Brain cystathionase

Since methionine has several pathways open to it, it is essential to know what factors control the direction that its metabolism takes. Studies in young adults have shown that the utilization of methyl groups is normally accounted for chiefly by creatinine formation. This reaction consumes more 5-adenosylmethionine than all other transmethylations together. However, examination of enzyme activities from these two pathways in fetal animals leads to the conclusion that remethylation preponderates over transsulfuration. Indeed, since y-cystathionase activity is immeasurable in human fetal liver and brain, not only is the remethylation sequence favored, but also cysteine then becomes an essential amino acid for the fetus and infant. 

Cystathioninuria, a deficiency of cystathionase, is a much rarer and less clearly defined disorder . While the disease has frequently been associated with mental retardation, this may only refiect the type of individual with which testing most frequently occurs. Patients with normal mental function are also known. Nonetheless, the high levels of cystathionine in brain and the mental defects associated with its faulty metabolism, have led to speculation that this thioether has some special role in nervous function. In tissues from at least one patient, there was evidence that the defect was in pyridoxal phosphate binding by cystathionase and that normal enzyme activity could be achieved at abnormally high levels of coenzyme. This is often quoted as the classical example of a binding or K mutant, but not all patients with the disorder give the same effect. 

The major developmental change which takes place In both brain and liver is the postnatal activation of the transsulfuration pathway of methionine metabolism. The net result of this pathway is the transfer of the sulfur atom from homocysteine to the carbon skeleton of serine to form cysteine. This conversion is mediated by two enzymes cystathionine synthase (L-serine hydro-lyase adding homocysteine, EC 4.2.1.22) which catalyzes the 3-activation of serine and the addition of homocysteine to form the thio-ether, cystathionine cystathionase (EC 4.4.1.1) which catalyzes the y-cleavage of cystathionine to form cysteine (Fig. 1). Both of these enzymes catalyze reactions other than those described above although their importance vivo is uncertain (Tallan et al., 1974). In mature mammals, activities both of cystathionine synthase and of cystathionase are present in brain and liver, although cystathionase activity in... 

Heinonen, K., 1973, Studies on cystathionase activity in rat liver and brain during development, Biochem. J., 136 1011-1015. 

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