Inhibition of nucleic acid methylation by cordycepin. In vivo synthesis of S-3'-DEOXYADENOSYLMETHIONINE BY WI-L2 human lymphoblasts.

Cell-free extracts of cultured WI-L2 human lymphoblasts catalyze the synthesis of S-3'-deoxyadenosyl-~methionine (3"dAdoMet) and s-S'-[9-@-~-arabinofuranosyl)adenyl]-L-methionine (ara-AdoMet) from L-methionine and either cordycepin triphosphate (3'-deoxyadenosine 5'-triphosphate) or adenine 9-fl-D-arabinofuranoside 5'-triphosphate (ara-ATP), respectively.The enzyme responsible for this reaction is assumed to be ATP:L-methionine S-adenosyltransferase (EC 2.5.1.6).Cordycepin (3'-deoxyadenosine) is cytotoxic for WI- L2 and inhibits growth by 50% at 1.6 p ~. Intact WI-L2 cells treated with an adenosine deaminase inhibitor and cordycepin accumulate substantial amounts of 3'-dATP and 3'-dAdoMet, and smaller amounts of S-3'deoxyadenosyl-L-homocysteine and 3',5'-dideoxy-5'methylthioadenosine indicating that 3'-dAdoMet participates in transmethylation reactions and polyamine synthesis.Measurements of in vivo nucleic acid methylation show that cordycepin causes a marked inhibition of 2'-O-methyluridine, 2'-O-methylcytidine, 5methyluridine, and 5-methylcytidine accumulation in RNA and of 5-methylcytosine formation in DNA.These data suggest that 3'-deoxy analogues of S-adenosylmethionine and S-adenosylhomocysteine are responsible for the inhibition of nucleic acid methylation by cordycepin and that they contribute to the cytotoxic effects of this nucleoside antimetabolite.WI-L2 cells treated with adenine 9-P-D-arabinofuranoside accumulate very little ara-ATP and no detectable ara-AdoMet.