Cystathionine is an intermediate dipeptide within the process of transsulfuration. Transsulfuration is the main route for irreversible homocysteine disposal, glutathione production, and energy. The initial step involves the enzyme cystathionine β-synthase enzyme (CBS). This reaction requires nutrient cofactors such as vitamin B6 and iron. Cystathionine is then converted to cysteine, and eventually goes on to either make glutathione or feed the Kreb’s cycle. 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.
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- Lamers Y, Williamson J, Ralat M, et al. Moderate dietary vitamin B-6 restriction raises plasma glycine and cystathionine concentrations while minimally affecting the rates of glycine turnover and glycine cleavage in healthy men and women. J Nutr. 2009;139(3):452-460.
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Abnormalities within the methylation cycle can result in lower levels of cystathionine. Low levels of SAM, or methylation imbalances, result in the body preferentially deferring transsulfuration to maintain methylation. Because the CBS enzyme requires vitamin B6 as a cofactor, deficiencies in vitamin B6 may result in lower cystathionine.
Because cystathionine is an intermediate of the transsulfuration pathway, elevation of this biomarker may indicate a downstream backup of the transsulfuration pathway. Conversion of cystathionine to glutathione, or other transsulfuration metabolites, requires necessary cofactors, such as vitamin B6, zinc, glycine, and magnesium. Therefore, transient elevations of this metabolite may indicate increased need for these cofactors.
Elevated cystathionine may be seen in individuals who have a CBS SNP which upregulates this enzyme and therefore upregulates the conversion of homocysteine to cystathionine.
Elevated S-adenosylmethionine (SAM) directly upregulates the CBS enzyme leading to higher cystathionine levels.
Dimethylglycine (DMG) or trimethylglycine (betaine) supplementation contribute to maintaining methylation. If the methylation cycle is adequate, transsulfuration is then upregulated. With this, supplementation of DMG or betaine have been associated with elevated cystathionine.
Elevated homocysteine may increase its metabolism into transsulfuration. Therefore, in vitamin B12 and folate deficiencies which result in high homocysteine, cystathionine might also be elevated.
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