Cystathionine is an intermediary metabolite that is formed in the sequential enzymatic conversion of methionine to cysteine. Cystathionine is normally detected at very low levels in plasma. It is found between homocysteine and cysteine and is formed by the enzyme cystathionine beta-synthase (CBS).
Serine and B-6 are required to produce cystathionine from homocysteine.
– A defect in cystathionine beta-synthase (=CBS) may elevate homocysteine and decrease both cystathionine and cysteine levels.
– Cystathionine levels are normally very low, but a disproportionately low cystathionine level in the presence of elevated homocysteine may be interpreted as being indicative of compromised CBS activity.
– Lower levels are likely due to low B2 and maybe also B6. This will limit flow into the transsulfuration pathway, which could limit glutathione synthesis. The low cystathionine would be consistent with low methionine and could also be caused by low magnesium or low B6 availability as P5P, the latter of which could be caused by low B2.
– Subnormal cystathionine is usually a nutritional condition that is readily corrected by adequate provision of serine and B-6/P-5-P. If dietary cysteine intake is insufficient, there could also be a cysteine deficiency that could be associated with subnormal intracellular glutathione and decreased antioxidant and detoxification (chemical and heavy metal) capacity.
There might be a B6 functional deficit as B6 or P-5-P is required for the conversion of cystathionine to cysteine. Hence low cysteine can result.
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