S-Ribosylhomocysteine Cleavage Enzyme from Escherichia coZi*

Cell-free extracts from Escherichia coli cleave the thioether linkage of S-ribosylhomocysteine and generate free homocysteine. Homocysteine was quantitatively recovered from reaction mixtures by ion exchange chromatography on Amberlite CG-120 resin. Free ribose was not a product of this cleavage; however, a compound initially derived from the ribose moiety of S-ribosylhomocysteine has been isolated by chromatography on Dowex l-X8 resin but has not been fully characterized. A 15-fold purification of the cleavage enzyme has been achieved by ammonium sulfate precipitation, Sephadex G-150 gel filtration, and ethyl alcohol precipitation. The thioether linkage of S-adenosylhomocysteine also was cleaved by crude cell-free extracts of E. coli. However, the ratio of specific activities, obtained with S-ribosylhomocysteine and S-adenosylhomocysteine as substrates during the purification procedure, showed the necessity for cleavage of the glycosyl bond of S-adenosylhomocysteine prior to cleavage of the thioether linkage. The pH optimum for stability of the S-ribosylhomocysteine cleavage enzyme is 7.6 with 0.1 M Tris-HCl buffer, and the Michaelis constant is 1.94 X 10m3 M for S-ribosylhomocysteine.