Inhibition of S-adenosyl-l-homocysteine hydrolase (EC 3.3.1.1) (adenosylhomocysteinase) has been suggested as contributing to the cytotoxic effects of 2′-deoxyadenosine and 9-β-d-arabinofuranosyladenine (ara-A). In the current work, the activity of adenosylhomocysteinase in extracts of asynchronous L1210 leukemia cells was inhibited in a time-dependent manner by 2′-deoxyadenosine and ara-A when assays were conducted in the presence of the inhibitor of adenosine deaminase, deoxycoformycin. Adenosylhomocysteinase was not inhibited by 9-β-d-xylofuranosyladenine, 3′-deoxyadenosine, 9-β-d-arabinofuranosylhypoxanthine, tubercidin, or xylotubercidin. When intact L1210 cells were exposed to growth-inhibitory levels of 2′-deoxyadenosine or ara-A, adenosylhomocysteinase activity was reduced to one-tenth of that of untreated cells, and intracellular levels of S-adenosyl-l-homocysteine were elevated, consistent with the suggestion that inhibition of adenosylhomocysteine contributes to cytotoxicity.

The specific activity of adenosylhomocysteinase was determined at various points during the replication cycle of HeLa S3 cells and Chinese hamster ovary cells under saturating conditions for both substrates (adenosine, l-homocysteine). Adenosylhomocysteinase activity did not change in either cell type as cells progressed from G1 through S phase of the replication cycle.

The relationship between cytotoxicity and the cell cycle was assessed for both adenosylhomocysteinase-inhibitory (ara-A) and noninhibitory (tubercidin, 9-β-d-xylofuranosyladenine, and 3′-deoxyadenosine) agents. High concentrations of the ara-A prodrug, 9-β-d-arabinofuranosyladenine 5′-monophosphate, were required to achieve cytotoxicity against synchronous HeLa cells, and there were no apparent differences in cytotoxicity between G1 and mid-S phase, suggesting that ara-A is not strongly phase specific. 9-β-d-Xylofuranosyladenine was somewhat more toxic during S phase than G1 phase, and tubercidin and 3′-deoxyadenosine exhibited similar toxicities.

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Supported by the National Cancer Institute of Canada, the Alberta Heritage Foundation for Medical Research, and the Alberta Heritage Savings Trust Fund: Applied Research—Cancer.

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