Several nucleoside analogues, some of which are potent inactivators of isolated S-adenosylhomocysteinase (AdoHcyase), were tested with respect to their effect on intracellular AdoHcyase and S-adenosylhomocysteine (AdoHcy) catabolism in intact rat hepatocytes. Among the analogues tested, 9-β-d-arabinofuranosyladenine and 9-β-d-arabinofuranosyl-3-deazaadenine were the most potent inactivators of intracellular AdoHcyase. Compounds like 2-chloroadenosine and carbocyclic adenosine are extremely efficient inactivators of the isolated enzyme, but these nucleosides exerted only a limited effect on the enzyme in intact hepatocytes. Only a moderate effect was observed with 2-chloro-3-deazaadenosine and 2′-deoxyadenosine, and a high concentration of 5′-deoxy-5′-methylthioadenosine was required to inactivate the enzyme. There was a correlation between inactivation and accumulation of AdoHcy. Carbocyclic-3-deazaadenosine caused no inactivation of AdoHcyase in liver cells but caused a massive accumulation of AdoHcy, suggesting that this analogue functions as a reversible inhibitor of the enzyme. Residual enzyme activity was observed with all the nucleoside analogues tested. The intracellular enzyme inactivated in the presence of 2′-deoxyadenosine and 9-β-d-arabinofuranosyladenine was readily recovered when the cells were transferred to fresh medium containing adenosine deaminase, but reactivation of the enzyme activity after treatment of the cells with 2-chloroadenosine and 5′-deoxy-5′-methylthioadenosine was also observed. The inactivation of intracellular enzyme induced by 2-chloro-3-deazaadenosine, 9-β-d-arabinofuranosyl-3-deazaadenine, and carbocyclic adenosine was essentially irreversible under the conditions of the experiments. Factors determining the effect of nucleoside analogues on intracellular AdoHcyase are discussed.

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Supported by grants from the Norwegian Society for Fighting Cancer.

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