purines, due to reversal of the initial inhibition of purine synthesis or to degradation of nucleic acids within the cell with partial reuse of nucleic acid

Addition of 1 @Mmethotrexate to cultures of L5178Y cells results in an initial inhibition ofthymidine, uridine, and leucine incorporation into acid-insoluble material followed, after about 10 hr. by a partial recovery in the extent of incorporation of these precursors. Acid-soluble adenosine triphosphate and guanosine triphosphate concentrations are greatly reduced initially, but guanosine triphosphate con centrations appear to recover partially by 10 hr. Acid soluble uridine triphosphate and cytidine triphosphate con centrations initially increase after methotrexate treatment but then, with time, they too decline. Hypoxanthine and guanine are more effective than is adenine in overcoming the methotrexate-induced inhibition of thymidine incor poration. These results suggest that, in the presence of methotrexate, guanine nucleotides become limiting for nucleic acid synthesis before adenine nucleotides do. The block of purine de novo synthesis in L5178Y cells by methotrexate is almost complete and is not reversed with time. This suggests that the additional purine nucleotides that are available for nucleic acid synthesis 8 to 10 hr after addition of methotrexate are being derived from nucleic acid breakdown. Consistent with this is the observed re duction in the number of polyribosomes and hence, pre sumably, in messenger RNA levels.