono-hydroxylated member |
| Hyodeoxycholic acid | HDCA | 83-49-8 | 3α, 6α | The porcine signature — see below |
| 7-Ketolithocholic acid | 7-KLCA | 4651-67-6 | 3α, 7-oxo | UDCA impurity F; the intermediate between CDCA and UDCA |
| Tauroursodeoxycholic acid | TUDCA | 14605-22-2 | 3α, 7β (as UDCA) | UDCA with taurine conjugated at C24 |
| Ursocholic acid | — | — | 3α, 7β, 12α | UDCA impurity D; the 12α relative of UDCA |
How to read the pattern
The useful move is to treat the positions as columns and the molecules as rows, then read across. Set it out that way and the relationships stop being arbitrary.
A few things fall out of the grid immediately.
The 3-position is always occupied. Every member of this family carries a 3α hydroxyl. When you see a substance described with a 3β configuration, you are looking at something else — and note that 3β,7β-dihydroxy-5β-cholan-24-oic acid appears in the UDCA monograph as impurity H, which is exactly that kind of near-miss molecule.
The 7-position is where the UDCA family lives. Three of the entries above differ only at C7: CDCA has 7α, UDCA has 7β, and 7-KLCA has a ketone there. That is the whole difference between the starting material, the product, and the intermediate in one of the main industrial routes. It also explains why impurity A and impurity F sit so close to the active substance in every sense.
The 12-position splits off the cholic acid branch. Add a 12α hydroxyl to CDCA and you get cholic acid. Add it to UDCA and you get ursocholic acid. Remove it from cholic acid and you get DCA. This is why CA, DCA, CDCA and the 12α-related impurities appear together in the diagrams of anyone who works with bile acid chemistry.
Remove hydroxyls and you walk down to lithocholic acid. One hydroxyl left at C3, and you have the molecule that all bile-acid specification sheets single out. It is also the endpoint of the direction in which this family gets less water-soluble, which is a helpful way to remember where it sits.
Where the 6-position comes in, and why it matters for sourcing
Hyodeoxycholic acid breaks the pattern. Its second hydroxyl is at C6, not C7, and C6 hydroxylation is characteristic of pig bile. This is the structural reason that material derived from porcine bile looks different analytically from material derived from other species, and it is the same reason the Chinese Pharmacopoeia monograph for dried pig bile powder can identify the material by a porcine-specific marker substance.
If you buy porcine-derived bile material, HDCA and related 6-hydroxylated bile acids are what you expect to see. If you buy material of another declared origin and find them, that is a species question rather than a quality question, and the traceability side of it is covered in our note on porcine origin and traceability. The monograph requirements themselves are in the article on dried pig bile powder under the Chinese Pharmacopoeia.
Conjugation: the second axis
Everything above describes the free acids. Bile acids also occur conjugated, most commonly with taurine or glycine at the C24 carboxyl. Conjugation does not change the steroid skeleton or the hydroxyl pattern; it changes the molecule's behaviour enough that it becomes a different substance for specification purposes.
Tauroursodeoxycholic acid is the clearest case. Take UDCA and conjugate taurine at C24 and you have TUDCA, with a different CAS number and no pharmacopoeial monograph of its own. That single fact changes what a specification can be built on, which is the subject of our note on specifying TUDCA without a monograph.
The conjugated forms also matter on the impurity side of porcine material, because the pig bile monograph expresses its assay against a conjugated marker rather than a free acid. Different marker, different number, same material — a point that trips up buyers regularly.
Which molecule lives in which specification
Useful shorthand if you are cross-referencing certificates. UDCA, CDCA and the two porcine-derived pharmacopoeial materials each have their own monograph and their own limits. 7-KLCA appears both as a substance in its own right and as UDCA impurity F. CA, DCA, LCA and ursocholic acid appear principally as named or described impurities inside the UDCA and CDCA monographs rather than as compendial substances in their own right.
The full impurity letter list, with the identity behind each letter, is in the A–I reference. The material we supply across the range is listed on the bile-acid page, and requirements for a specific molecule can be raised through a quotation request.
Frequently asked questions
What is the difference between UDCA and CDCA?
Configuration at carbon 7. Both are 3α-hydroxy C24 bile acids; CDCA has a 7α hydroxyl and UDCA has a 7β hydroxyl. The two are epimers, with the same molecular formula.
What is 7-ketolithocholic acid?
A C24 bile acid with a 3α hydroxyl and a ketone at carbon 7. It is UDCA impurity F and the intermediate in the epimerisation of CDCA to UDCA.
Why does hyodeoxycholic acid matter?
Because its second hydroxyl is at C6, and C6 hydroxylation is characteristic of porcine bile. It is a structural marker of porcine origin rather than a general member of the human bile acid pattern.
How is TUDCA related to UDCA?
TUDCA is UDCA with taurine conjugated at the C24 carboxyl group. Same steroid skeleton and hydroxyl pattern, different substance, different CAS number, and no monograph of its own.
Which bile acids appear as UDCA impurities?
Nine letters, A to I. CDCA is A, cholic acid is B, lithocholic acid is C, ursocholic acid is D, deoxycholic acid is E, 7-KLCA is F, a methyl ester of UDCA is G, 3β,7β-dihydroxy-5β-cholan-24-oic acid is H, and I is a triol derivative.
Are all bile acids 5β?
The ones in this family are, which is what the 5β in names like 5β-cholan-24-oic acid indicates. The 5α series exists but describes different substances with different properties.
One way to use this page
If you are looking at a certificate and a substance name does not match what you expected, check the position and configuration at C3, C7 and C12 before assuming the supplier has made a mistake. In this family, those three columns explain most of what looks like an error and is not.
If you want the source documents rather than my summary of them: PubChem, the European Pharmacopoeia and the United States Pharmacopeia. Editions and limit values move, so read the version in force now.
This article is a structural and nomenclature reference for bile acids, written for research, quality and procurement professionals. It states no use, dose or indication for any substance and no order quantity or lead time. CAS numbers and hydroxyl patterns should be confirmed against the current pharmacopoeial monograph or a primary structure database before use in any specification.