Nadolska-Lutyk J, Balinska M, Paszewski A
Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw.
Eur J Biochem. 1989 Apr 15;181(1):231-5. doi: 10.1111/j.1432-1033.1989.tb14716.x.
In Aspergillus nidulans homocysteine can be metabolized both to cysteine and methionine. Mutants impaired in the main pathway of cysteine synthesis or in the sulphate assimilation pathway show a low pool of glutathione and elevated levels of homocysteine synthase and of the homocysteine-to-cysteine pathway enzymes. On the other hand, the level of methionine synthase and other enzymes of folate metabolism is depressed in these mutants. This anticoordinated regulation provides a mechanism controlling the partition of homocysteine between the two diverging pathways. Homocysteine synthase was found derepressed, along with folate enzymes, in a strain carrying a mutation which suppresses mutations in metA, metB and metG genes. These results indicate that homocysteine synthase can be regarded as the enzyme of an alternative pathway of methionine synthesis and strongly suggest that the regulatory mechanisms governing sulphur-containing amino acid and folate metabolisms are interrelated.
在构巢曲霉中，同型半胱氨酸既可以代谢生成半胱氨酸，也可以代谢生成甲硫氨酸。在半胱氨酸合成的主要途径或硫酸盐同化途径中受损的突变体，其谷胱甘肽池较低，同型半胱氨酸合成酶以及同型半胱氨酸转化为半胱氨酸途径的酶水平升高。另一方面，这些突变体中甲硫氨酸合成酶和叶酸代谢的其他酶水平降低。这种反协同调节提供了一种控制同型半胱氨酸在两条不同途径之间分配的机制。在一个携带抑制metA、metB和metG基因突变的突变菌株中，发现同型半胱氨酸合成酶与叶酸酶一起被去抑制。这些结果表明，同型半胱氨酸合成酶可被视为甲硫氨酸合成替代途径的酶，并强烈提示，调控含硫氨基酸和叶酸代谢的机制是相互关联的。
