Thursday, November 3, 2016

Homocysteine metabolism notes for USMLE


Let's do a quick and dirty Q&A post :D

Homocysteine metabolism
What are the clinical manifestations of homocystinuria?

Early atherosclerosis
Lens subluxation
Mitral regurgitation
Marfanoid stature

Elevated homocysteine can lead to hypercoaguable state.

What are the types of homocystinuria?

Type 1: Cystathionine synthase deficiency
Type 2: Decreased affinity of B6 for CS
Type 3: Homocysteine methyltransferase deficiency

Which vitamins are involved in homocysteine metabolism?
Vitamin B6 (Pyridoxine)
Vitamin B9 (Folic acid)
Vitamin B12 (Cyanocobalamine)

What is seen in patients with B9 and B12 deficiency?
Increased homocysteine levels will be seen in both B9 and B12 deficiency.
(Only B12 deficiency will cause an increase in methylmalonic acid concentration, B9 deficiency will not.)

That's all!
Balalala :D


  1. Can you please explain how only B12 deficiency will cause rise in methylmalonic acid and not B9 deficiency

  2. B12 is also a cofactor in the conversion of MMA --> Succinyl CoA during the breakdown of Odd chain FA's and branched AA's. B9 is not part of this pathway.


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