[CANCER RESEARCH 33. 2425 2430, October 1973) Furine Nucleotide Metabolism and Nucleotide Pool Sizes in Synchronized Lymphoma L5178Y Cells1 Floyd F. Snyder, J. Frank Henderson, S. C. Kim, A. R. P. Peterson, and L. W. Brox University of Alberta Cancer Research Unit (McEachern Laboratory) and Department of Biochemistry. University oíAlberta, Edmonton, Alberta. T6G 2EÃŒ,Canada SUMMARY MATERIALS AND METHODS Mouse lymphoma L5178Y cells in culture were syn Cell Cultures. Mouse lymphoma LSI78Y cells were chronized by exposure to thymidine and Colcemid. At four routinely grown in suspension cultures with Fischer's me times during the cell cycle, various aspects of purine metab dium (Grand Island Biological Co., Grand Island, N. Y.) olism were examined with the use of radioactive purine supplemented with 10% horse serum, streptomycin (100 bases. In all experiments the uptake and conversion of bases Mg/ml), and penicillin (100 units/ml) (17). Cell numbers to nucleotides was rate limiting for ribonucleoside triphos- were determined with Coulter Model F or B electronic phate synthesis. Cell cycle variations in the apparent en particle counters. The cells were synchronized by 5 hr of zyme activities of the purine phosphoribosyltransferases, exposure to 2 mM thymidine followed by 5 hr of exposure adenosine monophosphate deaminase and guanosine mono- to 0.02 /ig Colcemid per ml (Grand Island Biological Co.). phosphate reducíase,were observed. Purine and pyrimidine This procedure is similar to that described by Doida and nucleotide pool sizes were determined by high-pressure Okada (9). After resuspension in fresh medium, between liquid chromatography. After correction for increases in 70 and 80% of the cells present underwent synchronous cell volume, it was observed that nicotinamide adenine di- division (Chart 1). nucleotide, adenosine triphosphate, and guanosine triphos- Incubation with Purine Bases. Adenine-8-14C (39.3 phate concentrations remained relatively constant through mCi/mmole), guanine-8-14C (4^1.8 mCi/mmole), and hy- out the cell cycle, whereas uridine diphosphate-glucose, poxanthine-8-14C (49.4 mCi/mmole) were obtained from cytidine triphosphate, and uridine triphosphate concentra Schwarz/Mann, Orangeburg, N. Y. Duplicate 2.5-ml cul tions appeared to decrease in the S and G2 phases. ture samples were transferred to stoppered 16- x 100-mm tubes and maintained at 37°for 20 min; radioactive purine bases were then added. We terminated incubations 30 min INTRODUCTION later by pouring the cultures into chilled centrifuge tubes, centrifuging down the cells in a clinical centrifuge, and ex A delayed lethal effect of 6-mercaptopurine on cultured tracting the cells with 0.010 ml of 0.4 M perchloric acid. mouse lymphoma LSI78Y cells has recently been reported After neutralization with 0.010 ml of 7 M KOH, the entire (16), and it was concluded that this implies involvement of sample was spotted on polyethyleneimine-cellulose plates 6-mercaptopurine with macromolecular synthesis. Con (Polygram Cel 300 PEI; Brinkman Instruments, Westbury, sistent with this suggestion was the observation that the N. Y.). The purine ribo- and deoxyribonucleotides were 6-mercaptopurine toxicity was limited primarily to the separated by 2-dimensional chromatography (14) by a S phase of synchronized lymphoma LSI78Y cells (A. R. P. modification of the method of Crabtree and Henderson (8). Paterson, unpublished observations). However, it was of Analysis of Acid-soluble Nucleotides. Portions of syn interest to examine the possibility that this cell cycle spec chronous lymphoma LSI78Y cultures containing about ificity of 6-mercaptopurine might also be related to varia 1.5 x IO7 cells were taken at various times and poured tion in the activities of enzymes of purine metabolism or of into cooled centrifuge tubes. After 3 min in an ice-water concentrations of purine nucleotides through the cell cycle. bath with frequent mixing, the cells were collected by cen- As there was very little information available as to such trifugation and extracted with 0.3 ml of 0.4 M perchloric variations in mammalian cells, we therefore examined acid for 15 min at 4°.The supernatant was removed after various aspects of purine metabolism, using synchronized centrifugation, and the pellet was reextracted with 0.1 ml lymphoma LSI78Y cells, by utilizing radioactively labeled of 0.4 M perchloric acid. The combined supernatants were purine bases, while purine and pyrimidine nucleotide pools neutralized to between pH 6 and 7 with 7 N KOH. The were determined directly by means of high-pressure liquid KC1O4 was removed by centrifugation, and the extract was chromatography. concentrated to about 0.15 ml by 10% CO2 gently bubbled through the solution at 4°.A known amount of UTP-14C 'This work was supported by the National Cancer Institute of Canada. was added with the initial perchloric acid so that recoveries Received February 23, 1973; accepted June 22, 