Reactivation of S-adenosylhomocysteine hydrolase activity in cells exposed to 9-beta-D-arabinofuranosyladenine.

9-beta-D-Arabinofuranosyladenine (ara-A) inactivates isolated S-adenosyl-L-homocysteine (AdoHcy) hydrolase (EC 3.3.1.1) as well as AdoHcy hydrolase in intact cells. Whereas the inactivation in cell-free systems is an irreversible process, the AdoHcy hydrolase activity in rat hepatocytes exposed to ara-A gradually recovered upon prolonged incubation of the cells in a medium devoid of ara-A. This process, tentatively termed reactivation of the enzyme, was nearly totally dependent on a high level of adenosine deaminase in the extracellular medium, which induced a decrease in intracellular content of adenosine as well as ara-A. Reactivation of intracellular enzyme was inhibited by adenosine deaminase inhibitors [2'-deoxycoformycin and erythro-9-(2-hydroxy-3-nonyl)adenine] and the synthetic substrate for AdoHcy hydrolase, 3-deazaadenosine. An inhibitor of protein synthesis (cycloheximide) was without effect. Homocysteine, which protected the intracellular AdoHcy hydrolase against inactivation by ara-A, induced no reactivation of the enzyme. The half-life of the intracellular ara-A-AdoHcy hydrolase complex was about 90 min and was not affected by adenosine deaminase, 3-deazaadenosine, or homocysteine added to the cell suspension. However, the rate of elimination of the complex in the hepatocytes exceeded the rate of reactivation of AdoHcy hydrolase. Thus, the elimination process accounted for the reactivation, but not correlation between these two processes was observed. Reactivation of intracellular AdoHcy hydrolase caused a pronounced fall in cellular content of AdoHcy. The possibility that reduced cellular level of AdoHcy induced the reactivation of AdoHcy hydrolase seemed unlikely. This statement was based on the observation that reactivation was observed also under conditions of high concentrations of AdoHcy (obtained by the addition of homocysteine to the cell suspension). Reactivation of AdoHcy hydrolase with a concomitant decrease in cellular level of AdoHcy could also be demonstrated with mouse plasmacytoma (MPC-11) cells and mouse fibroblasts (L-929) exposed to ara-A, but the reactivation process was far less pronounced than with hepatocytes.