Homocystinuria due to cystathionine beta-synthase (CBS) deficiency is treated with pyridoxine (vitamin B6) in responsive patients. CBS catalyzes the condensation of homocysteine with serine to form cystathionine. In the trans-sulfuration pathway, the sulfur atom originally from methionine appears in which final product?
- A Cysteine
- B Taurine
- C Glutathione
- D All of the above — methionine sulfur flows through cysteine to taurine and glutathione ✓
Explanation
In the trans-sulfuration pathway, methionine donates its sulfur through the following sequence: methionine → S-adenosylmethionine (SAM) → S-adenosylhomocysteine → homocysteine → (CBS reaction with serine) → cystathionine → (cystathionase) → cysteine + alpha-ketobutyrate. Cysteine retains the sulfur originally from methionine. Cysteine is then a precursor for taurine (via cysteine sulfinic acid pathway) and glutathione (gamma-Glu-Cys-Gly tripeptide). Therefore, the sulfur from methionine ultimately appears in cysteine, taurine, and glutathione. This explains why cysteine becomes 'conditionally essential' in CBS deficiency — it can no longer be synthesized from homocysteine, and dietary supplementation of cysteine is recommended.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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Written and medically reviewed by the StethoPrep medical team.