S-adenosylhomocysteine(SAH) is known to be a potent inhibitor of S-adenosylmethionine(SAM)-mediated reactions, of which SAH itself is a product. The immediate metabolic fate of SAH involves its hydrolysis to adenosine and L-homocysteine by the enzyme SAH hydrolase, but the reversibility of this reaction and its extremely low K9 in the hydrolytic direction suggest that under certain conditions of adenosine excess, SAH might accumulate with significant cytotoxic effects. We have used a model system consisting of cultured S49 mouse lymphoma cells together with the adenosine deaminase(ADA) inhibitor, erythro-9-(2-hydroxy-3-nonyl) adenine(EHNA), to determine whether SAH is a mediator of adenosine cytotoxicity.

Cells rendered resistant to adenosine-induced pyrimidine starvation by the addition of exogenous uridine or by the mutational loss of adenosine kinase are still sensitive to adenosine at concentrations> 15 PM. We find that this effect is appreciably enhanced by the addition of L-homocysteine thiolactone to the culture medium. Cytotoxic concentrations of adenosine also cause significant elevations in intracellular levels of SAH, which are increased an additional several fold by 100 FM exogenous L-homocysteine thiolactone. A fair correlation exists between a single time point determination of intracellular SAH and the degree of growth inhibition after 72 hr, but complicated time-dependent variations in SAH make it difficult to compare results obtained in the absence and presence of exogenous L-homocysteine thiolactone. In vivo DNA methylation in S49 cells is markedly inhibited by exposure of cells to concentrations of adenosine known to cause uridine-resistant cytotoxicity. This inhibition of methylation has been measured with short-term pulses of radiolabel, and correlates well with intracellular concentrations of SAH at all tested combinations of