6] is widely recognized for its importance in the inactivation of homocysteine by serving as coenzyme in the form of PLP for two degradative enzymes (cystathionine [beta]-synthase and cystathionine [gamma]-lyase) in the transsulfuration
pathway for homocysteine removal (Selhub, 1999; Miles and Kraus, 2004).
Is the increase in serum cystathionine levels in patients with liver cirrhosis a consequence of impaired homocysteine transsulfuration
at the level of gamma-cystathionase?
Alternatively, homocysteine can be directed to the transsulfuration
pathway and used in the synthesis of GSH (Figure 1).
Homocysteine can be regenerated to methionine via the remethylation pathway or irreversibly degraded to cysteine via transsulfuration
SAH then is broken down by SAH hydrolase (SAHH) to form homocysteine, which can either enter a set of reactions called the transsulfuration
pathway to form glutathione (GSH) or be remethylated to form methionine (Grillo and Colombatto 2008).
Mudd SH, Levy HL, Kraus JP Disorders of transsulfuration
Firstly, Hcy react with serine to form Ctn by catalytic action of cystathionine [beta]-synthase (CBS) through the transsulfuration
Sulfur metabolites drain excessively through the transsulfuration
pathway and are excreted.
According to the transsulfuration
data, it can be concluded that the higher cystine formation with HMTBA compared to DLM could explain the higher glutathione level observed in the present study.
Both pathways require vitamin-derived cofactors, pyridoxine for transsulfuration
and both folate and cobalamin in the methionine cycle.
Hcy metabolized through the transsulfuration
and transmethylation pathways, where folate, vitamin B-12 and vitamin B-6 are involved as a substrate or cofactor.
There are two pathways for the metabolism of homocysteine: 1) vitamin B6 dependent transsulfuration
, which leads to formation of cysteine irreversibly, 2) folate and vitamin B12 dependent remethylation to form methionine by help of methionine synthase.