Transsulfuration is the main route for irreversible homocysteine disposal, and begins by replacing a serine hydroxyl group with a sulfhydryl group to form cystathionine, using the cystathionine β-synthase enzyme (CBS). This reaction requires nutrient cofactors such as vitamin B6 and iron.
Cystathionine is a dipeptide which is then converted to cysteine using the enzyme cystathionine gamma- lyase (CTH). Currently, there is no known source or physiologic function for cystathionine other than serving as a transsulfuration intermediate. Some literature suggests that cystathionine may exert protection against endoplasmic reticulum stress-induced tissue damage and cell death, but studies are sparse. [L]
Because cystathionine is an intermediate of the transsulfuration pathway, elevation of this biomarker may indicate a backup of the transsulfuration pathway. Conversion of cystathionine to glutathione requires necessary cofactors, such as vitamin B6, zinc, glycine, and magnesium. Therefore, transient elevations of this metabolite may indicate increased need for these cofactors.
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References:
Maclean KN, Greiner LS, Evans JR, et al. Cystathionine protects against endoplasmic reticulum stress-induced lipid accumulation, tissue injury, and apoptotic cell death. J Biol Chem. 2012;287(38):31994-32005. [L]
Blom HJ, Smulders Y. Overview of homocysteine and folate metabolism. With special references to cardiovascular disease and neural tube defects. J Inher Metab Dis. 2011;34(1):75-81. [L]
DeStefano Vea. Linkage disequilibrium at the cystathionine beta-synthase (CBS) locus and the association between genetic variation at the CBS locus and plasma levels of homocysteine. Ann Human Genet. 1998;62(6):481-490. [L]
Obeid R. The metabolic burden of methyl donor deficiency with focus on the betaine homocysteine methyltransferase pathway. Nutrients. 2013;5(9):3481-3495. [L]
Stabler S, et al. Elevation of serum cystathionine levels in patients with cobalamin and folate deficiency. Blood. 1993;81(12):3404-3413 [L]
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Wu X-Y, Lu L. Vitamin B6 deficiency, genome instability and cancer. Asian Pac J Cancer Prev. 2012;13(11):5333-5338. [L]
WHAT DOES IT MEAN IF YOUR CYSTATHIONINE RESULT IS TOO LOW?
– Vitamin B6 cofactor need for the CBS enzyme [L]
WHAT DOES IT MEAN IF YOUR CYSTATHIONINE RESULT IS TOO HIGH?
– Elevated SAM which directly upregulates CBS enzyme [L]
– DMG,Betaine supplementation [L]
– High oxidative stress or inflammation [L]
– Vitamin B6 cofactor need for CTH enzyme [L]
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