Thymidine (Thd) was phosphorylated to dTTP also at 0°C, both Ehrlich ascites tumor cells and human tonsillar lymphocytes, but was not incorporated into DNA. The uptake and phosphorylation of 14C-Thd into the pools showed regular kinetics (Km 6,6 uM), and the main metabolite was dTTP (75-84%) both at 0°C, and 37°C. Similarly, deoxycytidine (dCyd) was also transported and phosphorylated to nucleotides (76%) at low temperature, but no incorporation into DNA and phospholipid precursor liponucleotides were labeled with 5-3H- dCyd, 51% of the total pool radioactivity was found in liponucleotides. Transport and phosphorylation of deoxynucleosides seem to be tightly coordinated at both temperatures, which processes are directly coupled to membrane-phospholipid and DNA biosynthesis, but only at physiological temperature while they seem 'uncoupled' at low temperature. The fact that nucleoside phosphorylation occurs also at low temperature has implications for several experimental techniques used in cell biology.
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