Cysteine
In a patient with cystathionine-β-synthase (CBS) deficiency, cysteine becomes a conditionally essential amino acid because the enzymatic pathway that normally synthesizes it from homocysteine is blocked.
Biochemical Rationale
Under normal circumstances, cysteine is synthesized endogenously through the transsulfuration pathway:
- Methionine → Homocysteine → Cystathionine (via CBS) → Cysteine
When CBS is deficient, this pathway is interrupted at the critical step converting homocysteine to cystathionine 1. The patient accumulates homocysteine but cannot produce cysteine from methionine/homocysteine, making dietary cysteine intake necessary.
Clinical Management Context
The 2017 guidelines for CBS deficiency management emphasize that severely affected patients require a low-methionine diet 1. This dietary restriction further limits the precursor availability for cysteine synthesis, making exogenous cysteine supplementation even more critical to prevent deficiency.
Key management principles include:
- Maintaining plasma total homocysteine below 100 μmol/L (ideally below 120 μmol/L) 1
- Treatment with pyridoxine (for responsive patients), betaine, and/or dietary methionine restriction 1, 2
- Because methionine intake is restricted, the normal pathway for cysteine synthesis is doubly compromised
Why Other Options Are Incorrect
- Methionine: Remains an essential amino acid but does not become essential—it already is essential
- Histidine: Essential amino acid unrelated to the transsulfuration pathway
- Valine: Branched-chain essential amino acid with no connection to homocysteine metabolism
- Alanine: Non-essential amino acid synthesized through transamination reactions independent of CBS
Clinical Pitfall
Do not confuse the accumulation of homocysteine (the substrate before the blocked enzyme) with the deficiency of cysteine (the end product after the blocked enzyme). The metabolic block creates a "traffic jam" upstream while causing shortage downstream 3.