Studies on the Utilization of Asparagine by Mouse Leukemia Cells2

Asparaginase-sensitive (EARAD1) and asparaginase-resistant (RADA1) mouse leukemia cells were incubated with RPMl 1630 medium containing 15% fetal calf serum and 14C-, amide-15N-L-asparagine, amide-15N-L-glutamine, or 15NH4CI for 24 or 48 hours. The protein and nucleic acid fractions were then isolated and the incorporation of isotopes was determined. In both tumors the incorporation of asparagine carbon and amide nitrogen into nucleic acid was very low, but there was substantial incorporation into protein. The incorporation of glutamine amide nitrogen into nucleic acid and protein was considerable, whereas the incorporation of 15N from 15N-ammonia into nucleic acid and protein was very low. In in vivo experiments mice bearing EARAD1 or RADA1 tumors were given injections of amide-15N-gIutamine, amide-15N-asparagine, or 15NH4CI, and the protein and nucleic acid fractions were isolated 24 hours later. There was substantial incorporation of 15N from glutamine amide and asparagine amide into proteins but such incorporation was only about 10% of that observed in the in vitro experiments. The incorporation of asparagine amide nitrogen into nucleic acids was much less than that of glutamine amide nitrogen, and there was appreciable incorporation of N from ammonium into both nucleic acids and proteins.