Della Ragione F, Porcelli M, Cartenì-Farina M, Zappia V, Pegg A E
Biochem J. 1985 Dec 1;232(2):335-41. doi: 10.1042/bj2320335.
S-Adenosylhomocysteine/5'-methylthioadenosine nucleosidase (EC 3.2.2.9) was purified to homogeneity from Escherichia coli to a final specific activity of 373 mumol of 5'-methylthioadenosine cleaved/min per mg of protein. Affinity chromatography on S-formycinylhomocysteine-Sepharose is the key step of the purification procedure. The enzyme, responsible for the cleavage of the glycosidic bond of both S-adenosylhomocysteine and 5'-methylthioadenosine, was partially characterized. The apparent Km for 5'-methylthioadenosine is 0.4 microM, and that for S-adenosylhomocysteine is 4.3 microM. The maximal rate of cleavage of S-adenosylhomocysteine is approx. 40% of that of 5'-methylthioadenosine. Some 25 analogues of the two naturally occurring thioethers were studied as potential substrates or inhibitors of the enzyme. Except for the analogues modified in the 5'-position of the ribose moiety or the 2-position of the purine ring, none of the compounds tested was effective as a substrate. Moreover, 5'-methylthioformycin, 5'-chloroformycin, S-formycinylhomocysteine, 5'-methylthiotubercidin and S-tubercidinylhomocysteine were powerful inhibitors of the enzyme activity. The results obtained allow the hypothesis of a mechanism of enzymic catalysis requiring as a key step the protonation of N-7 of the purine ring.
S-腺苷高半胱氨酸/5'-甲硫基腺苷核苷酶（EC 3.2.2.9）从大肠杆菌中纯化至同质，最终比活性为每毫克蛋白质每分钟裂解373微摩尔5'-甲硫基腺苷。在S-甲酰肌苷高半胱氨酸-琼脂糖上进行亲和层析是纯化过程的关键步骤。该酶负责裂解S-腺苷高半胱氨酸和5'-甲硫基腺苷的糖苷键，已对其进行了部分表征。5'-甲硫基腺苷的表观Km为0.4微摩尔，S-腺苷高半胱氨酸的表观Km为4.3微摩尔。S-腺苷高半胱氨酸的最大裂解速率约为5'-甲硫基腺苷的40%。研究了两种天然硫醚的约25种类似物作为该酶的潜在底物或抑制剂。除了在核糖部分的5'-位或嘌呤环的2'-位修饰的类似物外，所测试的化合物均无作为底物的活性。此外，5'-甲硫基肌苷、5'-氯肌苷、S-甲酰肌苷高半胱氨酸、5'-甲硫基结核菌素和S-结核菌素高半胱氨酸是该酶活性的强力抑制剂。所得结果支持了一种酶催化机制的假设，该机制需要嘌呤环N-7的质子化作为关键步骤。
