S-Adenosylhomocysteine/5′-methylthioadenosine nucleosidase (EC 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.

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