1973. of acid-soluble nucleotides could be determined from the OCTOBER 1973 2425 Downloaded from cancerres.aacrjournals.org on September 30, 2021. © 1973 American Association for Cancer Research. Snyder, Henderson, Kim, Paterson, and Brox xanthine and guanine was near maximal at 40 JÕM,while 28 that for adenine was still not saturated at 50 JIM. Because adenine is markedly toxic to L5178Y cells in culture at 24 concentrations above 20 pM, this concentration of all 3 pu- rines was chosen for the studies of synchronous cells. The metabolism of the purine bases in the culture me x ~E dium alone was examined by incubation of 20 /¿Madenine, guanine, or hypoxanthine for 30 min with medium without 16 cells. The purine bases were isolated by column chroma- tography on Dowex 50 and by subsequent 2-dimensional 9 JD chromatography on cellulose thin layers (8). In 30 min, 80% g t ÃŽ t D of the guanine originally present was converted to xanthine 12 and 3% of the hypoxanthine was converted to xanthine, but there was no detectable metabolism of the adenine. The 0 u guanine deaminase activity has been attributed to the horse serum present in the culture medium. Samples from a synchronous lymphoma L5178Y culture were taken at the 4 times shown on Chart 1 which corre 8 2468 10 12 14 sponded to periods in the GÃŒ,early S, late S, and G2 phases, as indicated by thymidine-labeling experiments hours (S. C. Kim and A. R. P. Paterson, unpublished observa Chart 1. Increase in cell number of synchronized L5178Y cells with time. After the thymidine and Colcemid treatments, the cells were re- tions). The G, phase sample was taken just after the in crease in cell number had reached a plateau, which was ap suspended at zero time in fresh medium to a concentration of about 100,000 cells/ml. Cell numbers were determined with a Coulter counter. proximately 2 hr after the release of the Colcemid block. Arrows, times at which cells were used for the purine base studies or the The subsequent samples were taken at 2-hr intervals. acid-soluble nucleotide determinations. Total nucleotide synthesis from each purine base during the 4 periods is shown in Table 1. Nucleotide synthesis final volume (determined by weight) and the recovered from adenine increased to a maximum during the late radioactivity. Using a triphosphate as a marker also pro S-phase period and then declined in the G2-phase period. vided an estimate of the extent of any breakdown of tri- Extents of nucleotide synthesis from hypoxanthine and phosphates due to the extraction or storage conditions. guanine were parallel but did not exhibit the magnitude of Essentially no breakdown was observed during extraction or variation seen with adenine. storage of the frozen neutralized extracts for 24 hr. The synthesis of individual nucleotides, expressed in nmoles/109 cells, for the four 30-min labeling periods is Chromatography of the acid-soluble extracts was per formed with a Varian Aerograph LCS-1000 liquid Chro also presented in Table 1. Most of the acid-soluble radio- matograph with a 3-m capillary column packed with Varian PA-38 pellicular anion-exchange resin. The column effluent was monitored at 254 nm with the use of a low-volume (8 ft\) flow cell with a 1-cm light path and a dual-pen recorder set for full-scale deflections of 0.002 and 0.008 absorbance unit. The acid-soluble nucleotides were separated, utilizing a linear gradient of potassium phosphate and potassium chloride at pH 4.5, essentially as described by Brown (5). The radioactivity profile of the UTP-MC marker was determined by collecting 1-min samples and counting the entire sample in Bray's phosphor solution (1). The individ ual nucleotides were quantitated by comparing the peak areas, which were determined by planimetry, with a series of standard areas obtained with authentic nucleotides. 0 40 RESULTS PURINE BASE Chart 2. Total nucleotide synthesis from purine bases in asynchronous The labeling of ATP and GTP from radioactive bases in L5178Y cultures. Cultures (2.5 ml) of exponentially growing cells (141,800 asynchronous lymphoma cells was linear for 30 min. Chart cells/ml) were incubated for 30 min with the indicated concentrations of 2 shows the relationship between total nucleotide synthesis radioactively labeled adenine (•),guanine (D), or hypoxanthine (O). and purine base concentration for exponentially growing The total nucleotides synthesized was determined after polyethyleneimine- lymphoma L5178Y cells. Nucleotide synthesis from hypo- cellulose thin-layer chromatography. 2426 CANCER RESEARCH VOL. 33 Downloaded from cancerres.aacrjournals.org on September 30, 2021. © 1973 American Association for Cancer Research. Downloaded from cancerres.aacrjournals.org on September 30, 2021. © 1973 American Association for Cancer Research. Snyder, Henderson, Kim, Paterson, and Brox Table 2 Apparent enzyme adivines during cell cycle The apparent en/yrne activities are determined as described in the text.