S-Adenosylhomocysteine hydrolase from hamster liver: purification and kinetic properties

3-Deazaadenosine is both an inhibitor of and a substrate for S-adenosylhomocysteine hydrolase. Its administration to rats results in the accumulation of both S-adenosylhomocysteine and 3-deazaadenosylhomocysteine in the liver and other tissues. In hamsters, however, the administration of 3-deazaadenosine results only in the accumulation of 3-deazaadenosylhomocysteine (P. K. Chiang and G. L. Cantoni (1979) Biochem. Pharmacol. 28, 1897). In order to investigate the possible reasons for this difference, S-adenosylhomocysteine hydrolase from hamster liver has been purified to homogeneity and some of its kinetic and physical parameters have been determined. The molecular weight of the native enzyme is 200,000 with a subunit molecular weight of 48,000. The Km's for adenosine and 3-deazaadenosine are about 1.0 microM, and the Vmax's are also similar. The Km for S-adenosylhomocysteine is 1.0 microM, or more than 10 times smaller than the Km of the rat liver enzyme. This difference in Km value may explain the differences in the response of rat and hamster liver to the administration of 3-deazaadenosine. S-Adenosylhomocysteine hydrolase from hamster liver exhibits an interesting kinetic property in that its activity can be affected bimodally by either adenosine or adenosine analogs. At very low concentrations of these analogs, the activity of S-adenosylhomocysteine hydrolase can be stimulated by 10-30%, and at higher concentrations these same analogs become competitive inhibitors.