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Vitamin B5 (pantothenic acid)
Probably the least commonly tested vitamin on Step1, but here’s what you need to know:
- It is a coenzyme-A (CoA) reactions.
- Deficiency can cause adrenal insufficiency and burning feet syndrome (paresthesias/dysesthesias).
That’s it. That’s all you need to know about B5.
Vitamin B6 (pyridoxine)
Depleted with isoniazid (INH) use for TB.
- The highest yield thing on Step1 to know about vitamin B6 is that isoniazid (INH) used for TB causes B6 deficiency. Seizures or neuropathy in the setting of TB Tx always = B6 deficiency on Step1. Always give pyridoxine with INH. And the USMLE will sometimes list “pyridoxine” instead of “vitamin B6” as an answer choice.
Cystathionine synthase requires B6 as a cofactor.
- Vitamin B6 is adjunct therapy for homocysteinuria, since the formation of cystathionine requires B6 as a cofactor.
- For more on this, see my post on homocysteinuria here.
Vitamin B6 is needed for heme synthesis.
- Deficiency of B6 leads to sideroblastic anemia.
- In the first step of heme synthesis, succinyl-CoA + glycine → δ-aminolevulinic acid, via vitamin B6.
B6 is needed for decarboxylase reactions.
- Histidine → histamine, via histidine decarboxylase + B6
- Glutamatic acid → GABA, via glutamic acid decarboxylase + B6
| Seizures caused by vitamin B6-deficiency secondary to INH use are due to decreased GABA. |
B6 is needed for B3 synthesis.
- As discussed here, B6 and B2 are needed for niacin biosynthesis.
B6 is used for hepatic transaminase reactions.
- Pyridoxine is also needed for transaminase reactions (i.e. the liver enzymes ALT and AST).
- Pyruvate + glutamate ↔ α-KG + alanine (via ALT and B6)
- OAA + glutamate ↔ α-KG + aspartate (via AST and B6)
Biotin
Required for carboxylase reactions.
- Acetyl-CoA → malonyl-CoA (via acetyl-CoA carboxylase + B7)
- Pyruvate → oxaloacetate (via pyruvate carboxylase + B7)
- Propionyl-CoA → methylmalonyl-CoA (via propionyl-CoA carboxylase + B7)
- I give a little more context on the latter two reactions here.
| Carboxylase reactions add CO2 to a molecule; decarboxylase reactions remove CO2. |
Deficiency is caused by consumption of raw egg whites
- Most common cause of B7 deficiency is consumption of raw egg whites (old-fashioned organic body builders).
- The whites contain avidin, a B7-binding molecule.
- Yolks contain lots of B7, so only isolated, ongoing, raw egg white consumption → B7 deficiency.
- Cooking egg whites denatures avidin.
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