Molecular Pharmacology, Vol 18, 483-490, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Evidence that the Inhibitory Effect of Adenosine, but not Cordycepin, on the Methylation of Nuclear RNA is Mediated by S-Adenosylhomocysteine Hydrolase

¹ Applied Pharmacology Section, Laboratory of Medicinal Chemistry and Biology, National Cancer Institute, Bethesda, Maryland 20205

The mechanism of action of adenosine, 2'-deoxyadenosine, and cordycepin (3'-deoxyadenosine) was explored in L1210 cells in vitro under conditions where their deamination was blocked by the adenosine deaminase inhibitor, 2'-deoxycoformycin. Cordycepin but not adenosine or 2'-deoxyadenosine inhibited the methylation of nuclear RNA in the presence of 2'-deoxycoformycin. Upon the addition of homocysteine, adenosine produced 65% inhibition of methylation of nuclear RNA, whereas 2'-deoxyadenosine was ineffective and the inhibitory effect of cordycepin was not potentiated. Under the latter conditions, RNA synthesis as measured by [¹⁴C]uridine incorporation was marginally affected (30% inhibition) by adenosine plus homocysteine, but markedly inhibited by 70% by cordycepin. Cordycepin inhibited 2'-O methylation of nuclear RNA to a greater extent than base methylation, while the combination of adenosine and homocysteine inhibited these sites to equal degrees. Moreover, cordycepin inhibited >18 S nuclear RNA four times as extensively as 4 S nuclear RNA, in comparison to the equal extents of inhibition of these two classes of nuclear RNA by adenosine plus homocysteine. A positive correlation was observed between the generation of intracellular S-adenosylhomocysteine in L1210 cells and the inhibition of methylation of nuclear RNA by adenosine plus homocysteine, but not by cordycepin. These results indicate that a significant amount of S-adenosylhomocysteine can be generated in the presence of adenosine and homocysteine, presumably via S-adenosylhomocysteine hydrolase, leading to marked inhibition of methylation of nuclear RNA in mouse lymphoid leukemia cells.

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ACKNOWLEDGMENT The authors wish to thank Mrs. Sylvia Rose for typing the manuscript.