Homocystinuria due to cystathionine beta-synthase (CBS) deficiency presents with lens dislocation (downward/inward), intellectual disability, osteoporosis, and thromboembolic events. Pyridoxine (B6) supplementation helps approximately 50% of patients because:

• A B6 increases dietary methionine absorption, reducing homocysteine load
• B B6 activates methionine synthase, diverting homocysteine to methionine
• C PLP is an essential cofactor for CBS; pharmacological doses of B6 can stabilize some mutant CBS proteins, increasing residual enzyme activity ✓
• D B6 directly reduces disulfide bonds in accumulated homocystine

Explanation

Cystathionine beta-synthase (CBS) requires pyridoxal phosphate (PLP) as a cofactor to catalyze the condensation of homocysteine and serine to form cystathionine. In approximately 50% of CBS-deficient patients, certain missense mutations produce a protein that still partially binds PLP but with reduced affinity; pharmacological doses of pyridoxine (precursor of PLP) can saturate these mutant enzymes and restore sufficient activity to reduce plasma homocysteine. B6-non-responsive patients require betaine supplementation to remethylate homocysteine to methionine via betaine-homocysteine methyltransferase.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.