Leukemia L1210 ascites cells sensitive to 6-thioguanine were incubated aerobically in a glucose-balanced salts medium supplemented with glutamine in the presence and and absence of the analog. The rates of incorporation and distribution of various isotopic precursors in this system were examined by radioassaying the various fractions of the tumor cells collected on membrane filters. This technic was extremely useful for the rapid assay of incorporation into tumor cells.

The incorporation of isotopic precursors represented net synthesis rather than exchange. Thioguanine decreased the uptake of formate-C14 and thymidine-C14H3 into the acid-insoluble fraction of the cells by about one-half and one-third, respectively, but did not affect incorporation of methionine-S35 or acetate-2-C14. The drug inhibited incorporation of formate-C14 into the nucleic acids, but proteins were only slightly affected. Molar activities of the isolated nucleic acid bases were reduced by the drug.

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This research was supported by Research Grant CY-2978 from the National Cancer Institute, United States Public Health Service, Bethesda, Maryland.

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